fix: implement Ace Lightning (2002) obscure behaviour

Reviewed-on: #7

.github/workflows/main.yml

@@ -0,0 +1,59 @@
+name: Nightly
+
+on:
+  push:
+    paths:
+      - "**.zig"
+    branches:
+      - main
+  schedule:
+    - cron: '0 0 * * *'
+  workflow_dispatch:
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        # os: [ubuntu-latest, windows-latest, macos-latest]
+        os: [ubuntu-latest, windows-latest]
+    runs-on: ${{matrix.os}}
+    steps:
+      - uses: goto-bus-stop/setup-zig@v2
+        with:
+          version: master
+      - name: prepare-linux
+        if: runner.os == 'Linux'
+        run: |
+            sudo apt update
+            sudo apt install libgtk-3-dev libsdl2-dev
+      - name: prepare-windows
+        if: runner.os == 'Windows'
+        run: |
+            vcpkg integrate install
+            vcpkg install sdl2:x64-windows
+            git config --global core.autocrlf false
+      - name: prepare-macos
+        if: runner.os == 'macOS'
+        run: |
+            brew install sdl2
+      - uses: actions/checkout@v3
+        with:
+          submodules: recursive
+      - name: build 
+        run: zig build -Doptimize=ReleaseSafe -Dcpu=baseline
+      - name: upload
+        uses: actions/upload-artifact@v3
+        with:
+          name: zba-${{matrix.os}}
+          path: zig-out/bin
+  lint:
+    runs-on: ubuntu-latest
+    steps: 
+      - uses: actions/checkout@v3
+        with:
+          submodules: recursive
+      - uses: goto-bus-stop/setup-zig@v2
+        with:
+          version: master
+      - run: zig fmt src/**/*.zig
+  

.gitignore

@@ -1,7 +1,7 @@
 /.vscode
 /bin
-/zig-cache
-/zig-out
+**/zig-cache
+**/zig-out
 /docs
 **/*.log
 **/*.bin
@@ -11,4 +11,8 @@
 /lib/SDL2
 
 # Any Custom Scripts for Debugging purposes
-*.sh
+*.sh
+
+
+# Dear ImGui
+**/imgui.ini

.gitmodules

@@ -13,3 +13,15 @@
 [submodule "lib/zig-toml"]
 	path = lib/zig-toml
 	url = https://github.com/aeronavery/zig-toml
+[submodule "lib/zba-gdbstub"]
+	path = lib/zba-gdbstub
+	url = https://git.musuka.dev/paoda/zba-gdbstub
+[submodule "lib/zgui"]
+	path = lib/zgui
+	url = https://git.musuka.dev/paoda/zgui
+[submodule "lib/nfd-zig"]
+	path = lib/nfd-zig
+	url = https://github.com/fabioarnold/nfd-zig
+[submodule "lib/zba-util"]
+	path = lib/zba-util
+	url = https://git.musuka.dev/paoda/zba-util.git

.vscode/extensions.json

@@ -1,8 +0,0 @@
-{
-    "recommendations": [
-        "augusterame.zls-vscode",
-        "usernamehw.errorlens",
-        "vadimcn.vscode-lldb",
-        "dan-c-underwood.arm"
-    ]
-}

README.md

@@ -1,81 +1,113 @@
 # ZBA (working title)
+
 A Game Boy Advance Emulator written in Zig ⚡!
 
+![ZBA running リズム天国](assets/screenshot.png)
+
 ## Scope
-I'm hardly the first to write a Game Boy Advance Emulator nor will I be the last. This project isn't going to compete with the GOATs like 
-[mGBA](https://github.com/mgba-emu) or [NanoBoyAdvance](https://github.com/nba-emu/NanoBoyAdvance). There aren't any interesting
-ideas either like in [DSHBA](https://github.com/DenSinH/DSHBA). 
 
-This is a simple (read: incomplete) for-fun long-term project. I hope to get "mostly there", which to me means that I'm not missing any major hardware
-features and the set of possible improvements would be in memory timing or in UI/UX. With respect to that goal, here's what's outstanding: 
+I'm hardly the first to write a Game Boy Advance Emulator nor will I be the last. This project isn't going to compete with the GOATs like [mGBA](https://github.com/mgba-emu) or [NanoBoyAdvance](https://github.com/nba-emu/NanoBoyAdvance). There aren't any interesting ideas either like in [DSHBA](https://github.com/DenSinH/DSHBA).
 
-### TODO 
-- [ ] Affine Sprites
+This is a simple (read: incomplete) for-fun long-term project. I hope to get "mostly there", which to me means that I'm not missing any major hardware features and the set of possible improvements would be in memory timing or in UI/UX. With respect to that goal, here's what's outstanding:
+
+### TODO
+
+- [x] Affine Sprites
 - [ ] Windowing (see [this branch](https://git.musuka.dev/paoda/zba/src/branch/window))
-- [ ] Shaders (see [this branch](https://git.musuka.dev/paoda/zba/src/branch/opengl))
 - [ ] Audio Resampler (Having issues with SDL2's)
-- [ ] Immediate Mode GUI
 - [ ] Refactoring for easy-ish perf boosts
 
-## Tests 
-- [x] [jsmolka's GBA Test Collection](https://github.com/jsmolka/gba-tests)
-    - [x] `arm.gba` and `thumb.gba`
-    - [x] `flash64.gba`, `flash128.gba`, `none.gba`, and `sram.gba`
-    - [x] `hello.gba`, `shades.gba`, and `stripes.gba`
-    - [x] `memory.gba`
-    - [x] `bios.gba`
-    - [x] `nes.gba`
-- [ ] [DenSinH's GBA ROMs](https://github.com/DenSinH/GBARoms)
-    - [x] `eeprom-test` and `flash-test`
-    - [x] `midikey2freq`
-    - [ ] `swi-tests-random`
-- [ ] [destoer's GBA Tests](https://github.com/destoer/gba_tests)
-    - [x] `cond_invalid.gba`
-    - [x] `dma_priority.gba`
-    - [x] `hello_world.gba`
-    - [x] `if_ack.gba`
-    - [ ] `line_timing.gba`
-    - [ ] `lyc_midline.gba`
-    - [ ] `window_midframe.gba`
-- [x] [ladystarbreeze's GBA Test Collection](https://github.com/ladystarbreeze/GBA-Test-Collection)
-    - [x] `retAddr.gba`
-    - [x] `helloWorld.gba`
-    - [x] `helloAudio.gba`
-- [x] [`armwrestler-gba-fixed.gba`](https://github.com/destoer/armwrestler-gba-fixed)
-- [x] [FuzzARM](https://github.com/DenSinH/FuzzARM)
+## Usage
+
+As it currently exists, ZBA is run from the terminal. In your console of choice, type `./zba --help` to see what you can do.
+
+I typically find myself typing `./zba -b ./bin/bios.bin` and then going to File -> Insert ROM to load the title of my choice.
+
+Need a BIOS? Why not try using the open-source [Cult-Of-GBA BIOS](https://github.com/Cult-of-GBA/BIOS) written by [fleroviux](https://github.com/fleroviux) and [DenSinH](https://github.com/DenSinH)?
+
+Finally it's worth noting that ZBA uses a TOML config file it'll store in your OS's data directory. See `example.toml` to learn about the defaults and what exactly you can mess around with.
+
+## Tests
+
+GBA Tests | [jsmolka](https://github.com/jsmolka/)
+--- | ---
+`arm.gba`,  `thumb.gba` | PASS
+`memory.gba`, `bios.gba` | PASS
+`flash64.gba`, `flash128.gba` | PASS
+`sram.gba` | PASS
+`none.gba` | PASS
+`hello.gba`, `shades.gba`, `stripes.gba` | PASS
+`nes.gba` | PASS
+
+GBARoms | [DenSinH](https://github.com/DenSinH/)
+--- | ---
+`eeprom-test`, `flash-test` | PASS
+`midikey2freq` | PASS
+`swi-tests-random` | FAIL
+
+gba_tests | [destoer](https://github.com/destoer/)
+--- | ---
+`cond_invalid.gba` | PASS
+`dma_priority.gba` | PASS
+`hello_world.gba` | PASS
+`if_ack.gba` | PASS
+`line_timing.gba` | FAIL
+`lyc_midline.gba` | FAIL
+`window_midframe.gba` | FAIL
+
+GBA Test Collection | [ladystarbreeze](https://github.com/ladystarbreeze)
+--- | ---
+`retAddr.gba` | PASS
+`helloWorld.gba` | PASS
+`helloAudio.gba` | PASS
+
+FuzzARM | [DenSinH](https://github.com/DenSinH/)
+--- | ---
+`main.gba` | PASS
+
+arm7wrestler GBA Fixed | [destoer](https://github.com/destoer)
+--- | ---
+`armwrestler-gba-fixed.gba` | PASS
 
 ## Resources
-* [GBATEK](https://problemkaputt.de/gbatek.htm)
-* [TONC](https://coranac.com/tonc/text/toc.htm)
-* [ARM Architecture Reference Manual](https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/third-party/ddi0100e_arm_arm.pdf)
-* [ARM7TDMI Data Sheet](https://www.dca.fee.unicamp.br/cursos/EA871/references/ARM/ARM7TDMIDataSheet.pdf)
+
+- [GBATEK](https://problemkaputt.de/gbatek.htm)
+- [TONC](https://coranac.com/tonc/text/toc.htm)
+- [ARM Architecture Reference Manual](https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/third-party/ddi0100e_arm_arm.pdf)
+- [ARM7TDMI Data Sheet](https://www.dca.fee.unicamp.br/cursos/EA871/references/ARM/ARM7TDMIDataSheet.pdf)
 
 ## Compiling
-Most recently built on Zig [0.10.0-dev.4324+c23b3e6fd](https://github.com/ziglang/zig/tree/c23b3e6fd)
+
+Most recently built on Zig [v0.11.0-dev.2168+322ace70f](https://github.com/ziglang/zig/tree/322ace70f)
 
 ### Dependencies
-* [SDL.zig](https://github.com/MasterQ32/SDL.zig)
-    * [SDL2](https://www.libsdl.org/download-2.0.php)
-* [zig-clap](https://github.com/Hejsil/zig-clap)
-* [known-folders](https://github.com/ziglibs/known-folders)
-* [zig-toml](https://github.com/aeronavery/zig-toml)
-* [zig-datetime](https://github.com/frmdstryr/zig-datetime)
-* [`bitfields.zig`](https://github.com/FlorenceOS/Florence/blob/aaa5a9e568/lib/util/bitfields.zig)
 
-`bitfields.zig` from [FlorenceOS](https://github.com/FlorenceOS) is included under `lib/util/bitfield.zig`.
+Dependency | Source
+--- | ---
+SDL.zig | <https://github.com/MasterQ32/SDL.zig>
+known-folders | <https://github.com/ziglibs/known-folders>
+nfd-zig | <https://github.com/fabioarnold/nfd-zig>
+zgui | <https://github.com/michal-z/zig-gamedev/tree/main/libs/zgui>
+zig-clap | <https://github.com/Hejsil/zig-clap>
+zig-datetime | <https://github.com/frmdstryr/zig-datetime>
+zig-toml | <https://github.com/aeronavery/zig-toml>
+`bitfields.zig` | [https://github.com/FlorenceOS/Florence](https://github.com/FlorenceOS/Florence/blob/aaa5a9e568/lib/util/bitfields.zig)
+`gl.zig` | <https://github.com/MasterQ32/zig-opengl>
 
-Use `git submodule update --init` from the project root to pull the git submodules `SDL.zig`, `zig-clap`, `known-folders`, `zig-toml` and `zig-datetime`
+Use `git submodule update --init` from the project root to pull the git relevant git submodules
 
-Be sure to provide SDL2 using: 
-* Linux: Your distro's package manager
-* MacOS: ¯\\\_(ツ)_/¯
-* Windows: [`vcpkg`](https://github.com/Microsoft/vcpkg) (install `sdl2:x64-windows`)
+Be sure to provide SDL2 using:
 
-`SDL.zig` will provide a helpful compile error if the zig compiler is unable to find SDL2. 
+- Linux: Your distro's package manager
+- macOS: ¯\\\_(ツ)_/¯ (try [this formula](https://formulae.brew.sh/formula/sdl2)?)
+- Windows: [`vcpkg`](https://github.com/Microsoft/vcpkg) (install `sdl2:x64-windows`)
 
-Once you've got all the dependencies, execute `zig build -Drelease-fast`. The executable is located at `zig-out/bin/`. 
+`SDL.zig` will provide a helpful compile error if the zig compiler is unable to find SDL2.
+
+Once you've got all the dependencies, execute `zig build -Doptimize=ReleaseSafe`. The executable is located at `zig-out/bin/`.
 
 ## Controls
+
 Key | Button
 --- | ---
 <kbd>X</kbd> | A

build.zig

@@ -1,45 +1,70 @@
 const std = @import("std");
+const builtin = @import("builtin");
+
 const Sdk = @import("lib/SDL.zig/Sdk.zig");
+const gdbstub = @import("lib/zba-gdbstub/build.zig");
+const zgui = @import("lib/zgui/build.zig");
+const nfd = @import("lib/nfd-zig/build.zig");
+
+pub fn build(b: *std.Build) void {
+    // Minimum Zig Version
+    const min_ver = std.SemanticVersion.parse("0.11.0-dev.2168+322ace70f") catch return; // https://github.com/ziglang/zig/commit/322ace70f
+    if (builtin.zig_version.order(min_ver).compare(.lt)) {
+        std.log.err("{s}", .{b.fmt("Zig v{} does not meet the minimum version requirement. (Zig v{})", .{ builtin.zig_version, min_ver })});
+        std.os.exit(1);
+    }
 
-pub fn build(b: *std.build.Builder) void {
-    // Standard target options allows the person running `zig build` to choose
-    // what target to build for. Here we do not override the defaults, which
-    // means any target is allowed, and the default is native. Other options
-    // for restricting supported target set are available.
     const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
 
-    // Standard release options allow the person running `zig build` to select
-    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall.
-    const mode = b.standardReleaseOptions();
+    const exe = b.addExecutable(.{
+        .name = "zba",
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
 
-    const exe = b.addExecutable("zba", "src/main.zig");
-    exe.setTarget(target);
+    exe.setMainPkgPath("."); // Necessary so that src/main.zig can embed example.toml
 
     // Known Folders (%APPDATA%, XDG, etc.)
-    exe.addPackagePath("known_folders", "lib/known-folders/known-folders.zig");
+    exe.addAnonymousModule("known_folders", .{ .source_file = .{ .path = "lib/known-folders/known-folders.zig" } });
 
     // DateTime Library
-    exe.addPackagePath("datetime", "lib/zig-datetime/src/main.zig");
+    exe.addAnonymousModule("datetime", .{ .source_file = .{ .path = "lib/zig-datetime/src/main.zig" } });
 
     // Bitfield type from FlorenceOS: https://github.com/FlorenceOS/
-    // exe.addPackage(.{ .name = "bitfield", .path = .{ .path = "lib/util/bitfield.zig" } });
-    exe.addPackagePath("bitfield", "lib/util/bitfield.zig");
+    exe.addAnonymousModule("bitfield", .{ .source_file = .{ .path = "lib/bitfield.zig" } });
 
     // Argument Parsing Library
-    exe.addPackagePath("clap", "lib/zig-clap/clap.zig");
+    exe.addAnonymousModule("clap", .{ .source_file = .{ .path = "lib/zig-clap/clap.zig" } });
 
     // TOML Library
-    exe.addPackagePath("toml", "lib/zig-toml/src/toml.zig");
+    exe.addAnonymousModule("toml", .{ .source_file = .{ .path = "lib/zig-toml/src/toml.zig" } });
 
     // OpenGL 3.3 Bindings
-    exe.addPackagePath("gl", "lib/gl.zig");
+    exe.addAnonymousModule("gl", .{ .source_file = .{ .path = "lib/gl.zig" } });
+
+    // ZBA utility code
+    exe.addAnonymousModule("zba-util", .{ .source_file = .{ .path = "lib/zba-util/src/lib.zig" } });
+
+    // gdbstub
+    exe.addModule("gdbstub", gdbstub.getModule(b));
+
+    // NativeFileDialog(ue) Bindings
+    exe.linkLibrary(nfd.makeLib(b, target, optimize));
+    exe.addModule("nfd", nfd.getModule(b));
 
     // Zig SDL Bindings: https://github.com/MasterQ32/SDL.zig
-    const sdk = Sdk.init(b);
+    const sdk = Sdk.init(b, null);
     sdk.link(exe, .dynamic);
-    exe.addPackage(sdk.getNativePackage("sdl2"));
+    exe.addModule("sdl2", sdk.getNativeModule());
+
+    // Dear ImGui bindings
+
+    // .shared option should stay in sync with SDL.zig call above where true == .dynamic, and false == .static
+    const zgui_pkg = zgui.package(b, target, optimize, .{ .options = .{ .backend = .sdl2_opengl3, .shared = true } });
+    zgui_pkg.link(exe);
 
-    exe.setBuildMode(mode);
     exe.install();
 
     const run_cmd = exe.run();
@@ -51,9 +76,11 @@ pub fn build(b: *std.build.Builder) void {
     const run_step = b.step("run", "Run the app");
     run_step.dependOn(&run_cmd.step);
 
-    const exe_tests = b.addTest("src/main.zig");
-    exe_tests.setTarget(target);
-    exe_tests.setBuildMode(mode);
+    const exe_tests = b.addTest(.{
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
 
     const test_step = b.step("test", "Run unit tests");
     test_step.dependOn(&exe_tests.step);

example.toml

@@ -1,7 +1,7 @@
 [Host]
 # Using nearest-neighbour scaling, how many times the native resolution 
 # of the game bow should the screen be?
-win_scale = 4
+win_scale = 3
 # Enable VSYNC on the UI thread
 vsync = true
 # Mute ZBA
@@ -9,9 +9,9 @@ mute = false
 
 [Guest]
 # Sync Emulation to Audio 
-audio_sync = false
+audio_sync = true
 # Sync Emulation to Video
-video_sync = false
+video_sync = true
 # Force RTC support
 force_rtc = false
 # Skip BIOS

src/config.zig

@@ -49,16 +49,19 @@ pub fn config() *const Config {
 }
 
 /// Reads a config file and then loads it into the global state
-pub fn load(allocator: Allocator, config_path: []const u8) !void {
-    var config_file = try std.fs.cwd().openFile(config_path, .{});
+pub fn load(allocator: Allocator, file_path: []const u8) !void {
+    var config_file = try std.fs.cwd().openFile(file_path, .{});
     defer config_file.close();
 
-    log.info("loaded from {s}", .{config_path});
+    log.info("loaded from {s}", .{file_path});
 
     const contents = try config_file.readToEndAlloc(allocator, try config_file.getEndPos());
     defer allocator.free(contents);
 
-    const table = try toml.parseContents(allocator, contents, null);
+    var parser = try toml.parseFile(allocator, file_path);
+    defer parser.deinit();
+
+    const table = try parser.parse();
     defer table.deinit();
 
     // TODO: Report unknown config options

src/core/Bus.zig

@@ -1,6 +1,5 @@
 const std = @import("std");
 
-const AudioDeviceId = @import("sdl2").SDL_AudioDeviceID;
 const Arm7tdmi = @import("cpu.zig").Arm7tdmi;
 const Bios = @import("bus/Bios.zig");
 const Ewram = @import("bus/Ewram.zig");
@@ -20,7 +19,7 @@ const log = std.log.scoped(.Bus);
 
 const createDmaTuple = @import("bus/dma.zig").create;
 const createTimerTuple = @import("bus/timer.zig").create;
-const rotr = @import("../util.zig").rotr;
+const rotr = @import("zba-util").rotr;
 
 const timings: [2][0x10]u8 = [_][0x10]u8{
     // BIOS, Unused, EWRAM, IWRAM, I/0, PALRAM, VRAM, OAM, ROM0, ROM0, ROM1, ROM1, ROM2, ROM2, SRAM, Unused
@@ -34,6 +33,11 @@ pub const fetch_timings: [2][0x10]u8 = [_][0x10]u8{
     [_]u8{ 1, 1, 6, 1, 1, 2, 2, 1, 4, 4, 4, 4, 4, 4, 8, 8 }, // 32-bit
 };
 
+// Fastmem Related
+const page_size = 1 * 0x400; // 1KiB
+const address_space_size = 0x1000_0000;
+const table_len = address_space_size / page_size;
+
 const Self = @This();
 
 pak: GamePak,
@@ -49,7 +53,16 @@ io: Io,
 cpu: *Arm7tdmi,
 sched: *Scheduler,
 
+read_table: *const [table_len]?*const anyopaque,
+write_tables: [2]*const [table_len]?*anyopaque,
+allocator: Allocator,
+
 pub fn init(self: *Self, allocator: Allocator, sched: *Scheduler, cpu: *Arm7tdmi, paths: FilePaths) !void {
+    const tables = try allocator.alloc(?*anyopaque, 3 * table_len); // Allocate all tables
+
+    const read_table = tables[0..table_len];
+    const write_tables = .{ tables[table_len .. 2 * table_len], tables[2 * table_len .. 3 * table_len] };
+
     self.* = .{
         .pak = try GamePak.init(allocator, cpu, paths.rom, paths.save),
         .bios = try Bios.init(allocator, paths.bios),
@@ -62,7 +75,17 @@ pub fn init(self: *Self, allocator: Allocator, sched: *Scheduler, cpu: *Arm7tdmi
         .io = Io.init(),
         .cpu = cpu,
         .sched = sched,
+
+        .read_table = read_table,
+        .write_tables = write_tables,
+        .allocator = allocator,
     };
+
+    self.fillReadTable(read_table);
+
+    // Internal Display Memory behaves differently on 8-bit reads
+    self.fillWriteTable(u32, write_tables[0]);
+    self.fillWriteTable(u8, write_tables[1]);
 }
 
 pub fn deinit(self: *Self) void {
@@ -71,58 +94,167 @@ pub fn deinit(self: *Self) void {
     self.pak.deinit();
     self.bios.deinit();
     self.ppu.deinit();
+
+    // This is so I can deallocate the original `allocator.alloc`. I have to re-make the type
+    // since I'm not keeping it around, This is very jank and bad though
+    // FIXME: please figure out another way
+    self.allocator.free(@ptrCast([*]const ?*anyopaque, self.read_table[0..])[0 .. 3 * table_len]);
     self.* = undefined;
 }
 
-pub fn dbgRead(self: *const Self, comptime T: type, address: u32) T {
-    const page = @truncate(u8, address >> 24);
-    const aligned_addr = forceAlign(T, address);
+pub fn reset(self: *Self) void {
+    self.bios.reset();
+    self.ppu.reset();
+    self.apu.reset();
+    self.iwram.reset();
+    self.ewram.reset();
 
-    return switch (page) {
-        // General Internal Memory
-        0x00 => blk: {
-            if (address < Bios.size)
-                break :blk self.bios.dbgRead(T, self.cpu.r[15], aligned_addr);
+    // https://github.com/ziglang/zig/issues/14705
+    {
+        comptime var i: usize = 0;
+        inline while (i < self.dma.len) : (i += 1) {
+            self.dma[0].reset();
+        }
+    }
 
-            break :blk self.openBus(T, address);
-        },
-        0x02 => self.ewram.read(T, aligned_addr),
-        0x03 => self.iwram.read(T, aligned_addr),
-        0x04 => self.readIo(T, address),
+    // https://github.com/ziglang/zig/issues/14705
+    {
+        comptime var i: usize = 0;
+        inline while (i < self.tim.len) : (i += 1) {
+            self.tim[0].reset();
+        }
+    }
 
-        // Internal Display Memory
-        0x05 => self.ppu.palette.read(T, aligned_addr),
-        0x06 => self.ppu.vram.read(T, aligned_addr),
-        0x07 => self.ppu.oam.read(T, aligned_addr),
-
-        // External Memory (Game Pak)
-        0x08...0x0D => self.pak.dbgRead(T, aligned_addr),
-        0x0E...0x0F => blk: {
-            const value = self.pak.backup.read(address);
-
-            const multiplier = switch (T) {
-                u32 => 0x01010101,
-                u16 => 0x0101,
-                u8 => 1,
-                else => @compileError("Backup: Unsupported read width"),
-            };
-
-            break :blk @as(T, value) * multiplier;
-        },
-        else => self.openBus(T, address),
-    };
+    self.io.reset();
 }
 
-fn readIo(self: *const Self, comptime T: type, unaligned_address: u32) T {
-    const maybe_value = io.read(self, T, forceAlign(T, unaligned_address));
-    return if (maybe_value) |value| value else self.openBus(T, unaligned_address);
+pub fn replaceGamepak(self: *Self, file_path: []const u8) !void {
+    // Note: `save_path` isn't owned by `Backup`
+    const save_path = self.pak.backup.save_path;
+    self.pak.deinit();
+
+    self.pak = try GamePak.init(self.allocator, self.cpu, file_path, save_path);
+
+    const read_ptr: *[table_len]?*const anyopaque = @constCast(self.read_table);
+    const write_ptrs: [2]*[table_len]?*anyopaque = .{ @constCast(self.write_tables[0]), @constCast(self.write_tables[1]) };
+
+    self.fillReadTable(read_ptr);
+    self.fillWriteTable(u32, write_ptrs[0]);
+    self.fillWriteTable(u8, write_ptrs[1]);
+}
+
+fn fillReadTable(self: *Self, table: *[table_len]?*const anyopaque) void {
+    const vramMirror = @import("ppu/Vram.zig").mirror;
+
+    for (table, 0..) |*ptr, i| {
+        const addr = @intCast(u32, page_size * i);
+
+        ptr.* = switch (addr) {
+            // General Internal Memory
+            0x0000_0000...0x0000_3FFF => null, // BIOS has it's own checks
+            0x0200_0000...0x02FF_FFFF => &self.ewram.buf[addr & 0x3FFFF],
+            0x0300_0000...0x03FF_FFFF => &self.iwram.buf[addr & 0x7FFF],
+            0x0400_0000...0x0400_03FF => null, // I/O
+
+            // Internal Display Memory
+            0x0500_0000...0x05FF_FFFF => &self.ppu.palette.buf[addr & 0x3FF],
+            0x0600_0000...0x06FF_FFFF => &self.ppu.vram.buf[vramMirror(addr)],
+            0x0700_0000...0x07FF_FFFF => &self.ppu.oam.buf[addr & 0x3FF],
+
+            // External Memory (Game Pak)
+            0x0800_0000...0x0DFF_FFFF => self.fillReadTableExternal(addr),
+            0x0E00_0000...0x0FFF_FFFF => null, // SRAM
+            else => null,
+        };
+    }
+}
+
+fn fillWriteTable(self: *Self, comptime T: type, table: *[table_len]?*const anyopaque) void {
+    comptime std.debug.assert(T == u32 or T == u16 or T == u8);
+    const vramMirror = @import("ppu/Vram.zig").mirror;
+
+    for (table, 0..) |*ptr, i| {
+        const addr = @intCast(u32, page_size * i);
+
+        ptr.* = switch (addr) {
+            // General Internal Memory
+            0x0000_0000...0x0000_3FFF => null, // BIOS has it's own checks
+            0x0200_0000...0x02FF_FFFF => &self.ewram.buf[addr & 0x3FFFF],
+            0x0300_0000...0x03FF_FFFF => &self.iwram.buf[addr & 0x7FFF],
+            0x0400_0000...0x0400_03FF => null, // I/O
+
+            // Internal Display Memory
+            0x0500_0000...0x05FF_FFFF => if (T != u8) &self.ppu.palette.buf[addr & 0x3FF] else null,
+            0x0600_0000...0x06FF_FFFF => if (T != u8) &self.ppu.vram.buf[vramMirror(addr)] else null,
+            0x0700_0000...0x07FF_FFFF => if (T != u8) &self.ppu.oam.buf[addr & 0x3FF] else null,
+
+            // External Memory (Game Pak)
+            0x0800_0000...0x0DFF_FFFF => null, // ROM
+            0x0E00_0000...0x0FFF_FFFF => null, // SRAM
+            else => null,
+        };
+    }
+}
+
+fn fillReadTableExternal(self: *Self, addr: u32) ?*anyopaque {
+    // see `GamePak.zig` for more information about what conditions need to be true
+    // so that a simple pointer dereference isn't possible
+
+    std.debug.assert(addr & @as(u32, page_size - 1) == 0); // addr is guaranteed to be page-aligned
+
+    const start_addr = addr;
+    const end_addr = start_addr + page_size;
+
+    {
+        const data = start_addr <= 0x0800_00C4 and 0x0800_00C4 < end_addr; // GPIO Data
+        const direction = start_addr <= 0x0800_00C6 and 0x0800_00C6 < end_addr; // GPIO Direction
+        const control = start_addr <= 0x0800_00C8 and 0x0800_00C8 < end_addr; // GPIO Control
+
+        const has_gpio = data or direction or control;
+        const gpio_kind = self.pak.gpio.device.kind;
+
+        // There is a GPIO Device, and the current page contains at least one memory-mapped GPIO register
+        if (gpio_kind != .None and has_gpio) return null;
+    }
+
+    if (self.pak.backup.kind == .Eeprom) {
+        if (self.pak.buf.len > 0x100_000) {
+            // We are using a "large" EEPROM which means that if the below check is true
+            // this page has an address that's reserved for the EEPROM and therefore must
+            // be handled in slowmem
+            if (addr & 0x1FF_FFFF > 0x1FF_FEFF) return null;
+        } else {
+            // We are using a "small" EEPROM which means that if the below check is true
+            // (that is, we're in the 0xD address page) then we must handle at least one
+            // address in this page in slowmem
+            if (@truncate(u4, addr >> 24) == 0xD) return null;
+        }
+    }
+
+    // Finally, the GamePak has some unique behaviour for reads past the end of the ROM,
+    // so those will be handled by slowmem as well
+    const masked_addr = addr & 0x1FF_FFFF;
+    if (masked_addr >= self.pak.buf.len) return null;
+
+    return &self.pak.buf[masked_addr];
+}
+
+fn readIo(self: *const Self, comptime T: type, address: u32) T {
+    return io.read(self, T, address) orelse self.openBus(T, address);
 }
 
 fn openBus(self: *const Self, comptime T: type, address: u32) T {
+    @setCold(true);
     const r15 = self.cpu.r[15];
 
     const word = blk: {
         // If Arm, get the most recently fetched instruction (PC + 8)
+        //
+        // FIXME: This is most likely a faulty assumption.
+        // I think what *actually* happens is that the Bus has a latch for the most
+        // recently fetched piece of data, which is then returned during Open Bus (also DMA open bus?)
+        // I can "get away" with this because it's very statistically likely that the most recently latched value is
+        // the most recently fetched instruction by the pipeline
         if (!self.cpu.cpsr.t.read()) break :blk self.cpu.pipe.stage[1].?;
 
         const page = @truncate(u8, r15 >> 24);
@@ -169,88 +301,240 @@ fn openBus(self: *const Self, comptime T: type, address: u32) T {
         }
     };
 
-    return @truncate(T, rotr(u32, word, 8 * (address & 3)));
+    return @truncate(T, word);
 }
 
-pub fn read(self: *Self, comptime T: type, address: u32) T {
-    const page = @truncate(u8, address >> 24);
-    const aligned_addr = forceAlign(T, address);
+pub fn read(self: *Self, comptime T: type, unaligned_address: u32) T {
+    const bits = @typeInfo(std.math.IntFittingRange(0, page_size - 1)).Int.bits;
+    const page = unaligned_address >> bits;
+    const offset = unaligned_address & (page_size - 1);
 
-    self.sched.tick += timings[@boolToInt(T == u32)][@truncate(u4, page)];
+    // whether or not we do this in slowmem or fastmem, we should advance the scheduler
+    self.sched.tick += timings[@boolToInt(T == u32)][@truncate(u4, unaligned_address >> 24)];
+
+    // We're doing some serious out-of-bounds open-bus reads
+    if (page >= table_len) return self.openBus(T, unaligned_address);
+
+    if (self.read_table[page]) |some_ptr| {
+        // We have a pointer to a page, cast the pointer to it's underlying type
+        const Ptr = [*]const T;
+        const ptr = @ptrCast(Ptr, @alignCast(@alignOf(std.meta.Child(Ptr)), some_ptr));
+
+        // Note: We don't check array length, since we force align the
+        // lower bits of the address as the GBA would
+        return ptr[forceAlign(T, offset) / @sizeOf(T)];
+    }
+
+    return self.slowRead(T, unaligned_address);
+}
+
+pub fn dbgRead(self: *const Self, comptime T: type, unaligned_address: u32) T {
+    const bits = @typeInfo(std.math.IntFittingRange(0, page_size - 1)).Int.bits;
+    const page = unaligned_address >> bits;
+    const offset = unaligned_address & (page_size - 1);
+
+    // We're doing some serious out-of-bounds open-bus reads
+    if (page >= table_len) return self.openBus(T, unaligned_address);
+
+    if (self.read_table[page]) |some_ptr| {
+        // We have a pointer to a page, cast the pointer to it's underlying type
+        const Ptr = [*]const T;
+        const ptr = @ptrCast(Ptr, @alignCast(@alignOf(std.meta.Child(Ptr)), some_ptr));
+
+        // Note: We don't check array length, since we force align the
+        // lower bits of the address as the GBA would
+        return ptr[forceAlign(T, offset) / @sizeOf(T)];
+    }
+
+    return self.dbgSlowRead(T, unaligned_address);
+}
+
+fn slowRead(self: *Self, comptime T: type, unaligned_address: u32) T {
+    @setCold(true);
+
+    const page = @truncate(u8, unaligned_address >> 24);
+    const address = forceAlign(T, unaligned_address);
 
     return switch (page) {
         // General Internal Memory
         0x00 => blk: {
             if (address < Bios.size)
-                break :blk self.bios.read(T, self.cpu.r[15], aligned_addr);
+                break :blk self.bios.read(T, self.cpu.r[15], unaligned_address);
 
             break :blk self.openBus(T, address);
         },
-        0x02 => self.ewram.read(T, aligned_addr),
-        0x03 => self.iwram.read(T, aligned_addr),
+        0x02 => unreachable, // completely handled by fastmeme
+        0x03 => unreachable, // completely handled by fastmeme
         0x04 => self.readIo(T, address),
 
         // Internal Display Memory
-        0x05 => self.ppu.palette.read(T, aligned_addr),
-        0x06 => self.ppu.vram.read(T, aligned_addr),
-        0x07 => self.ppu.oam.read(T, aligned_addr),
+        0x05 => unreachable, // completely handled by fastmeme
+        0x06 => unreachable, // completely handled by fastmeme
+        0x07 => unreachable, // completely handled by fastmeme
 
         // External Memory (Game Pak)
-        0x08...0x0D => self.pak.read(T, aligned_addr),
-        0x0E...0x0F => blk: {
-            const value = self.pak.backup.read(address);
-
-            const multiplier = switch (T) {
-                u32 => 0x01010101,
-                u16 => 0x0101,
-                u8 => 1,
-                else => @compileError("Backup: Unsupported read width"),
-            };
-
-            break :blk @as(T, value) * multiplier;
-        },
+        0x08...0x0D => self.pak.read(T, address),
+        0x0E...0x0F => self.readBackup(T, unaligned_address),
         else => self.openBus(T, address),
     };
 }
 
-pub fn write(self: *Self, comptime T: type, address: u32, value: T) void {
-    const page = @truncate(u8, address >> 24);
-    const aligned_addr = forceAlign(T, address);
+fn dbgSlowRead(self: *const Self, comptime T: type, unaligned_address: u32) T {
+    const page = @truncate(u8, unaligned_address >> 24);
+    const address = forceAlign(T, unaligned_address);
 
-    self.sched.tick += timings[@boolToInt(T == u32)][@truncate(u4, page)];
+    return switch (page) {
+        // General Internal Memory
+        0x00 => blk: {
+            if (address < Bios.size)
+                break :blk self.bios.dbgRead(T, self.cpu.r[15], unaligned_address);
+
+            break :blk self.openBus(T, address);
+        },
+        0x02 => unreachable, // handled by fastmem
+        0x03 => unreachable, // handled by fastmem
+        0x04 => self.readIo(T, address),
+
+        // Internal Display Memory
+        0x05 => unreachable, // handled by fastmem
+        0x06 => unreachable, // handled by fastmem
+        0x07 => unreachable, // handled by fastmem
+
+        // External Memory (Game Pak)
+        0x08...0x0D => self.pak.dbgRead(T, address),
+        0x0E...0x0F => self.readBackup(T, unaligned_address),
+        else => self.openBus(T, address),
+    };
+}
+
+fn readBackup(self: *const Self, comptime T: type, unaligned_address: u32) T {
+    const value = self.pak.backup.read(unaligned_address);
+
+    const multiplier = switch (T) {
+        u32 => 0x01010101,
+        u16 => 0x0101,
+        u8 => 1,
+        else => @compileError("Backup: Unsupported read width"),
+    };
+
+    return @as(T, value) * multiplier;
+}
+
+pub fn write(self: *Self, comptime T: type, unaligned_address: u32, value: T) void {
+    const bits = @typeInfo(std.math.IntFittingRange(0, page_size - 1)).Int.bits;
+    const page = unaligned_address >> bits;
+    const offset = unaligned_address & (page_size - 1);
+
+    // whether or not we do this in slowmem or fastmem, we should advance the scheduler
+    self.sched.tick += timings[@boolToInt(T == u32)][@truncate(u4, unaligned_address >> 24)];
+
+    // We're doing some serious out-of-bounds open-bus writes, they do nothing though
+    if (page >= table_len) return;
+
+    if (self.write_tables[@boolToInt(T == u8)][page]) |some_ptr| {
+        // We have a pointer to a page, cast the pointer to it's underlying type
+        const Ptr = [*]T;
+        const ptr = @ptrCast(Ptr, @alignCast(@alignOf(std.meta.Child(Ptr)), some_ptr));
+
+        // Note: We don't check array length, since we force align the
+        // lower bits of the address as the GBA would
+        ptr[forceAlign(T, offset) / @sizeOf(T)] = value;
+    } else {
+        // we can return early if this is an 8-bit OAM write
+        if (T == u8 and @truncate(u8, unaligned_address >> 24) == 0x07) return;
+
+        self.slowWrite(T, unaligned_address, value);
+    }
+}
+
+/// Mostly Identical to `Bus.write`, slowmeme is handled by `Bus.dbgSlowWrite`
+pub fn dbgWrite(self: *Self, comptime T: type, unaligned_address: u32, value: T) void {
+    const bits = @typeInfo(std.math.IntFittingRange(0, page_size - 1)).Int.bits;
+    const page = unaligned_address >> bits;
+    const offset = unaligned_address & (page_size - 1);
+
+    // We're doing some serious out-of-bounds open-bus writes, they do nothing though
+    if (page >= table_len) return;
+
+    if (self.write_tables[@boolToInt(T == u8)][page]) |some_ptr| {
+        // We have a pointer to a page, cast the pointer to it's underlying type
+        const Ptr = [*]T;
+        const ptr = @ptrCast(Ptr, @alignCast(@alignOf(std.meta.Child(Ptr)), some_ptr));
+
+        // Note: We don't check array length, since we force align the
+        // lower bits of the address as the GBA would
+        ptr[forceAlign(T, offset) / @sizeOf(T)] = value;
+    } else {
+        // we can return early if this is an 8-bit OAM write
+        if (T == u8 and @truncate(u8, unaligned_address >> 24) == 0x07) return;
+
+        self.dbgSlowWrite(T, unaligned_address, value);
+    }
+}
+
+fn slowWrite(self: *Self, comptime T: type, unaligned_address: u32, value: T) void {
+    @setCold(true);
+
+    const page = @truncate(u8, unaligned_address >> 24);
+    const address = forceAlign(T, unaligned_address);
 
     switch (page) {
         // General Internal Memory
-        0x00 => self.bios.write(T, aligned_addr, value),
-        0x02 => self.ewram.write(T, aligned_addr, value),
-        0x03 => self.iwram.write(T, aligned_addr, value),
-        0x04 => io.write(self, T, aligned_addr, value),
+        0x00 => self.bios.write(T, address, value),
+        0x02 => unreachable, // completely handled by fastmem
+        0x03 => unreachable, // completely handled by fastmem
+        0x04 => io.write(self, T, address, value),
 
         // Internal Display Memory
-        0x05 => self.ppu.palette.write(T, aligned_addr, value),
-        0x06 => self.ppu.vram.write(T, self.ppu.dispcnt, aligned_addr, value),
-        0x07 => self.ppu.oam.write(T, aligned_addr, value),
+        0x05 => self.ppu.palette.write(T, address, value),
+        0x06 => self.ppu.vram.write(T, self.ppu.dispcnt, address, value),
+        0x07 => unreachable, // completely handled by fastmem
 
         // External Memory (Game Pak)
-        0x08...0x0D => self.pak.write(T, self.dma[3].word_count, aligned_addr, value),
-        0x0E...0x0F => {
-            const rotate_by = switch (T) {
-                u32 => address & 3,
-                u16 => address & 1,
-                u8 => 0,
-                else => @compileError("Backup: Unsupported write width"),
-            };
-
-            self.pak.backup.write(address, @truncate(u8, rotr(T, value, 8 * rotate_by)));
-        },
+        0x08...0x0D => self.pak.write(T, self.dma[3].word_count, address, value),
+        0x0E...0x0F => self.pak.backup.write(unaligned_address, @truncate(u8, rotr(T, value, 8 * rotateBy(T, unaligned_address)))),
         else => {},
     }
 }
 
-fn forceAlign(comptime T: type, address: u32) u32 {
+fn dbgSlowWrite(self: *Self, comptime T: type, unaligned_address: u32, value: T) void {
+    @setCold(true);
+
+    const page = @truncate(u8, unaligned_address >> 24);
+    const address = forceAlign(T, unaligned_address);
+
+    switch (page) {
+        // General Internal Memory
+        0x00 => self.bios.write(T, address, value),
+        0x02 => unreachable, // completely handled by fastmem
+        0x03 => unreachable, // completely handled by fastmem
+        0x04 => return, // FIXME: Let debug writes mess with I/O
+
+        // Internal Display Memory
+        0x05 => self.ppu.palette.write(T, address, value),
+        0x06 => self.ppu.vram.write(T, self.ppu.dispcnt, address, value),
+        0x07 => unreachable, // completely handled by fastmem
+
+        // External Memory (Game Pak)
+        0x08...0x0D => return, // FIXME: Debug Write to Backup/GPIO w/out messing with state
+        0x0E...0x0F => return, // FIXME: Debug Write to Backup w/out messing with state
+        else => {},
+    }
+}
+
+inline fn rotateBy(comptime T: type, address: u32) u32 {
     return switch (T) {
-        u32 => address & 0xFFFF_FFFC,
-        u16 => address & 0xFFFF_FFFE,
+        u32 => address & 3,
+        u16 => address & 1,
+        u8 => 0,
+        else => @compileError("Unsupported write width"),
+    };
+}
+
+pub inline fn forceAlign(comptime T: type, address: u32) u32 {
+    return switch (T) {
+        u32 => address & ~@as(u32, 3),
+        u16 => address & ~@as(u32, 1),
         u8 => address,
         else => @compileError("Bus: Invalid read/write type"),
     };

src/core/apu.zig

@@ -3,8 +3,6 @@ const SDL = @import("sdl2");
 const io = @import("bus/io.zig");
 const util = @import("../util.zig");
 
-const AudioDeviceId = SDL.SDL_AudioDeviceID;
-
 const Arm7tdmi = @import("cpu.zig").Arm7tdmi;
 const Scheduler = @import("scheduler.zig").Scheduler;
 const ToneSweep = @import("apu/ToneSweep.zig");
@@ -14,133 +12,216 @@ const Noise = @import("apu/Noise.zig");
 
 const SoundFifo = std.fifo.LinearFifo(u8, .{ .Static = 0x20 });
 
-const intToBytes = @import("../util.zig").intToBytes;
-const setHi = @import("../util.zig").setHi;
-const setLo = @import("../util.zig").setLo;
+const getHalf = util.getHalf;
+const setHalf = util.setHalf;
 
 const log = std.log.scoped(.APU);
 
-pub const host_sample_rate = 1 << 15;
+pub const host_rate = @import("../platform.zig").sample_rate;
+pub const host_format = @import("../platform.zig").sample_format;
 
 pub fn read(comptime T: type, apu: *const Apu, addr: u32) ?T {
-    const byte = @truncate(u8, addr);
+    const byte_addr = @truncate(u8, addr);
 
     return switch (T) {
-        u16 => switch (byte) {
-            0x60 => apu.ch1.sound1CntL(),
-            0x62 => apu.ch1.sound1CntH(),
+        u32 => switch (byte_addr) {
+            0x60 => @as(T, apu.ch1.sound1CntH()) << 16 | apu.ch1.sound1CntL(),
             0x64 => apu.ch1.sound1CntX(),
             0x68 => apu.ch2.sound2CntL(),
             0x6C => apu.ch2.sound2CntH(),
-
-            0x70 => apu.ch3.select.raw & 0xE0, // SOUND3CNT_L
-            0x72 => apu.ch3.sound3CntH(),
-            0x74 => apu.ch3.freq.raw & 0x4000, // SOUND3CNT_X
-
+            0x70 => @as(T, apu.ch3.sound3CntH()) << 16 | apu.ch3.sound3CntL(),
+            0x74 => apu.ch3.sound3CntX(),
             0x78 => apu.ch4.sound4CntL(),
             0x7C => apu.ch4.sound4CntH(),
-
-            0x80 => apu.psg_cnt.raw & 0xFF77, // SOUNDCNT_L
-            0x82 => apu.dma_cnt.raw & 0x770F, // SOUNDCNT_H
+            0x80 => @as(T, apu.dma_cnt.raw) << 16 | apu.psg_cnt.raw, // SOUNDCNT_H, SOUNDCNT_L
             0x84 => apu.soundCntX(),
+            0x88 => apu.bias.raw, // SOUNDBIAS, high is unused
+            0x8C => null,
+            0x90, 0x94, 0x98, 0x9C => apu.ch3.wave_dev.read(T, apu.ch3.select, addr),
+            0xA0 => null, // FIFO_A
+            0xA4 => null, // FIFO_B
+            else => util.io.read.err(T, log, "unaligned {} read from 0x{X:0>8}", .{ T, addr }),
+        },
+        u16 => switch (byte_addr) {
+            0x60 => apu.ch1.sound1CntL(),
+            0x62 => apu.ch1.sound1CntH(),
+            0x64 => apu.ch1.sound1CntX(),
+            0x66 => 0x0000, // suite.gba expects 0x0000, not 0xDEAD
+            0x68 => apu.ch2.sound2CntL(),
+            0x6A => 0x0000,
+            0x6C => apu.ch2.sound2CntH(),
+            0x6E => 0x0000,
+            0x70 => apu.ch3.sound3CntL(),
+            0x72 => apu.ch3.sound3CntH(),
+            0x74 => apu.ch3.sound3CntX(),
+            0x76 => 0x0000,
+            0x78 => apu.ch4.sound4CntL(),
+            0x7A => 0x0000,
+            0x7C => apu.ch4.sound4CntH(),
+            0x7E => 0x0000,
+            0x80 => apu.soundCntL(),
+            0x82 => apu.soundCntH(),
+            0x84 => apu.soundCntX(),
+            0x86 => 0x0000,
             0x88 => apu.bias.raw, // SOUNDBIAS
+            0x8A => 0x0000,
+            0x8C, 0x8E => null,
+            0x90, 0x92, 0x94, 0x96, 0x98, 0x9A, 0x9C, 0x9E => apu.ch3.wave_dev.read(T, apu.ch3.select, addr),
+            0xA0, 0xA2 => null, // FIFO_A
+            0xA4, 0xA6 => null, // FIFO_B
+            else => util.io.read.err(T, log, "unaligned {} read from 0x{X:0>8}", .{ T, addr }),
+        },
+        u8 => switch (byte_addr) {
+            0x60, 0x61 => @truncate(T, @as(u16, apu.ch1.sound1CntL()) >> getHalf(byte_addr)),
+            0x62, 0x63 => @truncate(T, apu.ch1.sound1CntH() >> getHalf(byte_addr)),
+            0x64, 0x65 => @truncate(T, apu.ch1.sound1CntX() >> getHalf(byte_addr)),
+            0x66, 0x67 => 0x00, // assuming behaviour is identical to that of 16-bit reads
+            0x68, 0x69 => @truncate(T, apu.ch2.sound2CntL() >> getHalf(byte_addr)),
+            0x6A, 0x6B => 0x00,
+            0x6C, 0x6D => @truncate(T, apu.ch2.sound2CntH() >> getHalf(byte_addr)),
+            0x6E, 0x6F => 0x00,
+            0x70, 0x71 => @truncate(T, @as(u16, apu.ch3.sound3CntL()) >> getHalf(byte_addr)), // SOUND3CNT_L
+            0x72, 0x73 => @truncate(T, apu.ch3.sound3CntH() >> getHalf(byte_addr)),
+            0x74, 0x75 => @truncate(T, apu.ch3.sound3CntX() >> getHalf(byte_addr)), // SOUND3CNT_L
+            0x76, 0x77 => 0x00,
+            0x78, 0x79 => @truncate(T, apu.ch4.sound4CntL() >> getHalf(byte_addr)),
+            0x7A, 0x7B => 0x00,
+            0x7C, 0x7D => @truncate(T, apu.ch4.sound4CntH() >> getHalf(byte_addr)),
+            0x7E, 0x7F => 0x00,
+            0x80, 0x81 => @truncate(T, apu.soundCntL() >> getHalf(byte_addr)), // SOUNDCNT_L
+            0x82, 0x83 => @truncate(T, apu.soundCntH() >> getHalf(byte_addr)), // SOUNDCNT_H
+            0x84, 0x85 => @truncate(T, @as(u16, apu.soundCntX()) >> getHalf(byte_addr)),
+            0x86, 0x87 => 0x00,
+            0x88, 0x89 => @truncate(T, apu.bias.raw >> getHalf(byte_addr)), // SOUNDBIAS
+            0x8A, 0x8B => 0x00,
+            0x8C...0x8F => null,
             0x90...0x9F => apu.ch3.wave_dev.read(T, apu.ch3.select, addr),
-            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+            0xA0, 0xA1, 0xA2, 0xA3 => null, // FIFO_A
+            0xA4, 0xA5, 0xA6, 0xA7 => null, // FIFO_B
+            else => util.io.read.err(T, log, "unexpected {} read from 0x{X:0>8}", .{ T, addr }),
         },
-        u8 => switch (byte) {
-            0x60 => apu.ch1.sound1CntL(), // NR10
-            0x62 => apu.ch1.duty.raw, // NR11
-            0x63 => apu.ch1.envelope.raw, // NR12
-            0x68 => apu.ch2.duty.raw, // NR21
-            0x69 => apu.ch2.envelope.raw, // NR22
-            0x73 => apu.ch3.vol.raw, // NR32
-            0x79 => apu.ch4.envelope.raw, // NR42
-            0x7C => apu.ch4.poly.raw, // NR43
-            0x81 => @truncate(u8, apu.psg_cnt.raw >> 8), // NR51
-            0x84 => apu.soundCntX(),
-            0x89 => @truncate(u8, apu.bias.raw >> 8), // SOUNDBIAS_H
-            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
-        },
-        u32 => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         else => @compileError("APU: Unsupported read width"),
     };
 }
 
 pub fn write(comptime T: type, apu: *Apu, addr: u32, value: T) void {
-    const byte = @truncate(u8, addr);
+    const byte_addr = @truncate(u8, addr);
+
+    if (byte_addr <= 0x81 and !apu.cnt.apu_enable.read()) return;
 
     switch (T) {
-        u32 => switch (byte) {
-            0x60 => apu.ch1.setSound1Cnt(value),
-            0x64 => apu.ch1.setSound1CntX(&apu.fs, @truncate(u16, value)),
-            0x68 => apu.ch2.setSound2CntL(@truncate(u16, value)),
-            0x6C => apu.ch2.setSound2CntH(&apu.fs, @truncate(u16, value)),
-            0x70 => apu.ch3.setSound3Cnt(value),
-            0x74 => apu.ch3.setSound3CntX(&apu.fs, @truncate(u16, value)),
-            0x78 => apu.ch4.setSound4CntL(@truncate(u16, value)),
-            0x7C => apu.ch4.setSound4CntH(&apu.fs, @truncate(u16, value)),
+        u32 => {
+            // 0x80 and 0x81 handled in setSoundCnt
+            if (byte_addr < 0x80 and !apu.cnt.apu_enable.read()) return;
 
-            0x80 => apu.setSoundCnt(value),
-            // WAVE_RAM
-            0x90...0x9F => apu.ch3.wave_dev.write(T, apu.ch3.select, addr, value),
-            0xA0 => apu.chA.push(value), // FIFO_A
-            0xA4 => apu.chB.push(value), // FIFO_B
-            else => util.io.write.undef(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
+            switch (byte_addr) {
+                0x60 => apu.ch1.setSound1Cnt(value),
+                0x64 => apu.ch1.setSound1CntX(&apu.fs, @truncate(u16, value)),
+
+                0x68 => apu.ch2.setSound2CntL(@truncate(u16, value)),
+                0x6C => apu.ch2.setSound2CntH(&apu.fs, @truncate(u16, value)),
+
+                0x70 => apu.ch3.setSound3Cnt(value),
+                0x74 => apu.ch3.setSound3CntX(&apu.fs, @truncate(u16, value)),
+
+                0x78 => apu.ch4.setSound4CntL(@truncate(u16, value)),
+                0x7C => apu.ch4.setSound4CntH(&apu.fs, @truncate(u16, value)),
+
+                0x80 => apu.setSoundCnt(value),
+                0x84 => apu.setSoundCntX(value >> 7 & 1 == 1),
+                0x88 => apu.bias.raw = @truncate(u16, value),
+                0x8C => {},
+
+                0x90, 0x94, 0x98, 0x9C => apu.ch3.wave_dev.write(T, apu.ch3.select, addr, value),
+                0xA0 => apu.chA.push(value), // FIFO_A
+                0xA4 => apu.chB.push(value), // FIFO_B
+                else => util.io.write.undef(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
+            }
         },
-        u16 => switch (byte) {
-            0x60 => apu.ch1.setSound1CntL(@truncate(u8, value)), // SOUND1CNT_L
-            0x62 => apu.ch1.setSound1CntH(value),
-            0x64 => apu.ch1.setSound1CntX(&apu.fs, value),
+        u16 => {
+            if (byte_addr <= 0x81 and !apu.cnt.apu_enable.read()) return;
 
-            0x68 => apu.ch2.setSound2CntL(value),
-            0x6C => apu.ch2.setSound2CntH(&apu.fs, value),
+            switch (byte_addr) {
+                0x60 => apu.ch1.setSound1CntL(@truncate(u8, value)), // SOUND1CNT_L
+                0x62 => apu.ch1.setSound1CntH(value),
+                0x64 => apu.ch1.setSound1CntX(&apu.fs, value),
+                0x66 => {},
 
-            0x70 => apu.ch3.setSound3CntL(@truncate(u8, value)),
-            0x72 => apu.ch3.setSound3CntH(value),
-            0x74 => apu.ch3.setSound3CntX(&apu.fs, value),
+                0x68 => apu.ch2.setSound2CntL(value),
+                0x6A => {},
+                0x6C => apu.ch2.setSound2CntH(&apu.fs, value),
+                0x6E => {},
 
-            0x78 => apu.ch4.setSound4CntL(value),
-            0x7C => apu.ch4.setSound4CntH(&apu.fs, value),
+                0x70 => apu.ch3.setSound3CntL(@truncate(u8, value)),
+                0x72 => apu.ch3.setSound3CntH(value),
+                0x74 => apu.ch3.setSound3CntX(&apu.fs, value),
+                0x76 => {},
 
-            0x80 => apu.psg_cnt.raw = value, // SOUNDCNT_L
-            0x82 => apu.setSoundCntH(value),
-            0x84 => apu.setSoundCntX(value >> 7 & 1 == 1),
-            0x88 => apu.bias.raw = value, // SOUNDBIAS
-            // WAVE_RAM
-            0x90...0x9F => apu.ch3.wave_dev.write(T, apu.ch3.select, addr, value),
-            else => util.io.write.undef(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
+                0x78 => apu.ch4.setSound4CntL(value),
+                0x7A => {},
+                0x7C => apu.ch4.setSound4CntH(&apu.fs, value),
+                0x7E => {},
+
+                0x80 => apu.setSoundCntL(value),
+                0x82 => apu.setSoundCntH(value),
+                0x84 => apu.setSoundCntX(value >> 7 & 1 == 1),
+                0x86 => {},
+                0x88 => apu.bias.raw = value, // SOUNDBIAS
+                0x8A, 0x8C, 0x8E => {},
+
+                0x90, 0x92, 0x94, 0x96, 0x98, 0x9A, 0x9C, 0x9E => apu.ch3.wave_dev.write(T, apu.ch3.select, addr, value),
+                0xA0, 0xA2 => log.err("Tried to write 0x{X:0>4}{} to FIFO_A", .{ value, T }),
+                0xA4, 0xA6 => log.err("Tried to write 0x{X:0>4}{} to FIFO_B", .{ value, T }),
+                else => util.io.write.undef(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
+            }
         },
-        u8 => switch (byte) {
-            0x60 => apu.ch1.setSound1CntL(value),
-            0x62 => apu.ch1.setNr11(value),
-            0x63 => apu.ch1.setNr12(value),
-            0x64 => apu.ch1.setNr13(value),
-            0x65 => apu.ch1.setNr14(&apu.fs, value),
+        u8 => {
+            if (byte_addr <= 0x81 and !apu.cnt.apu_enable.read()) return;
 
-            0x68 => apu.ch2.setNr21(value),
-            0x69 => apu.ch2.setNr22(value),
-            0x6C => apu.ch2.setNr23(value),
-            0x6D => apu.ch2.setNr24(&apu.fs, value),
+            switch (byte_addr) {
+                0x60 => apu.ch1.setSound1CntL(value),
+                0x61 => {},
+                0x62 => apu.ch1.setNr11(value),
+                0x63 => apu.ch1.setNr12(value),
+                0x64 => apu.ch1.setNr13(value),
+                0x65 => apu.ch1.setNr14(&apu.fs, value),
+                0x66, 0x67 => {},
 
-            0x70 => apu.ch3.setSound3CntL(value), // NR30
-            0x72 => apu.ch3.setNr31(value),
-            0x73 => apu.ch3.vol.raw = value, // NR32
-            0x74 => apu.ch3.setNr33(value),
-            0x75 => apu.ch3.setNr34(&apu.fs, value),
+                0x68 => apu.ch2.setNr21(value),
+                0x69 => apu.ch2.setNr22(value),
+                0x6A, 0x6B => {},
+                0x6C => apu.ch2.setNr23(value),
+                0x6D => apu.ch2.setNr24(&apu.fs, value),
+                0x6E, 0x6F => {},
 
-            0x78 => apu.ch4.setNr41(value),
-            0x79 => apu.ch4.setNr42(value),
-            0x7C => apu.ch4.poly.raw = value, // NR 43
-            0x7D => apu.ch4.setNr44(&apu.fs, value),
+                0x70 => apu.ch3.setSound3CntL(value), // NR30
+                0x71 => {},
+                0x72 => apu.ch3.setNr31(value),
+                0x73 => apu.ch3.vol.raw = value, // NR32
+                0x74 => apu.ch3.setNr33(value),
+                0x75 => apu.ch3.setNr34(&apu.fs, value),
+                0x76, 0x77 => {},
 
-            0x80 => apu.setNr50(value),
-            0x81 => apu.setNr51(value),
-            0x82 => apu.setSoundCntH(setLo(u16, apu.dma_cnt.raw, value)),
-            0x83 => apu.setSoundCntH(setHi(u16, apu.dma_cnt.raw, value)),
-            0x84 => apu.setSoundCntX(value >> 7 & 1 == 1), // NR52
-            0x89 => apu.setSoundBiasH(value),
-            0x90...0x9F => apu.ch3.wave_dev.write(T, apu.ch3.select, addr, value),
-            else => util.io.write.undef(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, addr }),
+                0x78 => apu.ch4.setNr41(value),
+                0x79 => apu.ch4.setNr42(value),
+                0x7A, 0x7B => {},
+                0x7C => apu.ch4.poly.raw = value, // NR 43
+                0x7D => apu.ch4.setNr44(&apu.fs, value),
+                0x7E, 0x7F => {},
+
+                0x80, 0x81 => apu.setSoundCntL(setHalf(u16, apu.psg_cnt.raw, byte_addr, value)),
+                0x82, 0x83 => apu.setSoundCntH(setHalf(u16, apu.dma_cnt.raw, byte_addr, value)),
+                0x84 => apu.setSoundCntX(value >> 7 & 1 == 1),
+                0x85 => {},
+                0x86, 0x87 => {},
+                0x88, 0x89 => apu.bias.raw = setHalf(u16, apu.bias.raw, byte_addr, value), // SOUNDBIAS
+                0x8A...0x8F => {},
+
+                0x90...0x9F => apu.ch3.wave_dev.write(T, apu.ch3.select, addr, value),
+                0xA0...0xA3 => log.err("Tried to write 0x{X:0>2}{} to FIFO_A", .{ value, T }),
+                0xA4...0xA7 => log.err("Tried to write 0x{X:0>2}{} to FIFO_B", .{ value, T }),
+                else => util.io.write.undef(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, addr }),
+            }
         },
         else => @compileError("APU: Unsupported write width"),
     }
@@ -189,7 +270,7 @@ pub const Apu = struct {
             .bias = .{ .raw = 0x0200 },
 
             .sampling_cycle = 0b00,
-            .stream = SDL.SDL_NewAudioStream(SDL.AUDIO_U16, 2, 1 << 15, SDL.AUDIO_U16, 2, host_sample_rate).?,
+            .stream = SDL.SDL_NewAudioStream(SDL.AUDIO_U16, 2, 1 << 15, host_format, 2, host_rate).?,
             .sched = sched,
 
             .capacitor = 0,
@@ -197,29 +278,71 @@ pub const Apu = struct {
             .is_buffer_full = false,
         };
 
-        sched.push(.SampleAudio, apu.interval());
-        sched.push(.{ .ApuChannel = 0 }, @import("apu/signal/Square.zig").interval);
-        sched.push(.{ .ApuChannel = 1 }, @import("apu/signal/Square.zig").interval);
-        sched.push(.{ .ApuChannel = 2 }, @import("apu/signal/Wave.zig").interval);
-        sched.push(.{ .ApuChannel = 3 }, @import("apu/signal/Lfsr.zig").interval);
-        sched.push(.FrameSequencer, FrameSequencer.interval);
+        Self.initEvents(apu.sched, apu.interval());
 
         return apu;
     }
 
-    fn reset(self: *Self) void {
+    fn initEvents(scheduler: *Scheduler, apu_interval: u64) void {
+        scheduler.push(.SampleAudio, apu_interval);
+        scheduler.push(.{ .ApuChannel = 0 }, @import("apu/signal/Square.zig").interval);
+        scheduler.push(.{ .ApuChannel = 1 }, @import("apu/signal/Square.zig").interval);
+        scheduler.push(.{ .ApuChannel = 2 }, @import("apu/signal/Wave.zig").interval);
+        scheduler.push(.{ .ApuChannel = 3 }, @import("apu/signal/Lfsr.zig").interval);
+        scheduler.push(.FrameSequencer, FrameSequencer.interval);
+    }
+
+    /// Used when resetting the emulator
+    pub fn reset(self: *Self) void {
+        // FIXME: These reset functions are meant to emulate obscure APU behaviour. Write proper emu reset fns
         self.ch1.reset();
         self.ch2.reset();
         self.ch3.reset();
         self.ch4.reset();
+
+        self.chA.reset();
+        self.chB.reset();
+
+        self.psg_cnt = .{ .raw = 0 };
+        self.dma_cnt = .{ .raw = 0 };
+        self.cnt = .{ .raw = 0 };
+        self.bias = .{ .raw = 0x200 };
+
+        self.sampling_cycle = 0;
+        self.fs.reset();
+
+        Self.initEvents(self.sched, self.interval());
+    }
+
+    /// Emulates the reset behaviour of the APU
+    fn _reset(self: *Self) void {
+        // All PSG Registers between 0x0400_0060..0x0400_0081 are zeroed
+        // 0x0400_0082 and 0x0400_0088 retain their values
+        self.ch1.reset();
+        self.ch2.reset();
+        self.ch3.reset();
+        self.ch4.reset();
+
+        // GBATEK says 4000060h..4000081h I take this to mean inclusive
+        self.psg_cnt.raw = 0x0000;
     }
 
     /// SOUNDCNT
     fn setSoundCnt(self: *Self, value: u32) void {
-        self.psg_cnt.raw = @truncate(u16, value);
+        if (self.cnt.apu_enable.read()) self.setSoundCntL(@truncate(u16, value));
         self.setSoundCntH(@truncate(u16, value >> 16));
     }
 
+    /// SOUNDCNT_L
+    pub fn soundCntL(self: *const Self) u16 {
+        return self.psg_cnt.raw & 0xFF77;
+    }
+
+    /// SOUNDCNT_L
+    pub fn setSoundCntL(self: *Self, value: u16) void {
+        self.psg_cnt.raw = value;
+    }
+
     /// SOUNDCNT_H
     pub fn setSoundCntH(self: *Self, value: u16) void {
         const new: io.DmaSoundControl = .{ .raw = value };
@@ -232,6 +355,11 @@ pub const Apu = struct {
         self.dma_cnt = new;
     }
 
+    /// SOUNDCNT_H
+    pub fn soundCntH(self: *const Self) u16 {
+        return self.dma_cnt.raw & 0x770F;
+    }
+
     /// NR52
     pub fn setSoundCntX(self: *Self, value: bool) void {
         self.cnt.apu_enable.write(value);
@@ -240,13 +368,16 @@ pub const Apu = struct {
             self.fs.step = 0; // Reset Frame Sequencer
 
             // Reset Square Wave Offsets
-            self.ch1.square.pos = 0;
-            self.ch2.square.pos = 0;
+            self.ch1.square.reset();
+            self.ch2.square.reset();
 
-            // Reset Wave Device Offsets
-            self.ch3.wave_dev.offset = 0;
+            // Reset Wave
+            self.ch3.wave_dev.reset();
+
+            // Rest Noise
+            self.ch4.lfsr.reset();
         } else {
-            self.reset();
+            self._reset();
         }
     }
 
@@ -262,20 +393,6 @@ pub const Apu = struct {
         return apu_enable << 7 | ch4_enable << 3 | ch3_enable << 2 | ch2_enable << 1 | ch1_enable;
     }
 
-    /// NR50
-    pub fn setNr50(self: *Self, byte: u8) void {
-        self.psg_cnt.raw = (self.psg_cnt.raw & 0xFF00) | byte;
-    }
-
-    /// NR51
-    pub fn setNr51(self: *Self, byte: u8) void {
-        self.psg_cnt.raw = @as(u16, byte) << 8 | (self.psg_cnt.raw & 0xFF);
-    }
-
-    pub fn setSoundBiasH(self: *Self, byte: u8) void {
-        self.bias.raw = (@as(u16, byte) << 8) | (self.bias.raw & 0xFF);
-    }
-
     pub fn sampleAudio(self: *Self, late: u64) void {
         self.sched.push(.SampleAudio, self.interval() -| late);
 
@@ -327,8 +444,8 @@ pub const Apu = struct {
         right += if (self.dma_cnt.chB_right.read()) chB_sample else 0;
 
         // Add SOUNDBIAS
-        // FIXME: Is SOUNDBIAS 9-bit or 10-bit?
-        const bias = @as(i16, self.bias.level.read()) << 1;
+        // FIXME: SOUNDBIAS is 10-bit but The waveform is centered around 0 if I treat it as 11-bit
+        const bias = @as(i16, self.bias.level.read()) << 2;
         left += bias;
         right += bias;
 
@@ -339,7 +456,6 @@ pub const Apu = struct {
         const ext_left = (clamped_left << 5) | (clamped_left >> 6);
         const ext_right = (clamped_right << 5) | (clamped_right >> 6);
 
-        // FIXME: This rarely happens
         if (self.sampling_cycle != self.bias.sampling_cycle.read()) self.replaceSDLResampler();
 
         _ = SDL.SDL_AudioStreamPut(self.stream, &[2]u16{ ext_left, ext_right }, 2 * @sizeOf(u16));
@@ -356,7 +472,7 @@ pub const Apu = struct {
         defer SDL.SDL_FreeAudioStream(old_stream);
 
         self.sampling_cycle = self.bias.sampling_cycle.read();
-        self.stream = SDL.SDL_NewAudioStream(SDL.AUDIO_U16, 2, @intCast(c_int, sample_rate), SDL.AUDIO_U16, 2, host_sample_rate).?;
+        self.stream = SDL.SDL_NewAudioStream(SDL.AUDIO_U16, 2, @intCast(c_int, sample_rate), host_format, 2, host_rate).?;
     }
 
     fn interval(self: *const Self) u64 {
@@ -405,11 +521,15 @@ pub const Apu = struct {
         if (!self.cnt.apu_enable.read()) return;
 
         if (@boolToInt(self.dma_cnt.chA_timer.read()) == tim_id) {
+            if (!self.chA.enabled) return;
+
             self.chA.updateSample();
             if (self.chA.len() <= 15) cpu.bus.dma[1].requestAudio(0x0400_00A0);
         }
 
         if (@boolToInt(self.dma_cnt.chB_timer.read()) == tim_id) {
+            if (!self.chB.enabled) return;
+
             self.chB.updateSample();
             if (self.chB.len() <= 15) cpu.bus.dma[2].requestAudio(0x0400_00A4);
         }
@@ -423,17 +543,31 @@ pub fn DmaSound(comptime kind: DmaSoundKind) type {
         fifo: SoundFifo,
         kind: DmaSoundKind,
         sample: i8,
+        enabled: bool,
 
         fn init() Self {
             return .{
                 .fifo = SoundFifo.init(),
                 .kind = kind,
                 .sample = 0,
+                .enabled = false,
             };
         }
 
+        /// Used when resetting hte emulator (not emulation code)
+        fn reset(self: *Self) void {
+            self.* = Self.init();
+        }
+
         pub fn push(self: *Self, value: u32) void {
-            self.fifo.write(&intToBytes(u32, value)) catch |e| log.err("{} Error: {}", .{ kind, e });
+            if (!self.enabled) self.enable();
+
+            self.fifo.write(std.mem.asBytes(&value)) catch |e| log.err("{} Error: {}", .{ kind, e });
+        }
+
+        fn enable(self: *Self) void {
+            @setCold(true);
+            self.enabled = true;
         }
 
         pub fn len(self: *const Self) usize {
@@ -456,13 +590,17 @@ const DmaSoundKind = enum {
 };
 
 pub const FrameSequencer = struct {
-    const interval = (1 << 24) / 512;
     const Self = @This();
+    pub const interval = (1 << 24) / 512;
 
-    step: u3,
+    step: u3 = 0,
 
     pub fn init() Self {
-        return .{ .step = 0 };
+        return .{};
+    }
+
+    pub fn reset(self: *Self) void {
+        self.* = .{};
     }
 
     pub fn tick(self: *Self) void {

src/core/apu/Noise.zig

@@ -49,10 +49,13 @@ pub fn init(sched: *Scheduler) Self {
 }
 
 pub fn reset(self: *Self) void {
-    self.len = 0;
-    self.envelope.raw = 0;
-    self.poly.raw = 0;
-    self.cnt.raw = 0;
+    self.len = 0; // NR41
+    self.envelope.raw = 0; // NR42
+    self.poly.raw = 0; // NR43
+    self.cnt.raw = 0; // NR44
+
+    self.len_dev.reset();
+    self.env_dev.reset();
 
     self.sample = 0;
     self.enabled = false;

src/core/apu/Tone.zig

@@ -43,9 +43,12 @@ pub fn init(sched: *Scheduler) Self {
 }
 
 pub fn reset(self: *Self) void {
-    self.duty.raw = 0;
-    self.envelope.raw = 0;
-    self.freq.raw = 0;
+    self.duty.raw = 0; // NR21
+    self.envelope.raw = 0; // NR22
+    self.freq.raw = 0; // NR32, NR24
+
+    self.len_dev.reset();
+    self.env_dev.reset();
 
     self.sample = 0;
     self.enabled = false;

src/core/apu/ToneSweep.zig

@@ -50,12 +50,14 @@ pub fn init(sched: *Scheduler) Self {
 }
 
 pub fn reset(self: *Self) void {
-    self.sweep.raw = 0;
-    self.sweep_dev.calc_performed = false;
+    self.sweep.raw = 0; // NR10
+    self.duty.raw = 0; // NR11
+    self.envelope.raw = 0; // NR12
+    self.freq.raw = 0; // NR13, NR14
 
-    self.duty.raw = 0;
-    self.envelope.raw = 0;
-    self.freq.raw = 0;
+    self.len_dev.reset();
+    self.sweep_dev.reset();
+    self.env_dev.reset();
 
     self.sample = 0;
     self.enabled = false;
@@ -92,10 +94,9 @@ pub fn sound1CntL(self: *const Self) u8 {
 pub fn setSound1CntL(self: *Self, value: u8) void {
     const new = io.Sweep{ .raw = value };
 
-    if (self.sweep.direction.read() and !new.direction.read()) {
-        // Sweep Negate bit has been cleared
-        // If At least 1 Sweep Calculation has been made since
-        // the last trigger, the channel is immediately disabled
+    if (!new.direction.read()) {
+        // If at least one (1) sweep calculation has been made with
+        // the negate bit set (since last trigger), disable the channel
 
         if (self.sweep_dev.calc_performed) self.enabled = false;
     }

src/core/apu/Wave.zig

@@ -42,10 +42,13 @@ pub fn init(sched: *Scheduler) Self {
 }
 
 pub fn reset(self: *Self) void {
-    self.select.raw = 0;
-    self.length = 0;
-    self.vol.raw = 0;
-    self.freq.raw = 0;
+    self.select.raw = 0; // NR30
+    self.length = 0; // NR31
+    self.vol.raw = 0; // NR32
+    self.freq.raw = 0; // NR33, NR34
+
+    self.len_dev.reset();
+    self.wave_dev.reset();
 
     self.sample = 0;
     self.enabled = false;
@@ -71,6 +74,11 @@ pub fn setSound3CntL(self: *Self, value: u8) void {
     if (!self.select.enabled.read()) self.enabled = false;
 }
 
+/// NR30
+pub fn sound3CntL(self: *const Self) u8 {
+    return self.select.raw & 0xE0;
+}
+
 /// NR31, NR32
 pub fn sound3CntH(self: *const Self) u16 {
     return @as(u16, self.length & 0xE0) << 8;
@@ -94,6 +102,11 @@ pub fn setSound3CntX(self: *Self, fs: *const FrameSequencer, value: u16) void {
     self.setNr34(fs, @truncate(u8, value >> 8));
 }
 
+/// NR33, NR34
+pub fn sound3CntX(self: *const Self) u16 {
+    return self.freq.raw & 0x4000;
+}
+
 /// NR33
 pub fn setNr33(self: *Self, byte: u8) void {
     self.freq.raw = (self.freq.raw & 0xFF00) | byte;

src/core/apu/device/Envelope.zig

@@ -3,12 +3,16 @@ const io = @import("../../bus/io.zig");
 const Self = @This();
 
 /// Period Timer
-timer: u3,
+timer: u3 = 0,
 /// Current Volume
-vol: u4,
+vol: u4 = 0,
 
 pub fn create() Self {
-    return .{ .timer = 0, .vol = 0 };
+    return .{};
+}
+
+pub fn reset(self: *Self) void {
+    self.* = .{};
 }
 
 pub fn tick(self: *Self, nrx2: io.Envelope) void {

src/core/apu/device/Length.zig

@@ -1,9 +1,13 @@
 const Self = @This();
 
-timer: u9,
+timer: u9 = 0,
 
 pub fn create() Self {
-    return .{ .timer = 0 };
+    return .{};
+}
+
+pub fn reset(self: *Self) void {
+    self.* = .{};
 }
 
 pub fn tick(self: *Self, enabled: bool, ch_enable: *bool) void {

src/core/apu/device/Sweep.zig

@@ -3,19 +3,18 @@ const ToneSweep = @import("../ToneSweep.zig");
 
 const Self = @This();
 
-timer: u8,
-enabled: bool,
-shadow: u11,
+timer: u8 = 0,
+enabled: bool = false,
+shadow: u11 = 0,
 
-calc_performed: bool,
+calc_performed: bool = false,
 
 pub fn create() Self {
-    return .{
-        .timer = 0,
-        .enabled = false,
-        .shadow = 0,
-        .calc_performed = false,
-    };
+    return .{};
+}
+
+pub fn reset(self: *Self) void {
+    self.* = .{};
 }
 
 pub fn tick(self: *Self, ch1: *ToneSweep) void {
@@ -24,7 +23,6 @@ pub fn tick(self: *Self, ch1: *ToneSweep) void {
     if (self.timer == 0) {
         const period = ch1.sweep.period.read();
         self.timer = if (period == 0) 8 else period;
-        if (!self.calc_performed) self.calc_performed = true;
 
         if (self.enabled and period != 0) {
             const new_freq = self.calculate(ch1.sweep, &ch1.enabled);
@@ -45,7 +43,10 @@ pub fn calculate(self: *Self, sweep: io.Sweep, ch_enable: *bool) u12 {
     const shadow_shifted = shadow >> sweep.shift.read();
     const decrease = sweep.direction.read();
 
-    const freq = if (decrease) shadow - shadow_shifted else shadow + shadow_shifted;
+    const freq = if (decrease) blk: {
+        self.calc_performed = true;
+        break :blk shadow - shadow_shifted;
+    } else shadow + shadow_shifted;
     if (freq > 0x7FF) ch_enable.* = false;
 
     return freq;

src/core/apu/signal/Lfsr.zig

@@ -1,9 +1,7 @@
+//! Linear Feedback Shift Register
 const io = @import("../../bus/io.zig");
 
-/// Linear Feedback Shift Register
 const Scheduler = @import("../../scheduler.zig").Scheduler;
-const FrameSequencer = @import("../../apu.zig").FrameSequencer;
-const Noise = @import("../Noise.zig");
 
 const Self = @This();
 pub const interval: u64 = (1 << 24) / (1 << 22);
@@ -21,6 +19,11 @@ pub fn create(sched: *Scheduler) Self {
     };
 }
 
+pub fn reset(self: *Self) void {
+    self.shift = 0;
+    self.timer = 0;
+}
+
 pub fn sample(self: *const Self) i8 {
     return if ((~self.shift & 1) == 1) 1 else -1;
 }
@@ -35,7 +38,7 @@ pub fn reload(self: *Self, poly: io.PolyCounter) void {
 }
 
 /// Scheduler Event Handler for LFSR Timer Expire
-/// FIXME: This gets called a lot, clogging up the Scheduler
+/// FIXME: This gets called a lot, slowing down the scheduler
 pub fn onLfsrTimerExpire(self: *Self, poly: io.PolyCounter, late: u64) void {
     // Obscure: "Using a noise channel clock shift of 14 or 15
     // results in the LFSR receiving no clocks."

src/core/apu/signal/Square.zig

@@ -2,7 +2,6 @@ const std = @import("std");
 const io = @import("../../bus/io.zig");
 
 const Scheduler = @import("../../scheduler.zig").Scheduler;
-const FrameSequencer = @import("../../apu.zig").FrameSequencer;
 const ToneSweep = @import("../ToneSweep.zig");
 const Tone = @import("../Tone.zig");
 
@@ -21,6 +20,11 @@ pub fn init(sched: *Scheduler) Self {
     };
 }
 
+pub fn reset(self: *Self) void {
+    self.timer = 0;
+    self.pos = 0;
+}
+
 /// Scheduler Event Handler for Square Synth Timer Expire
 pub fn onSquareTimerExpire(self: *Self, comptime T: type, nrx34: io.Frequency, late: u64) void {
     comptime std.debug.assert(T == ToneSweep or T == Tone);

src/core/apu/signal/Wave.zig

@@ -2,8 +2,6 @@ const std = @import("std");
 const io = @import("../../bus/io.zig");
 
 const Scheduler = @import("../../scheduler.zig").Scheduler;
-const FrameSequencer = @import("../../apu.zig").FrameSequencer;
-const Wave = @import("../Wave.zig");
 
 const buf_len = 0x20;
 pub const interval: u64 = (1 << 24) / (1 << 22);
@@ -40,6 +38,13 @@ pub fn init(sched: *Scheduler) Self {
     };
 }
 
+pub fn reset(self: *Self) void {
+    self.timer = 0;
+    self.offset = 0;
+
+    // sample buffer isn't reset because it's outside of the range of what NR52{7}'s effects
+}
+
 /// Reload internal Wave Timer
 pub fn reload(self: *Self, value: u11) void {
     self.sched.removeScheduledEvent(.{ .ApuChannel = 2 });

src/core/bus/Bios.zig

@@ -3,6 +3,9 @@ const std = @import("std");
 const Allocator = std.mem.Allocator;
 const log = std.log.scoped(.Bios);
 
+const rotr = @import("zba-util").rotr;
+const forceAlign = @import("../Bus.zig").forceAlign;
+
 /// Size of the BIOS in bytes
 pub const size = 0x4000;
 const Self = @This();
@@ -10,21 +13,37 @@ const Self = @This();
 buf: ?[]u8,
 allocator: Allocator,
 
-addr_latch: u32,
+addr_latch: u32 = 0,
 
-pub fn read(self: *Self, comptime T: type, r15: u32, addr: u32) T {
+// https://github.com/ITotalJustice/notorious_beeg/issues/106
+pub fn read(self: *Self, comptime T: type, r15: u32, address: u32) T {
     if (r15 < Self.size) {
+        const addr = forceAlign(T, address);
+
         self.addr_latch = addr;
         return self._read(T, addr);
     }
 
-    log.debug("Rejected read since r15=0x{X:0>8}", .{r15});
-    return @truncate(T, self._read(T, self.addr_latch + 8));
+    log.warn("Open Bus! Read from 0x{X:0>8}, but PC was 0x{X:0>8}", .{ address, r15 });
+    const value = self._read(u32, self.addr_latch);
+
+    return @truncate(T, rotr(u32, value, 8 * rotateBy(T, address)));
 }
 
-pub fn dbgRead(self: *const Self, comptime T: type, r15: u32, addr: u32) T {
-    if (r15 < Self.size) return self._read(T, addr);
-    return @truncate(T, self._read(T, self.addr_latch + 8));
+fn rotateBy(comptime T: type, address: u32) u32 {
+    return switch (T) {
+        u8 => address & 3,
+        u16 => address & 2,
+        u32 => 0,
+        else => @compileError("bios: unsupported read width"),
+    };
+}
+
+pub fn dbgRead(self: *const Self, comptime T: type, r15: u32, address: u32) T {
+    if (r15 < Self.size) return self._read(T, forceAlign(T, address));
+
+    const value = self._read(u32, self.addr_latch);
+    return @truncate(T, rotr(u32, value, 8 * rotateBy(T, address)));
 }
 
 /// Read without the GBA safety checks
@@ -43,18 +62,23 @@ pub fn write(_: *Self, comptime T: type, addr: u32, value: T) void {
 }
 
 pub fn init(allocator: Allocator, maybe_path: ?[]const u8) !Self {
-    const buf: ?[]u8 = if (maybe_path) |path| blk: {
-        const file = try std.fs.cwd().openFile(path, .{});
-        defer file.close();
+    if (maybe_path == null) return .{ .buf = null, .allocator = allocator };
+    const path = maybe_path.?;
 
-        break :blk try file.readToEndAlloc(allocator, try file.getEndPos());
-    } else null;
+    const buf = try allocator.alloc(u8, Self.size);
+    errdefer allocator.free(buf);
 
-    return Self{
-        .buf = buf,
-        .allocator = allocator,
-        .addr_latch = 0,
-    };
+    const file = try std.fs.cwd().openFile(path, .{});
+    defer file.close();
+
+    const file_len = try file.readAll(buf);
+    if (file_len != Self.size) log.err("Expected BIOS to be {}B, was {}B", .{ Self.size, file_len });
+
+    return Self{ .buf = buf, .allocator = allocator };
+}
+
+pub fn reset(self: *Self) void {
+    self.addr_latch = 0;
 }
 
 pub fn deinit(self: *Self) void {

src/core/bus/Ewram.zig

@@ -35,6 +35,10 @@ pub fn init(allocator: Allocator) !Self {
     };
 }
 
+pub fn reset(self: *Self) void {
+    std.mem.set(u8, self.buf, 0);
+}
+
 pub fn deinit(self: *Self) void {
     self.allocator.free(self.buf);
     self.* = undefined;

src/core/bus/GamePak.zig

@@ -1,10 +1,6 @@
 const std = @import("std");
 const config = @import("../../config.zig");
 
-const Bit = @import("bitfield").Bit;
-const Bitfield = @import("bitfield").Bitfield;
-const DateTime = @import("datetime").datetime.Datetime;
-
 const Arm7tdmi = @import("../cpu.zig").Arm7tdmi;
 const Backup = @import("backup.zig").Backup;
 const Gpio = @import("gpio.zig").Gpio;
@@ -109,14 +105,13 @@ pub fn dbgRead(self: *const Self, comptime T: type, address: u32) T {
 
         switch (T) {
             u32 => switch (address) {
-                // TODO: Do I even need to implement these?
+                // FIXME: Do I even need to implement these?
                 0x0800_00C4 => std.debug.panic("Handle 32-bit GPIO Data/Direction Reads", .{}),
                 0x0800_00C6 => std.debug.panic("Handle 32-bit GPIO Direction/Control Reads", .{}),
                 0x0800_00C8 => std.debug.panic("Handle 32-bit GPIO Control Reads", .{}),
                 else => {},
             },
             u16 => switch (address) {
-                // FIXME: What do 16-bit GPIO Reads look like?
                 0x0800_00C4 => return self.gpio.read(.Data),
                 0x0800_00C6 => return self.gpio.read(.Direction),
                 0x0800_00C8 => return self.gpio.read(.Control),
@@ -184,23 +179,30 @@ pub fn write(self: *Self, comptime T: type, word_count: u16, address: u32, value
     }
 }
 
-pub fn init(allocator: Allocator, cpu: *Arm7tdmi, rom_path: []const u8, save_path: ?[]const u8) !Self {
-    const file = try std.fs.cwd().openFile(rom_path, .{});
-    defer file.close();
+pub fn init(allocator: Allocator, cpu: *Arm7tdmi, maybe_rom: ?[]const u8, maybe_save: ?[]const u8) !Self {
+    const Device = Gpio.Device;
 
-    const file_buf = try file.readToEndAlloc(allocator, try file.getEndPos());
-    const title = file_buf[0xA0..0xAC].*;
-    const kind = Backup.guess(file_buf);
-    const device = if (config.config().guest.force_rtc) .Rtc else guessDevice(file_buf);
+    const items: struct { []u8, [12]u8, Backup.Kind, Device.Kind } = if (maybe_rom) |file_path| blk: {
+        const file = try std.fs.cwd().openFile(file_path, .{});
+        defer file.close();
 
-    logHeader(file_buf, &title);
+        const buffer = try file.readToEndAlloc(allocator, try file.getEndPos());
+        const title = buffer[0xA0..0xAC];
+        logHeader(buffer, title);
+
+        const device_kind = if (config.config().guest.force_rtc) .Rtc else guessDevice(buffer);
+
+        break :blk .{ buffer, title.*, Backup.guess(buffer), device_kind };
+    } else .{ try allocator.alloc(u8, 0), [_]u8{0} ** 12, .None, .None };
+
+    const title = items[1];
 
     return .{
-        .buf = file_buf,
+        .buf = items[0],
         .allocator = allocator,
         .title = title,
-        .backup = try Backup.init(allocator, kind, title, save_path),
-        .gpio = try Gpio.init(allocator, cpu, device),
+        .backup = try Backup.init(allocator, items[2], title, maybe_save),
+        .gpio = try Gpio.init(allocator, cpu, items[3]),
     };
 }
 
@@ -218,25 +220,24 @@ fn guessDevice(buf: []const u8) Gpio.Device.Kind {
     // Try to Guess if ROM uses RTC
     const needle = "RTC_V"; // I was told SIIRTC_V, though Pokemen Firered (USA) is a false negative
 
+    // TODO: Use new for loop syntax?
     var i: usize = 0;
     while ((i + needle.len) < buf.len) : (i += 1) {
         if (std.mem.eql(u8, needle, buf[i..(i + needle.len)])) return .Rtc;
     }
 
     // TODO: Detect other GPIO devices
-
     return .None;
 }
 
 fn logHeader(buf: []const u8, title: *const [12]u8) void {
-    const code = buf[0xAC..0xB0];
-    const maker = buf[0xB0..0xB2];
     const version = buf[0xBC];
 
     log.info("Title: {s}", .{title});
     if (version != 0) log.info("Version: {}", .{version});
-    log.info("Game Code: {s}", .{code});
-    log.info("Maker Code: {s}", .{maker});
+
+    log.info("Game Code: {s}", .{buf[0xAC..0xB0]});
+    log.info("Maker Code: {s}", .{buf[0xB0..0xB2]});
 }
 
 test "OOB Access" {

src/core/bus/Iwram.zig

@@ -35,6 +35,10 @@ pub fn init(allocator: Allocator) !Self {
     };
 }
 
+pub fn reset(self: *Self) void {
+    std.mem.set(u8, self.buf, 0);
+}
+
 pub fn deinit(self: *Self) void {
     self.allocator.free(self.buf);
     self.* = undefined;

src/core/bus/backup.zig

@@ -6,7 +6,6 @@ const Eeprom = @import("backup/eeprom.zig").Eeprom;
 const Flash = @import("backup/Flash.zig");
 
 const escape = @import("../../util.zig").escape;
-const span = @import("../../util.zig").span;
 
 const Needle = struct { str: []const u8, kind: Backup.Kind };
 const backup_kinds = [6]Needle{
@@ -33,7 +32,7 @@ pub const Backup = struct {
     flash: Flash,
     eeprom: Eeprom,
 
-    const Kind = enum {
+    pub const Kind = enum {
         Eeprom,
         Sram,
         Flash,
@@ -138,6 +137,7 @@ pub const Backup = struct {
         for (backup_kinds) |needle| {
             const needle_len = needle.str.len;
 
+            // TODO: Use new for loop syntax?
             var i: usize = 0;
             while ((i + needle_len) < rom.len) : (i += 1) {
                 if (std.mem.eql(u8, needle.str, rom[i..][0..needle_len])) return needle.kind;
@@ -151,8 +151,8 @@ pub const Backup = struct {
         const file_path = try self.savePath(allocator, path);
         defer allocator.free(file_path);
 
-        // FIXME: Don't rely on this lol
-        if (std.mem.eql(u8, file_path[file_path.len - 12 .. file_path.len], "untitled.sav")) {
+        const expected = "untitled.sav";
+        if (std.mem.eql(u8, file_path[file_path.len - expected.len .. file_path.len], expected)) {
             return log.err("ROM header lacks title, no save loaded", .{});
         }
 
@@ -195,7 +195,7 @@ pub const Backup = struct {
     }
 
     fn saveName(self: *const Self, allocator: Allocator) ![]const u8 {
-        const title_str = span(&escape(self.title));
+        const title_str = std.mem.sliceTo(&escape(self.title), 0);
         const name = if (title_str.len != 0) title_str else "untitled";
 
         return try std.mem.concat(allocator, u8, &[_][]const u8{ name, ".sav" });
@@ -205,6 +205,10 @@ pub const Backup = struct {
         const file_path = try self.savePath(allocator, path);
         defer allocator.free(file_path);
 
+        // FIXME: communicate edge case to the user?
+        if (std.mem.eql(u8, &self.title, "ACE LIGHTNIN"))
+            return;
+
         switch (self.kind) {
             .Sram, .Flash, .Flash1M, .Eeprom => {
                 const file = try std.fs.createFileAbsolute(file_path, .{});

src/core/bus/backup/eeprom.zig

@@ -63,7 +63,7 @@ pub const Eeprom = struct {
         }
 
         if (self.state == .RequestEnd) {
-            if (bit != 0) log.debug("EEPROM Request did not end in 0u1. TODO: is this ok?", .{});
+            // if (bit != 0) log.debug("EEPROM Request did not end in 0u1. TODO: is this ok?", .{});
             self.state = .Ready;
             return;
         }

src/core/bus/dma.zig

@@ -5,89 +5,140 @@ const DmaControl = @import("io.zig").DmaControl;
 const Bus = @import("../Bus.zig");
 const Arm7tdmi = @import("../cpu.zig").Arm7tdmi;
 
-pub const DmaTuple = std.meta.Tuple(&[_]type{ DmaController(0), DmaController(1), DmaController(2), DmaController(3) });
+pub const DmaTuple = struct { DmaController(0), DmaController(1), DmaController(2), DmaController(3) };
 const log = std.log.scoped(.DmaTransfer);
 
-const setHi = util.setHi;
-const setLo = util.setLo;
+const getHalf = util.getHalf;
+const setHalf = util.setHalf;
+const setQuart = util.setQuart;
+
+const rotr = @import("zba-util").rotr;
 
 pub fn create() DmaTuple {
     return .{ DmaController(0).init(), DmaController(1).init(), DmaController(2).init(), DmaController(3).init() };
 }
 
 pub fn read(comptime T: type, dma: *const DmaTuple, addr: u32) ?T {
-    const byte = @truncate(u8, addr);
+    const byte_addr = @truncate(u8, addr);
 
     return switch (T) {
-        u32 => switch (byte) {
-            0xB8 => @as(T, dma.*[0].cnt.raw) << 16,
-            0xC4 => @as(T, dma.*[1].cnt.raw) << 16,
-            0xD0 => @as(T, dma.*[2].cnt.raw) << 16,
-            0xDC => @as(T, dma.*[3].cnt.raw) << 16,
-            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+        u32 => switch (byte_addr) {
+            0xB0, 0xB4 => null, // DMA0SAD, DMA0DAD,
+            0xB8 => @as(T, dma.*[0].dmacntH()) << 16, // DMA0CNT_L is write-only
+            0xBC, 0xC0 => null, // DMA1SAD, DMA1DAD
+            0xC4 => @as(T, dma.*[1].dmacntH()) << 16, // DMA1CNT_L is write-only
+            0xC8, 0xCC => null, // DMA2SAD, DMA2DAD
+            0xD0 => @as(T, dma.*[2].dmacntH()) << 16, // DMA2CNT_L is write-only
+            0xD4, 0xD8 => null, // DMA3SAD, DMA3DAD
+            0xDC => @as(T, dma.*[3].dmacntH()) << 16, // DMA3CNT_L is write-only
+            else => util.io.read.err(T, log, "unaligned {} read from 0x{X:0>8}", .{ T, addr }),
         },
-        u16 => switch (byte) {
-            0xBA => dma.*[0].cnt.raw,
-            0xC6 => dma.*[1].cnt.raw,
-            0xD2 => dma.*[2].cnt.raw,
-            0xDE => dma.*[3].cnt.raw,
-            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+        u16 => switch (byte_addr) {
+            0xB0, 0xB2, 0xB4, 0xB6 => null, // DMA0SAD, DMA0DAD
+            0xB8 => 0x0000, // DMA0CNT_L, suite.gba expects 0x0000 instead of 0xDEAD
+            0xBA => dma.*[0].dmacntH(),
+
+            0xBC, 0xBE, 0xC0, 0xC2 => null, // DMA1SAD, DMA1DAD
+            0xC4 => 0x0000, // DMA1CNT_L
+            0xC6 => dma.*[1].dmacntH(),
+
+            0xC8, 0xCA, 0xCC, 0xCE => null, // DMA2SAD, DMA2DAD
+            0xD0 => 0x0000, // DMA2CNT_L
+            0xD2 => dma.*[2].dmacntH(),
+
+            0xD4, 0xD6, 0xD8, 0xDA => null, // DMA3SAD, DMA3DAD
+            0xDC => 0x0000, // DMA3CNT_L
+            0xDE => dma.*[3].dmacntH(),
+            else => util.io.read.err(T, log, "unaligned {} read from 0x{X:0>8}", .{ T, addr }),
+        },
+        u8 => switch (byte_addr) {
+            0xB0...0xB7 => null, // DMA0SAD, DMA0DAD
+            0xB8, 0xB9 => 0x00, // DMA0CNT_L
+            0xBA, 0xBB => @truncate(T, dma.*[0].dmacntH() >> getHalf(byte_addr)),
+
+            0xBC...0xC3 => null, // DMA1SAD, DMA1DAD
+            0xC4, 0xC5 => 0x00, // DMA1CNT_L
+            0xC6, 0xC7 => @truncate(T, dma.*[1].dmacntH() >> getHalf(byte_addr)),
+
+            0xC8...0xCF => null, // DMA2SAD, DMA2DAD
+            0xD0, 0xD1 => 0x00, // DMA2CNT_L
+            0xD2, 0xD3 => @truncate(T, dma.*[2].dmacntH() >> getHalf(byte_addr)),
+
+            0xD4...0xDB => null, // DMA3SAD, DMA3DAD
+            0xDC, 0xDD => 0x00, // DMA3CNT_L
+            0xDE, 0xDF => @truncate(T, dma.*[3].dmacntH() >> getHalf(byte_addr)),
+            else => util.io.read.err(T, log, "unexpected {} read from 0x{X:0>8}", .{ T, addr }),
         },
-        u8 => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         else => @compileError("DMA: Unsupported read width"),
     };
 }
 
 pub fn write(comptime T: type, dma: *DmaTuple, addr: u32, value: T) void {
-    const byte = @truncate(u8, addr);
+    const byte_addr = @truncate(u8, addr);
 
     switch (T) {
-        u32 => switch (byte) {
+        u32 => switch (byte_addr) {
             0xB0 => dma.*[0].setDmasad(value),
             0xB4 => dma.*[0].setDmadad(value),
             0xB8 => dma.*[0].setDmacnt(value),
+
             0xBC => dma.*[1].setDmasad(value),
             0xC0 => dma.*[1].setDmadad(value),
             0xC4 => dma.*[1].setDmacnt(value),
+
             0xC8 => dma.*[2].setDmasad(value),
             0xCC => dma.*[2].setDmadad(value),
             0xD0 => dma.*[2].setDmacnt(value),
+
             0xD4 => dma.*[3].setDmasad(value),
             0xD8 => dma.*[3].setDmadad(value),
             0xDC => dma.*[3].setDmacnt(value),
             else => util.io.write.undef(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
-        u16 => switch (byte) {
-            0xB0 => dma.*[0].setDmasad(setLo(u32, dma.*[0].sad, value)),
-            0xB2 => dma.*[0].setDmasad(setHi(u32, dma.*[0].sad, value)),
-            0xB4 => dma.*[0].setDmadad(setLo(u32, dma.*[0].dad, value)),
-            0xB6 => dma.*[0].setDmadad(setHi(u32, dma.*[0].dad, value)),
+        u16 => switch (byte_addr) {
+            0xB0, 0xB2 => dma.*[0].setDmasad(setHalf(u32, dma.*[0].sad, byte_addr, value)),
+            0xB4, 0xB6 => dma.*[0].setDmadad(setHalf(u32, dma.*[0].dad, byte_addr, value)),
             0xB8 => dma.*[0].setDmacntL(value),
             0xBA => dma.*[0].setDmacntH(value),
 
-            0xBC => dma.*[1].setDmasad(setLo(u32, dma.*[1].sad, value)),
-            0xBE => dma.*[1].setDmasad(setHi(u32, dma.*[1].sad, value)),
-            0xC0 => dma.*[1].setDmadad(setLo(u32, dma.*[1].dad, value)),
-            0xC2 => dma.*[1].setDmadad(setHi(u32, dma.*[1].dad, value)),
+            0xBC, 0xBE => dma.*[1].setDmasad(setHalf(u32, dma.*[1].sad, byte_addr, value)),
+            0xC0, 0xC2 => dma.*[1].setDmadad(setHalf(u32, dma.*[1].dad, byte_addr, value)),
             0xC4 => dma.*[1].setDmacntL(value),
             0xC6 => dma.*[1].setDmacntH(value),
 
-            0xC8 => dma.*[2].setDmasad(setLo(u32, dma.*[2].sad, value)),
-            0xCA => dma.*[2].setDmasad(setHi(u32, dma.*[2].sad, value)),
-            0xCC => dma.*[2].setDmadad(setLo(u32, dma.*[2].dad, value)),
-            0xCE => dma.*[2].setDmadad(setHi(u32, dma.*[2].dad, value)),
+            0xC8, 0xCA => dma.*[2].setDmasad(setHalf(u32, dma.*[2].sad, byte_addr, value)),
+            0xCC, 0xCE => dma.*[2].setDmadad(setHalf(u32, dma.*[2].dad, byte_addr, value)),
             0xD0 => dma.*[2].setDmacntL(value),
             0xD2 => dma.*[2].setDmacntH(value),
 
-            0xD4 => dma.*[3].setDmasad(setLo(u32, dma.*[3].sad, value)),
-            0xD6 => dma.*[3].setDmasad(setHi(u32, dma.*[3].sad, value)),
-            0xD8 => dma.*[3].setDmadad(setLo(u32, dma.*[3].dad, value)),
-            0xDA => dma.*[3].setDmadad(setHi(u32, dma.*[3].dad, value)),
+            0xD4, 0xD6 => dma.*[3].setDmasad(setHalf(u32, dma.*[3].sad, byte_addr, value)),
+            0xD8, 0xDA => dma.*[3].setDmadad(setHalf(u32, dma.*[3].dad, byte_addr, value)),
             0xDC => dma.*[3].setDmacntL(value),
             0xDE => dma.*[3].setDmacntH(value),
             else => util.io.write.undef(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
-        u8 => util.io.write.undef(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, addr }),
+        u8 => switch (byte_addr) {
+            0xB0, 0xB1, 0xB2, 0xB3 => dma.*[0].setDmasad(setQuart(dma.*[0].sad, byte_addr, value)),
+            0xB4, 0xB5, 0xB6, 0xB7 => dma.*[0].setDmadad(setQuart(dma.*[0].dad, byte_addr, value)),
+            0xB8, 0xB9 => dma.*[0].setDmacntL(setHalf(u16, dma.*[0].word_count, byte_addr, value)),
+            0xBA, 0xBB => dma.*[0].setDmacntH(setHalf(u16, dma.*[0].cnt.raw, byte_addr, value)),
+
+            0xBC, 0xBD, 0xBE, 0xBF => dma.*[1].setDmasad(setQuart(dma.*[1].sad, byte_addr, value)),
+            0xC0, 0xC1, 0xC2, 0xC3 => dma.*[1].setDmadad(setQuart(dma.*[1].dad, byte_addr, value)),
+            0xC4, 0xC5 => dma.*[1].setDmacntL(setHalf(u16, dma.*[1].word_count, byte_addr, value)),
+            0xC6, 0xC7 => dma.*[1].setDmacntH(setHalf(u16, dma.*[1].cnt.raw, byte_addr, value)),
+
+            0xC8, 0xC9, 0xCA, 0xCB => dma.*[2].setDmasad(setQuart(dma.*[2].sad, byte_addr, value)),
+            0xCC, 0xCD, 0xCE, 0xCF => dma.*[2].setDmadad(setQuart(dma.*[2].dad, byte_addr, value)),
+            0xD0, 0xD1 => dma.*[2].setDmacntL(setHalf(u16, dma.*[2].word_count, byte_addr, value)),
+            0xD2, 0xD3 => dma.*[2].setDmacntH(setHalf(u16, dma.*[2].cnt.raw, byte_addr, value)),
+
+            0xD4, 0xD5, 0xD6, 0xD7 => dma.*[3].setDmasad(setQuart(dma.*[3].sad, byte_addr, value)),
+            0xD8, 0xD9, 0xDA, 0xDB => dma.*[3].setDmadad(setQuart(dma.*[3].dad, byte_addr, value)),
+            0xDC, 0xDD => dma.*[3].setDmacntL(setHalf(u16, dma.*[3].word_count, byte_addr, value)),
+            0xDE, 0xDF => dma.*[3].setDmacntH(setHalf(u16, dma.*[3].cnt.raw, byte_addr, value)),
+            else => util.io.write.undef(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, addr }),
+        },
         else => @compileError("DMA: Unsupported write width"),
     }
 }
@@ -99,6 +150,7 @@ fn DmaController(comptime id: u2) type {
 
         const sad_mask: u32 = if (id == 0) 0x07FF_FFFF else 0x0FFF_FFFF;
         const dad_mask: u32 = if (id != 3) 0x07FF_FFFF else 0x0FFF_FFFF;
+        const WordCount = if (id == 3) u16 else u14;
 
         /// Write-only. The first address in a DMA transfer. (DMASAD)
         /// Note: use writeSrc instead of manipulating src_addr directly
@@ -107,17 +159,19 @@ fn DmaController(comptime id: u2) type {
         /// Note: Use writeDst instead of manipulatig dst_addr directly
         dad: u32,
         /// Write-only. The Word Count for the DMA Transfer (DMACNT_L)
-        word_count: if (id == 3) u16 else u14,
+        word_count: WordCount,
         /// Read / Write. DMACNT_H
         /// Note: Use writeControl instead of manipulating cnt directly.
         cnt: DmaControl,
 
+        /// Internal. The last successfully read value
+        data_latch: u32,
         /// Internal. Currrent Source Address
         sad_latch: u32,
         /// Internal. Current Destination Address
         dad_latch: u32,
         /// Internal. Word Count
-        _word_count: if (id == 3) u16 else u14,
+        _word_count: WordCount,
 
         /// Some DMA Transfers are enabled during Hblank / VBlank and / or
         /// have delays. Thefore bit 15 of DMACNT isn't actually something
@@ -134,11 +188,17 @@ fn DmaController(comptime id: u2) type {
                 // Internals
                 .sad_latch = 0,
                 .dad_latch = 0,
+                .data_latch = 0,
+
                 ._word_count = 0,
                 .in_progress = false,
             };
         }
 
+        pub fn reset(self: *Self) void {
+            self.* = Self.init();
+        }
+
         pub fn setDmasad(self: *Self, addr: u32) void {
             self.sad = addr & sad_mask;
         }
@@ -151,6 +211,10 @@ fn DmaController(comptime id: u2) type {
             self.word_count = @truncate(@TypeOf(self.word_count), halfword);
         }
 
+        pub fn dmacntH(self: *const Self) u16 {
+            return self.cnt.raw & if (id == 3) 0xFFE0 else 0xF7E0;
+        }
+
         pub fn setDmacntH(self: *Self, halfword: u16) void {
             const new = DmaControl{ .raw = halfword };
 
@@ -158,7 +222,7 @@ fn DmaController(comptime id: u2) type {
                 // Reload Internals on Rising Edge.
                 self.sad_latch = self.sad;
                 self.dad_latch = self.dad;
-                self._word_count = if (self.word_count == 0) std.math.maxInt(@TypeOf(self._word_count)) else self.word_count;
+                self._word_count = if (self.word_count == 0) std.math.maxInt(WordCount) else self.word_count;
 
                 // Only a Start Timing of 00 has a DMA Transfer immediately begin
                 self.in_progress = new.start_timing.read() == 0b00;
@@ -181,19 +245,31 @@ fn DmaController(comptime id: u2) type {
             const offset: u32 = if (transfer_type) @sizeOf(u32) else @sizeOf(u16);
 
             const mask = if (transfer_type) ~@as(u32, 3) else ~@as(u32, 1);
+            const sad_addr = self.sad_latch & mask;
+            const dad_addr = self.dad_latch & mask;
 
             if (transfer_type) {
-                cpu.bus.write(u32, self.dad_latch & mask, cpu.bus.read(u32, self.sad_latch & mask));
+                if (sad_addr >= 0x0200_0000) self.data_latch = cpu.bus.read(u32, sad_addr);
+                cpu.bus.write(u32, dad_addr, self.data_latch);
             } else {
-                cpu.bus.write(u16, self.dad_latch & mask, cpu.bus.read(u16, self.sad_latch & mask));
+                if (sad_addr >= 0x0200_0000) {
+                    const value: u32 = cpu.bus.read(u16, sad_addr);
+                    self.data_latch = value << 16 | value;
+                }
+
+                cpu.bus.write(u16, dad_addr, @truncate(u16, rotr(u32, self.data_latch, 8 * (dad_addr & 3))));
             }
 
-            switch (sad_adj) {
-                .Increment => self.sad_latch +%= offset,
-                .Decrement => self.sad_latch -%= offset,
-                // FIXME: Is just ignoring this ok?
-                .IncrementReload => log.err("{} is a prohibited adjustment on SAD", .{sad_adj}),
-                .Fixed => {},
+            switch (@truncate(u8, sad_addr >> 24)) {
+                // according to fleroviux, DMAs with a source address in ROM misbehave
+                // the resultant behaviour is that the source address will increment despite what DMAXCNT says
+                0x08...0x0D => self.sad_latch +%= offset, // obscure behaviour
+                else => switch (sad_adj) {
+                    .Increment => self.sad_latch +%= offset,
+                    .Decrement => self.sad_latch -%= offset,
+                    .IncrementReload => log.err("{} is a prohibited adjustment on SAD", .{sad_adj}),
+                    .Fixed => {},
+                },
             }
 
             switch (dad_adj) {
@@ -266,11 +342,8 @@ fn DmaController(comptime id: u2) type {
     };
 }
 
-pub fn pollDmaOnBlank(bus: *Bus, comptime kind: DmaKind) void {
-    bus.dma[0].poll(kind);
-    bus.dma[1].poll(kind);
-    bus.dma[2].poll(kind);
-    bus.dma[3].poll(kind);
+pub fn onBlanking(bus: *Bus, comptime kind: DmaKind) void {
+    inline for (0..4) |i| bus.dma[i].poll(kind);
 }
 
 const Adjustment = enum(u2) {

src/core/bus/gpio.zig

@@ -1,6 +1,5 @@
 const std = @import("std");
 const Bit = @import("bitfield").Bit;
-const Bitfield = @import("bitfield").Bitfield;
 const DateTime = @import("datetime").datetime.Datetime;
 
 const Arm7tdmi = @import("../cpu.zig").Arm7tdmi;
@@ -68,9 +67,11 @@ pub const Gpio = struct {
         log.info("Device: {}", .{kind});
 
         const self = try allocator.create(Self);
+        errdefer allocator.destroy(self);
+
         self.* = .{
             .data = 0b0000,
-            .direction = 0b1111, // TODO: What is GPIO DIrection set to by default?
+            .direction = 0b1111, // TODO: What is GPIO Direction set to by default?
             .cnt = 0b0,
 
             .device = switch (kind) {
@@ -292,13 +293,13 @@ pub const Clock = struct {
         self.cpu.sched.push(.RealTimeClock, (1 << 24) -| late); // Reschedule
 
         const now = DateTime.now();
-        self.year = bcd(u8, @intCast(u8, now.date.year - 2000));
-        self.month = bcd(u5, now.date.month);
-        self.day = bcd(u6, now.date.day);
-        self.weekday = bcd(u3, (now.date.weekday() + 1) % 7); // API is Monday = 0, Sunday = 6. We want Sunday = 0, Saturday = 6
-        self.hour = bcd(u6, now.time.hour);
-        self.minute = bcd(u7, now.time.minute);
-        self.second = bcd(u7, now.time.second);
+        self.year = bcd(@intCast(u8, now.date.year - 2000));
+        self.month = @truncate(u5, bcd(now.date.month));
+        self.day = @truncate(u6, bcd(now.date.day));
+        self.weekday = @truncate(u3, bcd((now.date.weekday() + 1) % 7)); // API is Monday = 0, Sunday = 6. We want Sunday = 0, Saturday = 6
+        self.hour = @truncate(u6, bcd(now.time.hour));
+        self.minute = @truncate(u7, bcd(now.time.minute));
+        self.second = @truncate(u7, bcd(now.time.second));
     }
 
     fn step(self: *Self, value: Data) u4 {
@@ -448,16 +449,8 @@ pub const Clock = struct {
     }
 };
 
-fn bcd(comptime T: type, value: u8) T {
-    var input = value;
-    var ret: u8 = 0;
-    var shift: u3 = 0;
-
-    while (input > 0) {
-        ret |= (input % 10) << (shift << 2);
-        shift += 1;
-        input /= 10;
-    }
-
-    return @truncate(T, ret);
+/// Converts an 8-bit unsigned integer to its BCD representation.
+/// Note: Algorithm only works for values between 0 and 99 inclusive.
+fn bcd(value: u8) u8 {
+    return ((value / 10) << 4) + (value % 10);
 }

src/core/bus/io.zig

@@ -1,18 +1,16 @@
 const std = @import("std");
-const builtin = @import("builtin");
 const timer = @import("timer.zig");
 const dma = @import("dma.zig");
 const apu = @import("../apu.zig");
+const ppu = @import("../ppu.zig");
 const util = @import("../../util.zig");
 
 const Bit = @import("bitfield").Bit;
 const Bitfield = @import("bitfield").Bitfield;
 const Bus = @import("../Bus.zig");
-const DmaController = @import("dma.zig").DmaController;
-const Scheduler = @import("../scheduler.zig").Scheduler;
 
-const setHi = util.setLo;
-const setLo = util.setHi;
+const getHalf = util.getHalf;
+const setHalf = util.setHalf;
 
 const log = std.log.scoped(.@"I/O");
 
@@ -24,20 +22,26 @@ pub const Io = struct {
     ie: InterruptEnable,
     irq: InterruptRequest,
     postflg: PostFlag,
+    waitcnt: WaitControl,
     haltcnt: HaltControl,
-    keyinput: KeyInput,
+    keyinput: AtomicKeyInput,
 
     pub fn init() Self {
         return .{
             .ime = false,
             .ie = .{ .raw = 0x0000 },
             .irq = .{ .raw = 0x0000 },
-            .keyinput = .{ .raw = 0x03FF },
+            .keyinput = AtomicKeyInput.init(.{ .raw = 0x03FF }),
+            .waitcnt = .{ .raw = 0x0000_0000 }, // Bit 15 == 0 for GBA
             .postflg = .FirstBoot,
             .haltcnt = .Execute,
         };
     }
 
+    pub fn reset(self: *Self) void {
+        self.* = Self.init();
+    }
+
     fn setIrqs(self: *Io, word: u32) void {
         self.ie.raw = @truncate(u16, word);
         self.irq.raw &= ~@truncate(u16, word >> 16);
@@ -48,9 +52,10 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) ?T {
     return switch (T) {
         u32 => switch (address) {
             // Display
-            0x0400_0000 => bus.ppu.dispcnt.raw,
-            0x0400_0004 => @as(T, bus.ppu.vcount.raw) << 16 | bus.ppu.dispstat.raw,
-            0x0400_0006 => @as(T, bus.ppu.bg[0].cnt.raw) << 16 | bus.ppu.vcount.raw,
+            0x0400_0000...0x0400_0054 => ppu.read(T, &bus.ppu, address),
+
+            // Sound
+            0x0400_0060...0x0400_00A4 => apu.read(T, &bus.apu, address),
 
             // DMA Transfers
             0x0400_00B0...0x0400_00DC => dma.read(T, &bus.dma, address),
@@ -68,26 +73,18 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) ?T {
             0x0400_0150 => util.io.read.todo(log, "Read {} from JOY_RECV", .{T}),
 
             // Interrupts
-            0x0400_0200 => @as(T, bus.io.irq.raw) << 16 | bus.io.ie.raw,
+            0x0400_0200 => @as(u32, bus.io.irq.raw) << 16 | bus.io.ie.raw,
+            0x0400_0204 => bus.io.waitcnt.raw,
             0x0400_0208 => @boolToInt(bus.io.ime),
+            0x0400_0300 => @enumToInt(bus.io.postflg),
             else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, address }),
         },
         u16 => switch (address) {
             // Display
-            0x0400_0000 => bus.ppu.dispcnt.raw,
-            0x0400_0004 => bus.ppu.dispstat.raw,
-            0x0400_0006 => bus.ppu.vcount.raw,
-            0x0400_0008 => bus.ppu.bg[0].cnt.raw,
-            0x0400_000A => bus.ppu.bg[1].cnt.raw,
-            0x0400_000C => bus.ppu.bg[2].cnt.raw,
-            0x0400_000E => bus.ppu.bg[3].cnt.raw,
-            0x0400_004C => util.io.read.todo(log, "Read {} from MOSAIC", .{T}),
-            0x0400_0050 => bus.ppu.bldcnt.raw,
-            0x0400_0052 => bus.ppu.bldalpha.raw,
-            0x0400_0054 => bus.ppu.bldy.raw,
+            0x0400_0000...0x0400_0054 => ppu.read(T, &bus.ppu, address),
 
             // Sound
-            0x0400_0060...0x0400_009E => apu.read(T, &bus.apu, address),
+            0x0400_0060...0x0400_00A6 => apu.read(T, &bus.apu, address),
 
             // DMA Transfers
             0x0400_00B0...0x0400_00DE => dma.read(T, &bus.dma, address),
@@ -99,32 +96,38 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) ?T {
             0x0400_0128 => util.io.read.todo(log, "Read {} from SIOCNT", .{T}),
 
             // Keypad Input
-            0x0400_0130 => bus.io.keyinput.raw,
+            0x0400_0130 => bus.io.keyinput.load(.Monotonic).raw,
 
             // Serial Communication 2
             0x0400_0134 => util.io.read.todo(log, "Read {} from RCNT", .{T}),
+            0x0400_0136 => 0x0000,
+            0x0400_0142 => 0x0000,
+            0x0400_015A => 0x0000,
 
             // Interrupts
             0x0400_0200 => bus.io.ie.raw,
             0x0400_0202 => bus.io.irq.raw,
-            0x0400_0204 => util.io.read.todo(log, "Read {} from WAITCNT", .{T}),
+            0x0400_0204 => bus.io.waitcnt.raw,
+            0x0400_0206 => 0x0000,
             0x0400_0208 => @boolToInt(bus.io.ime),
+            0x0400_020A => 0x0000,
+            0x0400_0300 => @enumToInt(bus.io.postflg),
+            0x0400_0302 => 0x0000,
             else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, address }),
         },
         u8 => return switch (address) {
             // Display
-            0x0400_0000 => @truncate(T, bus.ppu.dispcnt.raw),
-            0x0400_0004 => @truncate(T, bus.ppu.dispstat.raw),
-            0x0400_0005 => @truncate(T, bus.ppu.dispcnt.raw >> 8),
-            0x0400_0006 => @truncate(T, bus.ppu.vcount.raw),
-            0x0400_0008 => @truncate(T, bus.ppu.bg[0].cnt.raw),
-            0x0400_0009 => @truncate(T, bus.ppu.bg[0].cnt.raw >> 8),
-            0x0400_000A => @truncate(T, bus.ppu.bg[1].cnt.raw),
-            0x0400_000B => @truncate(T, bus.ppu.bg[1].cnt.raw >> 8),
+            0x0400_0000...0x0400_0055 => ppu.read(T, &bus.ppu, address),
 
             // Sound
             0x0400_0060...0x0400_00A7 => apu.read(T, &bus.apu, address),
 
+            // DMA Transfers
+            0x0400_00B0...0x0400_00DF => dma.read(T, &bus.dma, address),
+
+            // Timers
+            0x0400_0100...0x0400_010F => timer.read(T, &bus.tim, address),
+
             // Serial Communication 1
             0x0400_0128 => util.io.read.todo(log, "Read {} from SIOCNT_L", .{T}),
 
@@ -133,10 +136,20 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) ?T {
 
             // Serial Communication 2
             0x0400_0135 => util.io.read.todo(log, "Read {} from RCNT_H", .{T}),
+            0x0400_0136, 0x0400_0137 => 0x00,
+            0x0400_0142, 0x0400_0143 => 0x00,
+            0x0400_015A, 0x0400_015B => 0x00,
 
             // Interrupts
-            0x0400_0200 => @truncate(T, bus.io.ie.raw),
+            0x0400_0200, 0x0400_0201 => @truncate(T, bus.io.ie.raw >> getHalf(@truncate(u8, address))),
+            0x0400_0202, 0x0400_0203 => @truncate(T, bus.io.irq.raw >> getHalf(@truncate(u8, address))),
+            0x0400_0204, 0x0400_0205 => @truncate(T, bus.io.waitcnt.raw >> getHalf(@truncate(u8, address))),
+            0x0400_0206, 0x0400_0207 => 0x00,
+            0x0400_0208, 0x0400_0209 => @truncate(T, @as(u16, @boolToInt(bus.io.ime)) >> getHalf(@truncate(u8, address))),
+            0x0400_020A, 0x0400_020B => 0x00,
             0x0400_0300 => @enumToInt(bus.io.postflg),
+            0x0400_0301 => null,
+            0x0400_0302, 0x0400_0303 => 0x00,
             else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, address }),
         },
         else => @compileError("I/O: Unsupported read width"),
@@ -147,34 +160,7 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
     return switch (T) {
         u32 => switch (address) {
             // Display
-            0x0400_0000 => bus.ppu.dispcnt.raw = @truncate(u16, value),
-            0x0400_0004 => {
-                bus.ppu.dispstat.raw = @truncate(u16, value);
-                bus.ppu.vcount.raw = @truncate(u16, value >> 16);
-            },
-            0x0400_0008 => bus.ppu.setAdjCnts(0, value),
-            0x0400_000C => bus.ppu.setAdjCnts(2, value),
-            0x0400_0010 => bus.ppu.setBgOffsets(0, value),
-            0x0400_0014 => bus.ppu.setBgOffsets(1, value),
-            0x0400_0018 => bus.ppu.setBgOffsets(2, value),
-            0x0400_001C => bus.ppu.setBgOffsets(3, value),
-            0x0400_0020 => bus.ppu.aff_bg[0].writePaPb(value),
-            0x0400_0024 => bus.ppu.aff_bg[0].writePcPd(value),
-            0x0400_0028 => bus.ppu.aff_bg[0].setX(bus.ppu.dispstat.vblank.read(), value),
-            0x0400_002C => bus.ppu.aff_bg[0].setY(bus.ppu.dispstat.vblank.read(), value),
-            0x0400_0030 => bus.ppu.aff_bg[1].writePaPb(value),
-            0x0400_0034 => bus.ppu.aff_bg[1].writePcPd(value),
-            0x0400_0038 => bus.ppu.aff_bg[1].setX(bus.ppu.dispstat.vblank.read(), value),
-            0x0400_003C => bus.ppu.aff_bg[1].setY(bus.ppu.dispstat.vblank.read(), value),
-            0x0400_0040 => bus.ppu.win.setH(value),
-            0x0400_0044 => bus.ppu.win.setV(value),
-            0x0400_0048 => bus.ppu.win.setIo(value),
-            0x0400_004C => log.debug("Wrote 0x{X:0>8} to MOSAIC", .{value}),
-            0x0400_0050 => {
-                bus.ppu.bldcnt.raw = @truncate(u16, value);
-                bus.ppu.bldalpha.raw = @truncate(u16, value >> 16);
-            },
-            0x0400_0054 => bus.ppu.bldy.raw = @truncate(u16, value),
+            0x0400_0000...0x0400_0054 => ppu.write(T, &bus.ppu, address, value),
             0x0400_0058...0x0400_005C => {}, // Unused
 
             // Sound
@@ -210,65 +196,28 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
 
             // Interrupts
             0x0400_0200 => bus.io.setIrqs(value),
-            0x0400_0204 => log.debug("Wrote 0x{X:0>8} to WAITCNT", .{value}),
+            0x0400_0204 => bus.io.waitcnt.set(@truncate(u16, value)),
             0x0400_0208 => bus.io.ime = value & 1 == 1,
-            0x0400_020C...0x0400_021C => {}, // Unused
+            0x0400_0300 => {
+                bus.io.postflg = @intToEnum(PostFlag, value & 1);
+                bus.io.haltcnt = if (value >> 15 & 1 == 0) .Halt else @panic("TODO: Implement STOP");
+            },
             else => util.io.write.undef(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, address }),
         },
         u16 => switch (address) {
             // Display
-            0x0400_0000 => bus.ppu.dispcnt.raw = value,
-            0x0400_0004 => bus.ppu.dispstat.raw = value,
-            0x0400_0006 => {}, // vcount is read-only
-            0x0400_0008 => bus.ppu.bg[0].cnt.raw = value,
-            0x0400_000A => bus.ppu.bg[1].cnt.raw = value,
-            0x0400_000C => bus.ppu.bg[2].cnt.raw = value,
-            0x0400_000E => bus.ppu.bg[3].cnt.raw = value,
-            0x0400_0010 => bus.ppu.bg[0].hofs.raw = value, // TODO: Don't write out every HOFS / VOFS?
-            0x0400_0012 => bus.ppu.bg[0].vofs.raw = value,
-            0x0400_0014 => bus.ppu.bg[1].hofs.raw = value,
-            0x0400_0016 => bus.ppu.bg[1].vofs.raw = value,
-            0x0400_0018 => bus.ppu.bg[2].hofs.raw = value,
-            0x0400_001A => bus.ppu.bg[2].vofs.raw = value,
-            0x0400_001C => bus.ppu.bg[3].hofs.raw = value,
-            0x0400_001E => bus.ppu.bg[3].vofs.raw = value,
-            0x0400_0020 => bus.ppu.aff_bg[0].pa = @bitCast(i16, value),
-            0x0400_0022 => bus.ppu.aff_bg[0].pb = @bitCast(i16, value),
-            0x0400_0024 => bus.ppu.aff_bg[0].pc = @bitCast(i16, value),
-            0x0400_0026 => bus.ppu.aff_bg[0].pd = @bitCast(i16, value),
-            0x0400_0028 => bus.ppu.aff_bg[0].x = @bitCast(i32, setLo(u32, @bitCast(u32, bus.ppu.aff_bg[0].x), value)),
-            0x0400_002A => bus.ppu.aff_bg[0].x = @bitCast(i32, setHi(u32, @bitCast(u32, bus.ppu.aff_bg[0].x), value)),
-            0x0400_002C => bus.ppu.aff_bg[0].y = @bitCast(i32, setLo(u32, @bitCast(u32, bus.ppu.aff_bg[0].y), value)),
-            0x0400_002E => bus.ppu.aff_bg[0].y = @bitCast(i32, setHi(u32, @bitCast(u32, bus.ppu.aff_bg[0].y), value)),
-            0x0400_0030 => bus.ppu.aff_bg[1].pa = @bitCast(i16, value),
-            0x0400_0032 => bus.ppu.aff_bg[1].pb = @bitCast(i16, value),
-            0x0400_0034 => bus.ppu.aff_bg[1].pc = @bitCast(i16, value),
-            0x0400_0036 => bus.ppu.aff_bg[1].pd = @bitCast(i16, value),
-            0x0400_0038 => bus.ppu.aff_bg[1].x = @bitCast(i32, setLo(u32, @bitCast(u32, bus.ppu.aff_bg[1].x), value)),
-            0x0400_003A => bus.ppu.aff_bg[1].x = @bitCast(i32, setHi(u32, @bitCast(u32, bus.ppu.aff_bg[1].x), value)),
-            0x0400_003C => bus.ppu.aff_bg[1].y = @bitCast(i32, setLo(u32, @bitCast(u32, bus.ppu.aff_bg[1].y), value)),
-            0x0400_003E => bus.ppu.aff_bg[1].y = @bitCast(i32, setHi(u32, @bitCast(u32, bus.ppu.aff_bg[1].y), value)),
-            0x0400_0040 => bus.ppu.win.h[0].raw = value,
-            0x0400_0042 => bus.ppu.win.h[1].raw = value,
-            0x0400_0044 => bus.ppu.win.v[0].raw = value,
-            0x0400_0046 => bus.ppu.win.v[1].raw = value,
-            0x0400_0048 => bus.ppu.win.in.raw = value,
-            0x0400_004A => bus.ppu.win.out.raw = value,
-            0x0400_004C => log.debug("Wrote 0x{X:0>4} to MOSAIC", .{value}),
-            0x0400_0050 => bus.ppu.bldcnt.raw = value,
-            0x0400_0052 => bus.ppu.bldalpha.raw = value,
-            0x0400_0054 => bus.ppu.bldy.raw = value,
-            0x0400_004E, 0x0400_0056 => {}, // Not used
+            0x0400_0000...0x0400_0054 => ppu.write(T, &bus.ppu, address, value),
+            0x0400_0056 => {}, // Not used
 
             // Sound
-            0x0400_0060...0x0400_009E => apu.write(T, &bus.apu, address, value),
+            0x0400_0060...0x0400_00A6 => apu.write(T, &bus.apu, address, value),
 
             // Dma Transfers
             0x0400_00B0...0x0400_00DE => dma.write(T, &bus.dma, address, value),
 
             // Timers
             0x0400_0100...0x0400_010E => timer.write(T, &bus.tim, address, value),
-            0x0400_0114 => {}, // TODO: Gyakuten Saiban writes 0x8000 to 0x0400_0114
+            0x0400_0114 => {},
             0x0400_0110 => {}, // Not Used,
 
             // Serial Communication 1
@@ -292,27 +241,29 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             // Interrupts
             0x0400_0200 => bus.io.ie.raw = value,
             0x0400_0202 => bus.io.irq.raw &= ~value,
-            0x0400_0204 => log.debug("Wrote 0x{X:0>4} to WAITCNT", .{value}),
+            0x0400_0204 => bus.io.waitcnt.set(value),
+            0x0400_0206 => {},
             0x0400_0208 => bus.io.ime = value & 1 == 1,
-            0x0400_0206, 0x0400_020A => {}, // Not Used
+            0x0400_020A => {},
+            0x0400_0300 => {
+                bus.io.postflg = @intToEnum(PostFlag, value & 1);
+                bus.io.haltcnt = if (value >> 15 & 1 == 0) .Halt else @panic("TODO: Implement STOP");
+            },
             else => util.io.write.undef(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, address }),
         },
         u8 => switch (address) {
             // Display
-            0x0400_0004 => bus.ppu.dispstat.raw = setLo(u16, bus.ppu.dispstat.raw, value),
-            0x0400_0005 => bus.ppu.dispstat.raw = setHi(u16, bus.ppu.dispstat.raw, value),
-            0x0400_0008 => bus.ppu.bg[0].cnt.raw = setLo(u16, bus.ppu.bg[0].cnt.raw, value),
-            0x0400_0009 => bus.ppu.bg[0].cnt.raw = setHi(u16, bus.ppu.bg[0].cnt.raw, value),
-            0x0400_000A => bus.ppu.bg[1].cnt.raw = setLo(u16, bus.ppu.bg[1].cnt.raw, value),
-            0x0400_000B => bus.ppu.bg[1].cnt.raw = setHi(u16, bus.ppu.bg[1].cnt.raw, value),
-            0x0400_0048 => bus.ppu.win.in.raw = setLo(u16, bus.ppu.win.in.raw, value),
-            0x0400_0049 => bus.ppu.win.in.raw = setHi(u16, bus.ppu.win.in.raw, value),
-            0x0400_004A => bus.ppu.win.out.raw = setLo(u16, bus.ppu.win.out.raw, value),
-            0x0400_0054 => bus.ppu.bldy.raw = setLo(u16, bus.ppu.bldy.raw, value),
+            0x0400_0000...0x0400_0055 => ppu.write(T, &bus.ppu, address, value),
 
             // Sound
             0x0400_0060...0x0400_00A7 => apu.write(T, &bus.apu, address, value),
 
+            // Dma Transfers
+            0x0400_00B0...0x0400_00DF => dma.write(T, &bus.dma, address, value),
+
+            // Timers
+            0x0400_0100...0x0400_010F => timer.write(T, &bus.tim, address, value),
+
             // Serial Communication 1
             0x0400_0120 => log.debug("Wrote 0x{X:0>2} to SIODATA32_L_L", .{value}),
             0x0400_0128 => log.debug("Wrote 0x{X:0>2} to SIOCNT_L", .{value}),
@@ -322,9 +273,16 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             0x0400_0140 => log.debug("Wrote 0x{X:0>2} to JOYCNT_L", .{value}),
 
             // Interrupts
+            0x0400_0200, 0x0400_0201 => bus.io.ie.raw = setHalf(u16, bus.io.ie.raw, @truncate(u8, address), value),
             0x0400_0202 => bus.io.irq.raw &= ~@as(u16, value),
+            0x0400_0203 => bus.io.irq.raw &= ~@as(u16, value) << 8, // TODO: Is this good?
+            0x0400_0204, 0x0400_0205 => bus.io.waitcnt.set(setHalf(u16, @truncate(u16, bus.io.waitcnt.raw), @truncate(u8, address), value)),
+            0x0400_0206, 0x0400_0207 => {},
             0x0400_0208 => bus.io.ime = value & 1 == 1,
-            0x0400_0300 => bus.io.postflg = std.meta.intToEnum(PostFlag, value & 1) catch unreachable,
+            0x0400_0209 => {},
+            0x0400_020A, 0x0400_020B => {},
+
+            0x0400_0300 => bus.io.postflg = @intToEnum(PostFlag, value & 1),
             0x0400_0301 => bus.io.haltcnt = if (value >> 7 & 1 == 0) .Halt else std.debug.panic("TODO: Implement STOP", .{}),
 
             0x0400_0410 => log.debug("Wrote 0x{X:0>2} to the common yet undocumented 0x{X:0>8}", .{ value, address }),
@@ -363,14 +321,22 @@ pub const DisplayControl = extern union {
 
 /// Read / Write
 pub const DisplayStatus = extern union {
+    /// read-only
     vblank: Bit(u16, 0),
+    /// read-only
     hblank: Bit(u16, 1),
+    // read-only
     coincidence: Bit(u16, 2),
     vblank_irq: Bit(u16, 3),
     hblank_irq: Bit(u16, 4),
     vcount_irq: Bit(u16, 5),
     vcount_trigger: Bitfield(u16, 8, 8),
     raw: u16,
+
+    pub fn set(self: *DisplayStatus, value: u16) void {
+        const mask: u16 = 0x00C7; // set bits are read-only
+        self.raw = (self.raw & mask) | (value & ~mask);
+    }
 };
 
 /// Read Only
@@ -384,10 +350,10 @@ const InterruptEnable = extern union {
     vblank: Bit(u16, 0),
     hblank: Bit(u16, 1),
     coincidence: Bit(u16, 2),
-    tm0_overflow: Bit(u16, 3),
-    tm1_overflow: Bit(u16, 4),
-    tm2_overflow: Bit(u16, 5),
-    tm3_overflow: Bit(u16, 6),
+    tim0: Bit(u16, 3),
+    tim1: Bit(u16, 4),
+    tim2: Bit(u16, 5),
+    tim3: Bit(u16, 6),
     serial: Bit(u16, 7),
     dma0: Bit(u16, 8),
     dma1: Bit(u16, 9),
@@ -414,6 +380,31 @@ const KeyInput = extern union {
     raw: u16,
 };
 
+const AtomicKeyInput = struct {
+    const Self = @This();
+    const Ordering = std.atomic.Ordering;
+
+    inner: KeyInput,
+
+    pub fn init(value: KeyInput) Self {
+        return .{ .inner = value };
+    }
+
+    pub inline fn load(self: *const Self, comptime ordering: Ordering) KeyInput {
+        return .{ .raw = switch (ordering) {
+            .AcqRel, .Release => @compileError("not supported for atomic loads"),
+            else => @atomicLoad(u16, &self.inner.raw, ordering),
+        } };
+    }
+
+    pub inline fn store(self: *Self, value: u16, comptime ordering: Ordering) void {
+        switch (ordering) {
+            .AcqRel, .Acquire => @compileError("not supported for atomic stores"),
+            else => @atomicStore(u16, &self.inner.raw, value, ordering),
+        }
+    }
+};
+
 // Read / Write
 pub const BackgroundControl = extern union {
     priority: Bitfield(u16, 0, 2),
@@ -462,6 +453,8 @@ pub const BldY = extern union {
     raw: u16,
 };
 
+const u8WriteKind = enum { Hi, Lo };
+
 /// Write-only
 pub const WinH = extern union {
     x2: Bitfield(u16, 0, 8),
@@ -471,6 +464,8 @@ pub const WinH = extern union {
 
 /// Write-only
 pub const WinV = extern union {
+    const Self = @This();
+
     y2: Bitfield(u16, 0, 8),
     y1: Bitfield(u16, 8, 8),
     raw: u16,
@@ -479,20 +474,20 @@ pub const WinV = extern union {
 pub const WinIn = extern union {
     w0_bg: Bitfield(u16, 0, 4),
     w0_obj: Bit(u16, 4),
-    w0_colour: Bit(u16, 5),
+    w0_bld: Bit(u16, 5),
     w1_bg: Bitfield(u16, 8, 4),
     w1_obj: Bit(u16, 12),
-    w1_colour: Bit(u16, 13),
+    w1_bld: Bit(u16, 13),
     raw: u16,
 };
 
 pub const WinOut = extern union {
     out_bg: Bitfield(u16, 0, 4),
     out_obj: Bit(u16, 4),
-    out_colour: Bit(u16, 5),
+    out_bld: Bit(u16, 5),
     obj_bg: Bitfield(u16, 8, 4),
     obj_obj: Bit(u16, 12),
-    obj_colour: Bit(u16, 13),
+    obj_bld: Bit(u16, 13),
     raw: u16,
 };
 
@@ -661,3 +656,24 @@ pub const SoundBias = extern union {
     sampling_cycle: Bitfield(u16, 14, 2),
     raw: u16,
 };
+
+/// Read / Write
+pub const WaitControl = extern union {
+    sram_cnt: Bitfield(u16, 0, 2),
+    s0_first: Bitfield(u16, 2, 2),
+    s0_second: Bit(u16, 4),
+    s1_first: Bitfield(u16, 5, 2),
+    s1_second: Bit(u16, 7),
+    s2_first: Bitfield(u16, 8, 2),
+    s2_second: Bit(u16, 10),
+    phi_out: Bitfield(u16, 11, 2),
+
+    prefetch_enable: Bit(u16, 14),
+    pak_kind: Bit(u16, 15),
+    raw: u16,
+
+    pub fn set(self: *WaitControl, value: u16) void {
+        const mask: u16 = 0x8000; // set bits are read-only
+        self.raw = (self.raw & mask) | (value & ~mask);
+    }
+};

src/core/bus/timer.zig

@@ -2,68 +2,99 @@ const std = @import("std");
 const util = @import("../../util.zig");
 
 const TimerControl = @import("io.zig").TimerControl;
-const Io = @import("io.zig").Io;
 const Scheduler = @import("../scheduler.zig").Scheduler;
-const Event = @import("../scheduler.zig").Event;
 const Arm7tdmi = @import("../cpu.zig").Arm7tdmi;
 
-pub const TimerTuple = std.meta.Tuple(&[_]type{ Timer(0), Timer(1), Timer(2), Timer(3) });
+pub const TimerTuple = struct { Timer(0), Timer(1), Timer(2), Timer(3) };
 const log = std.log.scoped(.Timer);
 
+const getHalf = util.getHalf;
+const setHalf = util.setHalf;
+
 pub fn create(sched: *Scheduler) TimerTuple {
     return .{ Timer(0).init(sched), Timer(1).init(sched), Timer(2).init(sched), Timer(3).init(sched) };
 }
 
 pub fn read(comptime T: type, tim: *const TimerTuple, addr: u32) ?T {
-    const nybble = @truncate(u4, addr);
+    const nybble_addr = @truncate(u4, addr);
 
     return switch (T) {
-        u32 => switch (nybble) {
+        u32 => switch (nybble_addr) {
             0x0 => @as(T, tim.*[0].cnt.raw) << 16 | tim.*[0].timcntL(),
             0x4 => @as(T, tim.*[1].cnt.raw) << 16 | tim.*[1].timcntL(),
             0x8 => @as(T, tim.*[2].cnt.raw) << 16 | tim.*[2].timcntL(),
             0xC => @as(T, tim.*[3].cnt.raw) << 16 | tim.*[3].timcntL(),
-            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+            else => util.io.read.err(T, log, "unaligned {} read from 0x{X:0>8}", .{ T, addr }),
         },
-        u16 => switch (nybble) {
+        u16 => switch (nybble_addr) {
             0x0 => tim.*[0].timcntL(),
             0x2 => tim.*[0].cnt.raw,
+
             0x4 => tim.*[1].timcntL(),
             0x6 => tim.*[1].cnt.raw,
+
             0x8 => tim.*[2].timcntL(),
             0xA => tim.*[2].cnt.raw,
+
             0xC => tim.*[3].timcntL(),
             0xE => tim.*[3].cnt.raw,
-            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+            else => util.io.read.err(T, log, "unaligned {} read from 0x{X:0>8}", .{ T, addr }),
+        },
+        u8 => switch (nybble_addr) {
+            0x0, 0x1 => @truncate(T, tim.*[0].timcntL() >> getHalf(nybble_addr)),
+            0x2, 0x3 => @truncate(T, tim.*[0].cnt.raw >> getHalf(nybble_addr)),
+
+            0x4, 0x5 => @truncate(T, tim.*[1].timcntL() >> getHalf(nybble_addr)),
+            0x6, 0x7 => @truncate(T, tim.*[1].cnt.raw >> getHalf(nybble_addr)),
+
+            0x8, 0x9 => @truncate(T, tim.*[2].timcntL() >> getHalf(nybble_addr)),
+            0xA, 0xB => @truncate(T, tim.*[2].cnt.raw >> getHalf(nybble_addr)),
+
+            0xC, 0xD => @truncate(T, tim.*[3].timcntL() >> getHalf(nybble_addr)),
+            0xE, 0xF => @truncate(T, tim.*[3].cnt.raw >> getHalf(nybble_addr)),
         },
-        u8 => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         else => @compileError("TIM: Unsupported read width"),
     };
 }
 
 pub fn write(comptime T: type, tim: *TimerTuple, addr: u32, value: T) void {
-    const nybble = @truncate(u4, addr);
+    const nybble_addr = @truncate(u4, addr);
 
     return switch (T) {
-        u32 => switch (nybble) {
+        u32 => switch (nybble_addr) {
             0x0 => tim.*[0].setTimcnt(value),
             0x4 => tim.*[1].setTimcnt(value),
             0x8 => tim.*[2].setTimcnt(value),
             0xC => tim.*[3].setTimcnt(value),
             else => util.io.write.undef(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
-        u16 => switch (nybble) {
+        u16 => switch (nybble_addr) {
             0x0 => tim.*[0].setTimcntL(value),
             0x2 => tim.*[0].setTimcntH(value),
+
             0x4 => tim.*[1].setTimcntL(value),
             0x6 => tim.*[1].setTimcntH(value),
+
             0x8 => tim.*[2].setTimcntL(value),
             0xA => tim.*[2].setTimcntH(value),
+
             0xC => tim.*[3].setTimcntL(value),
             0xE => tim.*[3].setTimcntH(value),
             else => util.io.write.undef(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
-        u8 => util.io.write.undef(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, addr }),
+        u8 => switch (nybble_addr) {
+            0x0, 0x1 => tim.*[0].setTimcntL(setHalf(u16, tim.*[0]._reload, nybble_addr, value)),
+            0x2, 0x3 => tim.*[0].setTimcntH(setHalf(u16, tim.*[0].cnt.raw, nybble_addr, value)),
+
+            0x4, 0x5 => tim.*[1].setTimcntL(setHalf(u16, tim.*[1]._reload, nybble_addr, value)),
+            0x6, 0x7 => tim.*[1].setTimcntH(setHalf(u16, tim.*[1].cnt.raw, nybble_addr, value)),
+
+            0x8, 0x9 => tim.*[2].setTimcntL(setHalf(u16, tim.*[2]._reload, nybble_addr, value)),
+            0xA, 0xB => tim.*[2].setTimcntH(setHalf(u16, tim.*[2].cnt.raw, nybble_addr, value)),
+
+            0xC, 0xD => tim.*[3].setTimcntL(setHalf(u16, tim.*[3]._reload, nybble_addr, value)),
+            0xE, 0xF => tim.*[3].setTimcntH(setHalf(u16, tim.*[3].cnt.raw, nybble_addr, value)),
+        },
         else => @compileError("TIM: Unsupported write width"),
     };
 }
@@ -97,6 +128,12 @@ fn Timer(comptime id: u2) type {
             };
         }
 
+        pub fn reset(self: *Self) void {
+            const scheduler = self.sched;
+
+            self.* = Self.init(scheduler);
+        }
+
         /// TIMCNT_L Getter
         pub fn timcntL(self: *const Self) u16 {
             if (self.cnt.cascade.read() or !self.cnt.enabled.read()) return self._counter;
@@ -119,21 +156,36 @@ fn Timer(comptime id: u2) type {
         pub fn setTimcntH(self: *Self, halfword: u16) void {
             const new = TimerControl{ .raw = halfword };
 
-            // If Timer happens to be enabled, It will either be resheduled or disabled
-            self.sched.removeScheduledEvent(.{ .TimerOverflow = id });
+            if (self.cnt.enabled.read()) {
+                // timer was already enabled
 
-            if (self.cnt.enabled.read() and (new.cascade.read() or !new.enabled.read())) {
-                // Either through the cascade bit or the enable bit, the timer has effectively been disabled
-                // The Counter should hold whatever value it should have been at when it was disabled
-                self._counter +%= @truncate(u16, (self.sched.now() - self._start_timestamp) / self.frequency());
+                // If enabled falling edge or cascade falling edge, timer is paused
+                if (!new.enabled.read() or (!self.cnt.cascade.read() and new.cascade.read())) {
+                    self.sched.removeScheduledEvent(.{ .TimerOverflow = id });
+
+                    // Counter should hold the value it stopped at meaning we have to calculate it now
+                    self._counter +%= @truncate(u16, (self.sched.now() - self._start_timestamp) / self.frequency());
+                }
+
+                // the timer has always been enabled, but the cascade bit which was blocking the timer has been unset
+                if (new.enabled.read() and (self.cnt.cascade.read() and !new.cascade.read())) {
+                    // we want to reschedule the timer event, however we won't reload the counter.
+                    // the invariant here is that self._counter holds the already calculated paused value
+
+                    self.rescheduleTimerExpire(0);
+                }
+            } else {
+                // the timer was previously disabeld
+
+                if (new.enabled.read()) {
+                    // timer should start counting (with a reloaded counter value)
+                    self._counter = self._reload;
+
+                    // if cascade happens to be set, the timer doesn't actually do anything though
+                    if (!new.cascade.read()) self.rescheduleTimerExpire(0);
+                }
             }
 
-            // The counter is only reloaded on the rising edge of the enable bit
-            if (!self.cnt.enabled.read() and new.enabled.read()) self._counter = self._reload;
-
-            // If Timer is enabled and we're not cascading, we need to schedule an overflow event
-            if (new.enabled.read() and !new.cascade.read()) self.rescheduleTimerExpire(0);
-
             self.cnt.raw = halfword;
         }
 
@@ -159,23 +211,20 @@ fn Timer(comptime id: u2) type {
 
             // Perform Cascade Behaviour
             switch (id) {
-                0 => if (cpu.bus.tim[1].cnt.cascade.read()) {
-                    cpu.bus.tim[1]._counter +%= 1;
-                    if (cpu.bus.tim[1]._counter == 0) cpu.bus.tim[1].onTimerExpire(cpu, late);
+                inline 0, 1, 2 => |idx| {
+                    const next = idx + 1;
+
+                    if (cpu.bus.tim[next].cnt.cascade.read()) {
+                        cpu.bus.tim[next]._counter +%= 1;
+                        if (cpu.bus.tim[next]._counter == 0) cpu.bus.tim[next].onTimerExpire(cpu, late);
+                    }
                 },
-                1 => if (cpu.bus.tim[2].cnt.cascade.read()) {
-                    cpu.bus.tim[2]._counter +%= 1;
-                    if (cpu.bus.tim[2]._counter == 0) cpu.bus.tim[2].onTimerExpire(cpu, late);
-                },
-                2 => if (cpu.bus.tim[3].cnt.cascade.read()) {
-                    cpu.bus.tim[3]._counter +%= 1;
-                    if (cpu.bus.tim[3]._counter == 0) cpu.bus.tim[3].onTimerExpire(cpu, late);
-                },
-                3 => {}, // There is no Timer for TIM3 to "cascade" to,
+                3 => {}, // THere is no timer for TIM3 to cascade to
             }
 
             // Reschedule Timer if we're not cascading
-            if (!self.cnt.cascade.read()) {
+            // TIM0 cascade value is N/A
+            if (id == 0 or !self.cnt.cascade.read()) {
                 self._counter = self._reload;
                 self.rescheduleTimerExpire(late);
             }

src/core/cpu.zig

@@ -1,14 +1,13 @@
 const std = @import("std");
-const util = @import("../util.zig");
 
 const Bus = @import("Bus.zig");
 const Bit = @import("bitfield").Bit;
 const Bitfield = @import("bitfield").Bitfield;
 const Scheduler = @import("scheduler.zig").Scheduler;
-const FilePaths = @import("../util.zig").FilePaths;
 const Logger = @import("../util.zig").Logger;
 
 const File = std.fs.File;
+const log = std.log.scoped(.Arm7Tdmi);
 
 // ARM Instructions
 pub const arm = struct {
@@ -40,13 +39,12 @@ pub const arm = struct {
     }
 
     fn populate() [0x1000]InstrFn {
-        return comptime {
+        comptime {
             @setEvalBranchQuota(0xE000);
-            var ret = [_]InstrFn{und} ** 0x1000;
+            var table = [_]InstrFn{und} ** 0x1000;
 
-            var i: usize = 0;
-            while (i < ret.len) : (i += 1) {
-                ret[i] = switch (@as(u2, i >> 10)) {
+            for (&table, 0..) |*handler, i| {
+                handler.* = switch (@as(u2, i >> 10)) {
                     0b00 => if (i == 0x121) blk: {
                         break :blk branchExchange;
                     } else if (i & 0xFCF == 0x009) blk: {
@@ -108,8 +106,8 @@ pub const arm = struct {
                 };
             }
 
-            return ret;
-        };
+            return table;
+        }
     }
 };
 
@@ -137,13 +135,12 @@ pub const thumb = struct {
     }
 
     fn populate() [0x400]InstrFn {
-        return comptime {
+        comptime {
             @setEvalBranchQuota(5025); // This is exact
-            var ret = [_]InstrFn{und} ** 0x400;
+            var table = [_]InstrFn{und} ** 0x400;
 
-            var i: usize = 0;
-            while (i < ret.len) : (i += 1) {
-                ret[i] = switch (@as(u3, i >> 7 & 0x7)) {
+            for (&table, 0..) |*handler, i| {
+                handler.* = switch (@as(u3, i >> 7 & 0x7)) {
                     0b000 => if (i >> 5 & 0x3 == 0b11) blk: {
                         const I = i >> 4 & 1 == 1;
                         const is_sub = i >> 3 & 1 == 1;
@@ -231,13 +228,11 @@ pub const thumb = struct {
                 };
             }
 
-            return ret;
-        };
+            return table;
+        }
     }
 };
 
-const log = std.log.scoped(.Arm7Tdmi);
-
 pub const Arm7tdmi = struct {
     const Self = @This();
 
@@ -248,18 +243,64 @@ pub const Arm7tdmi = struct {
     cpsr: PSR,
     spsr: PSR,
 
-    /// Storage  for R8_fiq -> R12_fiq and their normal counterparts
-    /// e.g [r[0 + 8], fiq_r[0 + 8], r[1 + 8], fiq_r[1 + 8]...]
-    banked_fiq: [2 * 5]u32,
-
-    /// Storage for r13_<mode>, r14_<mode>
-    /// e.g. [r13, r14, r13_svc, r14_svc]
-    banked_r: [2 * 6]u32,
-
-    banked_spsr: [5]PSR,
+    bank: Bank,
 
     logger: ?Logger,
 
+    /// Bank of Registers from other CPU Modes
+    const Bank = struct {
+        /// Storage for r13_<mode>, r14_<mode>
+        /// e.g. [r13, r14, r13_svc, r14_svc]
+        r: [2 * 6]u32,
+
+        /// Storage  for R8_fiq -> R12_fiq and their normal counterparts
+        /// e.g [r[0 + 8], fiq_r[0 + 8], r[1 + 8], fiq_r[1 + 8]...]
+        fiq: [2 * 5]u32,
+
+        spsr: [5]PSR,
+
+        const Kind = enum(u1) {
+            R13 = 0,
+            R14,
+        };
+
+        pub fn create() Bank {
+            return .{
+                .r = [_]u32{0x00} ** 12,
+                .fiq = [_]u32{0x00} ** 10,
+                .spsr = [_]PSR{.{ .raw = 0x0000_0000 }} ** 5,
+            };
+        }
+
+        inline fn regIdx(mode: Mode, kind: Kind) usize {
+            const idx: usize = switch (mode) {
+                .User, .System => 0,
+                .Supervisor => 1,
+                .Abort => 2,
+                .Undefined => 3,
+                .Irq => 4,
+                .Fiq => 5,
+            };
+
+            return (idx * 2) + if (kind == .R14) @as(usize, 1) else 0;
+        }
+
+        inline fn spsrIdx(mode: Mode) usize {
+            return switch (mode) {
+                .Supervisor => 0,
+                .Abort => 1,
+                .Undefined => 2,
+                .Irq => 3,
+                .Fiq => 4,
+                else => std.debug.panic("[CPU/Mode] {} does not have a SPSR Register", .{mode}),
+            };
+        }
+
+        inline fn fiqIdx(i: usize, mode: Mode) usize {
+            return (i * 2) + if (mode == .Fiq) @as(usize, 1) else 0;
+        }
+    };
+
     pub fn init(sched: *Scheduler, bus: *Bus, log_file: ?std.fs.File) Self {
         return Self{
             .r = [_]u32{0x00} ** 16,
@@ -268,39 +309,17 @@ pub const Arm7tdmi = struct {
             .bus = bus,
             .cpsr = .{ .raw = 0x0000_001F },
             .spsr = .{ .raw = 0x0000_0000 },
-            .banked_fiq = [_]u32{0x00} ** 10,
-            .banked_r = [_]u32{0x00} ** 12,
-            .banked_spsr = [_]PSR{.{ .raw = 0x0000_0000 }} ** 5,
+            .bank = Bank.create(),
             .logger = if (log_file) |file| Logger.init(file) else null,
         };
     }
 
-    inline fn bankedIdx(mode: Mode, kind: BankedKind) usize {
-        const idx: usize = switch (mode) {
-            .User, .System => 0,
-            .Supervisor => 1,
-            .Abort => 2,
-            .Undefined => 3,
-            .Irq => 4,
-            .Fiq => 5,
-        };
+    // FIXME: Resetting disables logging (if enabled)
+    pub fn reset(self: *Self) void {
+        const bus_ptr = self.bus;
+        const scheduler_ptr = self.sched;
 
-        return (idx * 2) + if (kind == .R14) @as(usize, 1) else 0;
-    }
-
-    inline fn bankedSpsrIndex(mode: Mode) usize {
-        return switch (mode) {
-            .Supervisor => 0,
-            .Abort => 1,
-            .Undefined => 2,
-            .Irq => 3,
-            .Fiq => 4,
-            else => std.debug.panic("[CPU/Mode] {} does not have a SPSR Register", .{mode}),
-        };
-    }
-
-    inline fn bankedFiqIdx(i: usize, mode: Mode) usize {
-        return (i * 2) + if (mode == .Fiq) @as(usize, 1) else 0;
+        self.* = Self.init(scheduler_ptr, bus_ptr, null);
     }
 
     pub inline fn hasSPSR(self: *const Self) bool {
@@ -338,14 +357,14 @@ pub const Arm7tdmi = struct {
         switch (idx) {
             8...12 => {
                 if (current == .Fiq) {
-                    self.banked_fiq[bankedFiqIdx(idx - 8, .User)] = value;
+                    self.bank.fiq[Bank.fiqIdx(idx - 8, .User)] = value;
                 } else self.r[idx] = value;
             },
             13, 14 => switch (current) {
                 .User, .System => self.r[idx] = value,
                 else => {
-                    const kind = std.meta.intToEnum(BankedKind, idx - 13) catch unreachable;
-                    self.banked_r[bankedIdx(.User, kind)] = value;
+                    const kind = std.meta.intToEnum(Bank.Kind, idx - 13) catch unreachable;
+                    self.bank.r[Bank.regIdx(.User, kind)] = value;
                 },
             },
             else => self.r[idx] = value, // R0 -> R7  and R15
@@ -356,12 +375,12 @@ pub const Arm7tdmi = struct {
         const current = getModeChecked(self, self.cpsr.mode.read());
 
         return switch (idx) {
-            8...12 => if (current == .Fiq) self.banked_fiq[bankedFiqIdx(idx - 8, .User)] else self.r[idx],
+            8...12 => if (current == .Fiq) self.bank.fiq[Bank.fiqIdx(idx - 8, .User)] else self.r[idx],
             13, 14 => switch (current) {
                 .User, .System => self.r[idx],
                 else => blk: {
-                    const kind = std.meta.intToEnum(BankedKind, idx - 13) catch unreachable;
-                    break :blk self.banked_r[bankedIdx(.User, kind)];
+                    const kind = std.meta.intToEnum(Bank.Kind, idx - 13) catch unreachable;
+                    break :blk self.bank.r[Bank.regIdx(.User, kind)];
                 },
             },
             else => self.r[idx], // R0 -> R7  and R15
@@ -372,40 +391,38 @@ pub const Arm7tdmi = struct {
         const now = getModeChecked(self, self.cpsr.mode.read());
 
         // Bank R8 -> r12
-        var i: usize = 0;
-        while (i < 5) : (i += 1) {
-            self.banked_fiq[bankedFiqIdx(i, now)] = self.r[8 + i];
+        for (0..5) |i| {
+            self.bank.fiq[Bank.fiqIdx(i, now)] = self.r[8 + i];
         }
 
         // Bank r13, r14, SPSR
         switch (now) {
             .User, .System => {
-                self.banked_r[bankedIdx(now, .R13)] = self.r[13];
-                self.banked_r[bankedIdx(now, .R14)] = self.r[14];
+                self.bank.r[Bank.regIdx(now, .R13)] = self.r[13];
+                self.bank.r[Bank.regIdx(now, .R14)] = self.r[14];
             },
             else => {
-                self.banked_r[bankedIdx(now, .R13)] = self.r[13];
-                self.banked_r[bankedIdx(now, .R14)] = self.r[14];
-                self.banked_spsr[bankedSpsrIndex(now)] = self.spsr;
+                self.bank.r[Bank.regIdx(now, .R13)] = self.r[13];
+                self.bank.r[Bank.regIdx(now, .R14)] = self.r[14];
+                self.bank.spsr[Bank.spsrIdx(now)] = self.spsr;
             },
         }
 
         // Grab R8 -> R12
-        i = 0;
-        while (i < 5) : (i += 1) {
-            self.r[8 + i] = self.banked_fiq[bankedFiqIdx(i, next)];
+        for (0..5) |i| {
+            self.r[8 + i] = self.bank.fiq[Bank.fiqIdx(i, next)];
         }
 
         // Grab r13, r14, SPSR
         switch (next) {
             .User, .System => {
-                self.r[13] = self.banked_r[bankedIdx(next, .R13)];
-                self.r[14] = self.banked_r[bankedIdx(next, .R14)];
+                self.r[13] = self.bank.r[Bank.regIdx(next, .R13)];
+                self.r[14] = self.bank.r[Bank.regIdx(next, .R14)];
             },
             else => {
-                self.r[13] = self.banked_r[bankedIdx(next, .R13)];
-                self.r[14] = self.banked_r[bankedIdx(next, .R14)];
-                self.spsr = self.banked_spsr[bankedSpsrIndex(next)];
+                self.r[13] = self.bank.r[Bank.regIdx(next, .R13)];
+                self.r[14] = self.bank.r[Bank.regIdx(next, .R14)];
+                self.spsr = self.bank.spsr[Bank.spsrIdx(next)];
             },
         }
 
@@ -426,8 +443,8 @@ pub const Arm7tdmi = struct {
         self.r[13] = 0x0300_7F00;
         self.r[15] = 0x0800_0000;
 
-        self.banked_r[bankedIdx(.Irq, .R13)] = 0x0300_7FA0;
-        self.banked_r[bankedIdx(.Supervisor, .R13)] = 0x0300_7FE0;
+        self.bank.r[Bank.regIdx(.Irq, .R13)] = 0x0300_7FA0;
+        self.bank.r[Bank.regIdx(.Supervisor, .R13)] = 0x0300_7FE0;
 
         // self.cpsr.raw = 0x6000001F;
         self.cpsr.raw = 0x0000_001F;
@@ -457,29 +474,11 @@ pub const Arm7tdmi = struct {
     }
 
     pub fn stepDmaTransfer(self: *Self) bool {
-        const dma0 = &self.bus.dma[0];
-        const dma1 = &self.bus.dma[1];
-        const dma2 = &self.bus.dma[2];
-        const dma3 = &self.bus.dma[3];
-
-        if (dma0.in_progress) {
-            dma0.step(self);
-            return true;
-        }
-
-        if (dma1.in_progress) {
-            dma1.step(self);
-            return true;
-        }
-
-        if (dma2.in_progress) {
-            dma2.step(self);
-            return true;
-        }
-
-        if (dma3.in_progress) {
-            dma3.step(self);
-            return true;
+        inline for (0..4) |i| {
+            if (self.bus.dma[i].in_progress) {
+                self.bus.dma[i].step(self);
+                return true;
+            }
         }
 
         return false;
@@ -534,10 +533,10 @@ pub const Arm7tdmi = struct {
             std.debug.print("R{}: 0x{X:0>8}\tR{}: 0x{X:0>8}\tR{}: 0x{X:0>8}\tR{}: 0x{X:0>8}\n", .{ i, self.r[i], i_1, self.r[i_1], i_2, self.r[i_2], i_3, self.r[i_3] });
         }
         std.debug.print("cpsr: 0x{X:0>8} ", .{self.cpsr.raw});
-        prettyPrintPsr(&self.cpsr);
+        self.cpsr.toString();
 
         std.debug.print("spsr: 0x{X:0>8} ", .{self.spsr.raw});
-        prettyPrintPsr(&self.spsr);
+        self.spsr.toString();
 
         std.debug.print("pipeline: {??X:0>8}\n", .{self.pipe.stage});
 
@@ -555,97 +554,31 @@ pub const Arm7tdmi = struct {
 
         std.debug.panic(format, args);
     }
-
-    fn prettyPrintPsr(psr: *const PSR) void {
-        std.debug.print("[", .{});
-
-        if (psr.n.read()) std.debug.print("N", .{}) else std.debug.print("-", .{});
-        if (psr.z.read()) std.debug.print("Z", .{}) else std.debug.print("-", .{});
-        if (psr.c.read()) std.debug.print("C", .{}) else std.debug.print("-", .{});
-        if (psr.v.read()) std.debug.print("V", .{}) else std.debug.print("-", .{});
-        if (psr.i.read()) std.debug.print("I", .{}) else std.debug.print("-", .{});
-        if (psr.f.read()) std.debug.print("F", .{}) else std.debug.print("-", .{});
-        if (psr.t.read()) std.debug.print("T", .{}) else std.debug.print("-", .{});
-        std.debug.print("|", .{});
-        if (getMode(psr.mode.read())) |mode| std.debug.print("{s}", .{modeString(mode)}) else std.debug.print("---", .{});
-
-        std.debug.print("]\n", .{});
-    }
-
-    fn modeString(mode: Mode) []const u8 {
-        return switch (mode) {
-            .User => "usr",
-            .Fiq => "fiq",
-            .Irq => "irq",
-            .Supervisor => "svc",
-            .Abort => "abt",
-            .Undefined => "und",
-            .System => "sys",
-        };
-    }
-
-    fn mgbaLog(self: *const Self, file: *const File, opcode: u32) !void {
-        const thumb_fmt = "{X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} cpsr: {X:0>8} | {X:0>4}:\n";
-        const arm_fmt = "{X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} {X:0>8} cpsr: {X:0>8} | {X:0>8}:\n";
-        var buf: [0x100]u8 = [_]u8{0x00} ** 0x100; // this is larger than it needs to be
-
-        const r0 = self.r[0];
-        const r1 = self.r[1];
-        const r2 = self.r[2];
-        const r3 = self.r[3];
-        const r4 = self.r[4];
-        const r5 = self.r[5];
-        const r6 = self.r[6];
-        const r7 = self.r[7];
-        const r8 = self.r[8];
-        const r9 = self.r[9];
-        const r10 = self.r[10];
-        const r11 = self.r[11];
-        const r12 = self.r[12];
-        const r13 = self.r[13];
-        const r14 = self.r[14];
-        const r15 = self.r[15] -| if (self.cpsr.t.read()) 2 else @as(u32, 4);
-
-        const c_psr = self.cpsr.raw;
-
-        var log_str: []u8 = undefined;
-        if (self.cpsr.t.read()) {
-            if (opcode >> 11 == 0x1E) {
-                // Instruction 1 of a BL Opcode, print in ARM mode
-                const other_half = self.bus.debugRead(u16, self.r[15] - 2);
-                const bl_opcode = @as(u32, opcode) << 16 | other_half;
-
-                log_str = try std.fmt.bufPrint(&buf, arm_fmt, .{ r0, r1, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, r13, r14, r15, c_psr, bl_opcode });
-            } else {
-                log_str = try std.fmt.bufPrint(&buf, thumb_fmt, .{ r0, r1, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, r13, r14, r15, c_psr, opcode });
-            }
-        } else {
-            log_str = try std.fmt.bufPrint(&buf, arm_fmt, .{ r0, r1, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, r13, r14, r15, c_psr, opcode });
-        }
-
-        _ = try file.writeAll(log_str);
-    }
 };
 
-pub fn checkCond(cpsr: PSR, cond: u4) bool {
-    return switch (cond) {
-        0x0 => cpsr.z.read(), // EQ - Equal
-        0x1 => !cpsr.z.read(), // NE - Not equal
-        0x2 => cpsr.c.read(), // CS - Unsigned higher or same
-        0x3 => !cpsr.c.read(), // CC - Unsigned lower
-        0x4 => cpsr.n.read(), // MI - Negative
-        0x5 => !cpsr.n.read(), // PL - Positive or zero
-        0x6 => cpsr.v.read(), // VS - Overflow
-        0x7 => !cpsr.v.read(), // VC - No overflow
-        0x8 => cpsr.c.read() and !cpsr.z.read(), // HI - unsigned higher
-        0x9 => !cpsr.c.read() or cpsr.z.read(), // LS - unsigned lower or same
-        0xA => cpsr.n.read() == cpsr.v.read(), // GE - Greater or equal
-        0xB => cpsr.n.read() != cpsr.v.read(), // LT - Less than
-        0xC => !cpsr.z.read() and (cpsr.n.read() == cpsr.v.read()), // GT - Greater than
-        0xD => cpsr.z.read() or (cpsr.n.read() != cpsr.v.read()), // LE - Less than or equal
-        0xE => true, // AL - Always
-        0xF => false, // NV - Never (reserved in ARMv3 and up, but seems to have not changed?)
-    };
+const condition_lut = [_]u16{
+    0xF0F0, // EQ - Equal
+    0x0F0F, // NE - Not Equal
+    0xCCCC, // CS - Unsigned higher or same
+    0x3333, // CC - Unsigned lower
+    0xFF00, // MI - Negative
+    0x00FF, // PL - Positive or Zero
+    0xAAAA, // VS - Overflow
+    0x5555, // VC - No Overflow
+    0x0C0C, // HI - unsigned hierh
+    0xF3F3, // LS - unsigned lower or same
+    0xAA55, // GE - greater or equal
+    0x55AA, // LT - less than
+    0x0A05, // GT - greater than
+    0xF5FA, // LE - less than or equal
+    0xFFFF, // AL - always
+    0x0000, // NV - never
+};
+
+pub inline fn checkCond(cpsr: PSR, cond: u4) bool {
+    const flags = @truncate(u4, cpsr.raw >> 28);
+
+    return condition_lut[cond] & (@as(u16, 1) << flags) != 0;
 }
 
 const Pipeline = struct {
@@ -667,9 +600,7 @@ const Pipeline = struct {
     pub fn step(self: *Self, cpu: *Arm7tdmi, comptime T: type) ?u32 {
         comptime std.debug.assert(T == u32 or T == u16);
 
-        // FIXME: https://github.com/ziglang/zig/issues/12642
-        var opcode = self.stage[0];
-
+        const opcode = self.stage[0];
         self.stage[0] = self.stage[1];
         self.stage[1] = cpu.fetch(T, cpu.r[15]);
 
@@ -701,9 +632,25 @@ pub const PSR = extern union {
     z: Bit(u32, 30),
     n: Bit(u32, 31),
     raw: u32,
+
+    fn toString(self: PSR) void {
+        std.debug.print("[", .{});
+
+        if (self.n.read()) std.debug.print("N", .{}) else std.debug.print("-", .{});
+        if (self.z.read()) std.debug.print("Z", .{}) else std.debug.print("-", .{});
+        if (self.c.read()) std.debug.print("C", .{}) else std.debug.print("-", .{});
+        if (self.v.read()) std.debug.print("V", .{}) else std.debug.print("-", .{});
+        if (self.i.read()) std.debug.print("I", .{}) else std.debug.print("-", .{});
+        if (self.f.read()) std.debug.print("F", .{}) else std.debug.print("-", .{});
+        if (self.t.read()) std.debug.print("T", .{}) else std.debug.print("-", .{});
+        std.debug.print("|", .{});
+        if (getMode(self.mode.read())) |m| std.debug.print("{s}", .{m.toString()}) else std.debug.print("---", .{});
+
+        std.debug.print("]\n", .{});
+    }
 };
 
-const Mode = enum(u5) {
+pub const Mode = enum(u5) {
     User = 0b10000,
     Fiq = 0b10001,
     Irq = 0b10010,
@@ -711,11 +658,18 @@ const Mode = enum(u5) {
     Abort = 0b10111,
     Undefined = 0b11011,
     System = 0b11111,
-};
 
-const BankedKind = enum(u1) {
-    R13 = 0,
-    R14,
+    pub fn toString(self: Mode) []const u8 {
+        return switch (self) {
+            .User => "usr",
+            .Fiq => "fiq",
+            .Irq => "irq",
+            .Supervisor => "svc",
+            .Abort => "abt",
+            .Undefined => "und",
+            .System => "sys",
+        };
+    }
 };
 
 fn getMode(bits: u5) ?Mode {

src/core/cpu/arm/block_data_transfer.zig

@@ -57,7 +57,6 @@ pub fn blockDataTransfer(comptime P: bool, comptime U: bool, comptime S: bool, c
                     cpu.r[15] = bus.read(u32, und_addr);
                     cpu.pipe.reload(cpu);
                 } else {
-                    // FIXME: Should r15 on write be +12 ahead?
                     bus.write(u32, und_addr, cpu.r[15] + 4);
                 }
 

src/core/cpu/arm/branch.zig

@@ -1,10 +1,8 @@
-const std = @import("std");
-
 const Bus = @import("../../Bus.zig");
 const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
 const InstrFn = @import("../../cpu.zig").arm.InstrFn;
 
-const sext = @import("../../../util.zig").sext;
+const sext = @import("zba-util").sext;
 
 pub fn branch(comptime L: bool) InstrFn {
     return struct {

src/core/cpu/arm/data_processing.zig

@@ -24,7 +24,7 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime kind: u4) Ins
             if (!I and opcode >> 4 & 1 == 1) cpu.r[15] -= 4;
 
             var result: u32 = undefined;
-            var overflow: bool = undefined;
+            var overflow: u1 = undefined;
 
             // Perform Data Processing Logic
             switch (kind) {
@@ -62,7 +62,9 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime kind: u4) Ins
                     if (rd == 0xF)
                         return undefinedTestBehaviour(cpu);
 
-                    overflow = @addWithOverflow(u32, op1, op2, &result);
+                    const tmp = @addWithOverflow(op1, op2);
+                    result = tmp[0];
+                    overflow = tmp[1];
                 },
                 0xC => result = op1 | op2, // ORR
                 0xD => result = op2, // MOV
@@ -110,7 +112,7 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime kind: u4) Ins
                     // ADD, ADC Flags
                     cpu.cpsr.n.write(result >> 31 & 1 == 1);
                     cpu.cpsr.z.write(result == 0);
-                    cpu.cpsr.c.write(overflow);
+                    cpu.cpsr.c.write(overflow == 0b1);
                     cpu.cpsr.v.write(((op1 ^ result) & (op2 ^ result)) >> 31 & 1 == 1);
                 },
                 0x6, 0x7 => if (S and rd != 0xF) {
@@ -141,7 +143,7 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime kind: u4) Ins
                         cpu.cpsr.v.write(((op1 ^ result) & (~op2 ^ result)) >> 31 & 1 == 1);
                     } else if (kind == 0xB) {
                         // CMN specific
-                        cpu.cpsr.c.write(overflow);
+                        cpu.cpsr.c.write(overflow == 0b1);
                         cpu.cpsr.v.write(((op1 ^ result) & (op2 ^ result)) >> 31 & 1 == 1);
                     } else {
                         // TST, TEQ specific
@@ -162,19 +164,19 @@ pub fn sbc(left: u32, right: u32, old_carry: u1) u32 {
     return ret;
 }
 
-pub fn add(overflow: *bool, left: u32, right: u32) u32 {
-    var ret: u32 = undefined;
-    overflow.* = @addWithOverflow(u32, left, right, &ret);
-    return ret;
+pub fn add(overflow: *u1, left: u32, right: u32) u32 {
+    const ret = @addWithOverflow(left, right);
+    overflow.* = ret[1];
+
+    return ret[0];
 }
 
-pub fn adc(overflow: *bool, left: u32, right: u32, old_carry: u1) u32 {
-    var ret: u32 = undefined;
-    const first = @addWithOverflow(u32, left, right, &ret);
-    const second = @addWithOverflow(u32, ret, old_carry, &ret);
+pub fn adc(overflow: *u1, left: u32, right: u32, old_carry: u1) u32 {
+    const tmp = @addWithOverflow(left, right);
+    const ret = @addWithOverflow(tmp[0], old_carry);
+    overflow.* = tmp[1] | ret[1];
 
-    overflow.* = first or second;
-    return ret;
+    return ret[0];
 }
 
 fn undefinedTestBehaviour(cpu: *Arm7tdmi) void {

src/core/cpu/arm/half_signed_data_transfer.zig

@@ -1,11 +1,9 @@
-const std = @import("std");
-
 const Bus = @import("../../Bus.zig");
 const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
 const InstrFn = @import("../../cpu.zig").arm.InstrFn;
 
-const sext = @import("../../../util.zig").sext;
-const rotr = @import("../../../util.zig").rotr;
+const sext = @import("zba-util").sext;
+const rotr = @import("zba-util").rotr;
 
 pub fn halfAndSignedDataTransfer(comptime P: bool, comptime U: bool, comptime I: bool, comptime W: bool, comptime L: bool) InstrFn {
     return struct {
@@ -35,11 +33,8 @@ pub fn halfAndSignedDataTransfer(comptime P: bool, comptime U: bool, comptime I:
                     },
                     0b11 => {
                         // LDRSH
-                        result = if (address & 1 == 1) blk: {
-                            break :blk sext(u32, u8, bus.read(u8, address));
-                        } else blk: {
-                            break :blk sext(u32, u16, bus.read(u16, address));
-                        };
+                        const value = bus.read(u16, address);
+                        result = if (address & 1 == 1) sext(u32, u8, @truncate(u8, value >> 8)) else sext(u32, u16, value);
                     },
                     0b00 => unreachable, // SWP
                 }

src/core/cpu/arm/psr_transfer.zig

@@ -7,7 +7,7 @@ const PSR = @import("../../cpu.zig").PSR;
 
 const log = std.log.scoped(.PsrTransfer);
 
-const rotr = @import("../../../util.zig").rotr;
+const rotr = @import("zba-util").rotr;
 
 pub fn psrTransfer(comptime I: bool, comptime R: bool, comptime kind: u2) InstrFn {
     return struct {

src/core/cpu/arm/single_data_swap.zig

@@ -1,10 +1,8 @@
-const std = @import("std");
-
 const Bus = @import("../../Bus.zig");
 const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
 const InstrFn = @import("../../cpu.zig").arm.InstrFn;
 
-const rotr = @import("../../../util.zig").rotr;
+const rotr = @import("zba-util").rotr;
 
 pub fn singleDataSwap(comptime B: bool) InstrFn {
     return struct {

src/core/cpu/arm/single_data_transfer.zig

@@ -1,12 +1,9 @@
-const std = @import("std");
-const util = @import("../../../util.zig");
-
 const shifter = @import("../barrel_shifter.zig");
 const Bus = @import("../../Bus.zig");
 const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
 const InstrFn = @import("../../cpu.zig").arm.InstrFn;
 
-const rotr = @import("../../../util.zig").rotr;
+const rotr = @import("zba-util").rotr;
 
 pub fn singleDataTransfer(comptime I: bool, comptime P: bool, comptime U: bool, comptime B: bool, comptime W: bool, comptime L: bool) InstrFn {
     return struct {
@@ -14,9 +11,7 @@ pub fn singleDataTransfer(comptime I: bool, comptime P: bool, comptime U: bool,
             const rn = opcode >> 16 & 0xF;
             const rd = opcode >> 12 & 0xF;
 
-            // rn is r15 and L is not set, the PC is 12 ahead
-            const base = cpu.r[rn] + if (!L and rn == 0xF) 4 else @as(u32, 0);
-
+            const base = cpu.r[rn];
             const offset = if (I) shifter.immediate(false, cpu, opcode) else opcode & 0xFFF;
 
             const modified_base = if (U) base +% offset else base -% offset;

src/core/cpu/barrel_shifter.zig

@@ -1,9 +1,7 @@
-const std = @import("std");
-
 const Arm7tdmi = @import("../cpu.zig").Arm7tdmi;
 const CPSR = @import("../cpu.zig").PSR;
 
-const rotr = @import("../../util.zig").rotr;
+const rotr = @import("zba-util").rotr;
 
 pub fn exec(comptime S: bool, cpu: *Arm7tdmi, opcode: u32) u32 {
     var result: u32 = undefined;

src/core/cpu/thumb/alu.zig

@@ -21,7 +21,8 @@ pub fn fmt4(comptime op: u4) InstrFn {
             const op2 = cpu.r[rs];
 
             var result: u32 = undefined;
-            var overflow: bool = undefined;
+            var overflow: u1 = undefined;
+
             switch (op) {
                 0x0 => result = op1 & op2, // AND
                 0x1 => result = op1 ^ op2, // EOR
@@ -34,7 +35,12 @@ pub fn fmt4(comptime op: u4) InstrFn {
                 0x8 => result = op1 & op2, // TST
                 0x9 => result = 0 -% op2, // NEG
                 0xA => result = op1 -% op2, // CMP
-                0xB => overflow = @addWithOverflow(u32, op1, op2, &result), // CMN
+                0xB => {
+                    // CMN
+                    const tmp = @addWithOverflow(op1, op2);
+                    result = tmp[0];
+                    overflow = tmp[1];
+                },
                 0xC => result = op1 | op2, // ORR
                 0xD => result = @truncate(u32, @as(u64, op2) * @as(u64, op1)),
                 0xE => result = op1 & ~op2,
@@ -71,7 +77,7 @@ pub fn fmt4(comptime op: u4) InstrFn {
                     // ADC, CMN
                     cpu.cpsr.n.write(result >> 31 & 1 == 1);
                     cpu.cpsr.z.write(result == 0);
-                    cpu.cpsr.c.write(overflow);
+                    cpu.cpsr.c.write(overflow == 0b1);
                     cpu.cpsr.v.write(((op1 ^ result) & (op2 ^ result)) >> 31 & 1 == 1);
                 },
                 0x6 => {

src/core/cpu/thumb/block_data_transfer.zig

@@ -92,8 +92,7 @@ pub fn fmt15(comptime L: bool, comptime rb: u3) InstrFn {
 inline fn countRlist(opcode: u16) u32 {
     var count: u32 = 0;
 
-    comptime var i: u4 = 0;
-    inline while (i < 8) : (i += 1) {
+    inline for (0..8) |i| {
         if (opcode >> (7 - i) & 1 == 1) count += 1;
     }
 

src/core/cpu/thumb/branch.zig

@@ -3,7 +3,7 @@ const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
 const InstrFn = @import("../../cpu.zig").thumb.InstrFn;
 
 const checkCond = @import("../../cpu.zig").checkCond;
-const sext = @import("../../../util.zig").sext;
+const sext = @import("zba-util").sext;
 
 pub fn fmt16(comptime cond: u4) InstrFn {
     return struct {

src/core/cpu/thumb/data_processing.zig

@@ -1,5 +1,3 @@
-const std = @import("std");
-
 const Bus = @import("../../Bus.zig");
 const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
 const InstrFn = @import("../../cpu.zig").thumb.InstrFn;
@@ -66,7 +64,7 @@ pub fn fmt5(comptime op: u2, comptime h1: u1, comptime h2: u1) InstrFn {
             const op2 = cpu.r[rs];
 
             var result: u32 = undefined;
-            var overflow: bool = undefined;
+            var overflow: u1 = undefined;
             switch (op) {
                 0b00 => result = add(&overflow, op1, op2), // ADD
                 0b01 => result = op1 -% op2, // CMP
@@ -128,13 +126,13 @@ pub fn fmt2(comptime I: bool, is_sub: bool, rn: u3) InstrFn {
                 cpu.cpsr.v.write(((op1 ^ result) & (~op2 ^ result)) >> 31 & 1 == 1);
             } else {
                 // ADD
-                var overflow: bool = undefined;
+                var overflow: u1 = undefined;
                 const result = add(&overflow, op1, op2);
                 cpu.r[rd] = result;
 
                 cpu.cpsr.n.write(result >> 31 & 1 == 1);
                 cpu.cpsr.z.write(result == 0);
-                cpu.cpsr.c.write(overflow);
+                cpu.cpsr.c.write(overflow == 0b1);
                 cpu.cpsr.v.write(((op1 ^ result) & (op2 ^ result)) >> 31 & 1 == 1);
             }
         }
@@ -147,7 +145,7 @@ pub fn fmt3(comptime op: u2, comptime rd: u3) InstrFn {
             const op1 = cpu.r[rd];
             const op2: u32 = opcode & 0xFF; // Offset
 
-            var overflow: bool = undefined;
+            var overflow: u1 = undefined;
             const result: u32 = switch (op) {
                 0b00 => op2, // MOV
                 0b01 => op1 -% op2, // CMP
@@ -171,7 +169,7 @@ pub fn fmt3(comptime op: u2, comptime rd: u3) InstrFn {
                 },
                 0b10 => {
                     // ADD
-                    cpu.cpsr.c.write(overflow);
+                    cpu.cpsr.c.write(overflow == 0b1);
                     cpu.cpsr.v.write(((op1 ^ result) & (op2 ^ result)) >> 31 & 1 == 1);
                 },
             }

src/core/cpu/thumb/data_transfer.zig

@@ -1,11 +1,9 @@
-const std = @import("std");
-
 const Bus = @import("../../Bus.zig");
 const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
 const InstrFn = @import("../../cpu.zig").thumb.InstrFn;
 
-const rotr = @import("../../../util.zig").rotr;
-const sext = @import("../../../util.zig").sext;
+const rotr = @import("zba-util").rotr;
+const sext = @import("zba-util").sext;
 
 pub fn fmt6(comptime rd: u3) InstrFn {
     return struct {
@@ -46,11 +44,8 @@ pub fn fmt78(comptime op: u2, comptime T: bool) InstrFn {
                     },
                     0b11 => {
                         // LDRSH
-                        cpu.r[rd] = if (address & 1 == 1) blk: {
-                            break :blk sext(u32, u8, bus.read(u8, address));
-                        } else blk: {
-                            break :blk sext(u32, u16, bus.read(u16, address));
-                        };
+                        const value = bus.read(u16, address);
+                        cpu.r[rd] = if (address & 1 == 1) sext(u32, u8, @truncate(u8, value >> 8)) else sext(u32, u16, value);
                     },
                 }
             } else {

src/core/emu.zig

@@ -2,29 +2,29 @@ const std = @import("std");
 const SDL = @import("sdl2");
 const config = @import("../config.zig");
 
-const Bus = @import("Bus.zig");
 const Scheduler = @import("scheduler.zig").Scheduler;
 const Arm7tdmi = @import("cpu.zig").Arm7tdmi;
-const FpsTracker = @import("../util.zig").FpsTracker;
-const FilePaths = @import("../util.zig").FilePaths;
+const Tracker = @import("../util.zig").FpsTracker;
+const TwoWayChannel = @import("zba-util").TwoWayChannel;
 
 const Timer = std.time.Timer;
-const Thread = std.Thread;
-const Atomic = std.atomic.Atomic;
-const Allocator = std.mem.Allocator;
 
-// 228 Lines which consist of 308 dots (which are 4 cycles long)
-const cycles_per_frame: u64 = 228 * (308 * 4); //280896
-const clock_rate: u64 = 1 << 24; // 16.78MHz
+/// 4 Cycles in 1 dot
+const cycles_per_dot = 4;
 
-// TODO: Don't truncate this, be more accurate w/ timing
-// 59.6046447754ns (truncated to just 59ns)
-const clock_period: u64 = std.time.ns_per_s / clock_rate;
-const frame_period = (clock_period * cycles_per_frame);
+/// The GBA draws 228 Horizontal which each consist 308 dots
+/// (note: not all lines are visible)
+const cycles_per_frame = 228 * (308 * cycles_per_dot); //280896
 
-// 59.7275005696Hz
-pub const frame_rate = @intToFloat(f64, std.time.ns_per_s) /
-    ((@intToFloat(f64, std.time.ns_per_s) / @intToFloat(f64, clock_rate)) * @intToFloat(f64, cycles_per_frame));
+/// The GBA ARM7TDMI runs at 2^24 Hz
+const clock_rate = 1 << 24; // 16.78MHz
+
+/// The # of nanoseconds a frame should take
+const frame_period = (std.time.ns_per_s * cycles_per_frame) / clock_rate;
+
+/// Exact Value:  59.7275005696Hz
+/// The inverse of the frame period
+pub const frame_rate: f64 = @intToFloat(f64, clock_rate) / cycles_per_frame;
 
 const log = std.log.scoped(.Emulation);
 
@@ -35,28 +35,41 @@ const RunKind = enum {
     LimitedFPS,
 };
 
-pub fn run(quit: *Atomic(bool), scheduler: *Scheduler, cpu: *Arm7tdmi, tracker: *FpsTracker) void {
-    const audio_sync = config.config().guest.audio_sync;
+pub fn run(cpu: *Arm7tdmi, scheduler: *Scheduler, tracker: *Tracker, channel: *TwoWayChannel) void {
+    const audio_sync = config.config().guest.audio_sync and !config.config().host.mute;
     if (audio_sync) log.info("Audio sync enabled", .{});
 
     if (config.config().guest.video_sync) {
-        inner(.LimitedFPS, audio_sync, quit, scheduler, cpu, tracker);
+        inner(.LimitedFPS, audio_sync, cpu, scheduler, tracker, channel);
     } else {
-        inner(.UnlimitedFPS, audio_sync, quit, scheduler, cpu, tracker);
+        inner(.UnlimitedFPS, audio_sync, cpu, scheduler, tracker, channel);
     }
 }
 
-fn inner(comptime kind: RunKind, audio_sync: bool, quit: *Atomic(bool), scheduler: *Scheduler, cpu: *Arm7tdmi, tracker: ?*FpsTracker) void {
+fn inner(comptime kind: RunKind, audio_sync: bool, cpu: *Arm7tdmi, scheduler: *Scheduler, tracker: ?*Tracker, channel: *TwoWayChannel) void {
     if (kind == .UnlimitedFPS or kind == .LimitedFPS) {
         std.debug.assert(tracker != null);
         log.info("FPS tracking enabled", .{});
     }
 
+    var paused: bool = false;
+
     switch (kind) {
         .Unlimited, .UnlimitedFPS => {
             log.info("Emulation w/out video sync", .{});
 
-            while (!quit.load(.SeqCst)) {
+            while (true) {
+                if (channel.emu.pop()) |e| switch (e) {
+                    .Quit => break,
+                    .Resume => paused = false,
+                    .Pause => {
+                        paused = true;
+                        channel.gui.push(.Paused);
+                    },
+                };
+
+                if (paused) continue;
+
                 runFrame(scheduler, cpu);
                 audioSync(audio_sync, cpu.bus.apu.stream, &cpu.bus.apu.is_buffer_full);
 
@@ -68,7 +81,18 @@ fn inner(comptime kind: RunKind, audio_sync: bool, quit: *Atomic(bool), schedule
             var timer = Timer.start() catch @panic("failed to initalize std.timer.Timer");
             var wake_time: u64 = frame_period;
 
-            while (!quit.load(.SeqCst)) {
+            while (true) {
+                if (channel.emu.pop()) |e| switch (e) {
+                    .Quit => break,
+                    .Resume => paused = false,
+                    .Pause => {
+                        paused = true;
+                        channel.gui.push(.Paused);
+                    },
+                };
+
+                if (paused) continue;
+
                 runFrame(scheduler, cpu);
                 const new_wake_time = videoSync(&timer, wake_time);
 
@@ -94,7 +118,7 @@ pub fn runFrame(sched: *Scheduler, cpu: *Arm7tdmi) void {
         if (!cpu.stepDmaTransfer()) {
             if (cpu.isHalted()) {
                 // Fast-forward to next Event
-                sched.tick = sched.queue.peek().?.tick;
+                sched.tick = sched.nextTimestamp();
             } else {
                 cpu.step();
             }
@@ -105,6 +129,7 @@ pub fn runFrame(sched: *Scheduler, cpu: *Arm7tdmi) void {
 }
 
 fn audioSync(audio_sync: bool, stream: *SDL.SDL_AudioStream, is_buffer_full: *bool) void {
+    comptime std.debug.assert(@import("../platform.zig").sample_format == SDL.AUDIO_U16);
     const sample_size = 2 * @sizeOf(u16);
     const max_buf_size: c_int = 0x400;
 
@@ -132,11 +157,10 @@ fn videoSync(timer: *Timer, wake_time: u64) u64 {
 
 // TODO: Better sleep impl?
 fn sleep(timer: *Timer, wake_time: u64) ?u64 {
-    // const step = std.time.ns_per_ms * 10; // 10ms
     const timestamp = timer.read();
 
     // ns_late is non zero if we are late.
-    const ns_late = timestamp -| wake_time;
+    var ns_late = timestamp -| wake_time;
 
     // If we're more than a frame late, skip the rest of this loop
     // Recalculate what our new wake time should be so that we can
@@ -144,15 +168,17 @@ fn sleep(timer: *Timer, wake_time: u64) ?u64 {
     if (ns_late > frame_period) return timestamp + frame_period;
     const sleep_for = frame_period - ns_late;
 
-    // // Employ several sleep calls in periods of 10ms
-    // // By doing this the behaviour should average out to be
-    // // more consistent
-    // const loop_count = sleep_for / step; // How many groups of 10ms
+    const step = 2 * std.time.ns_per_ms; // Granularity of 2ms
+    const times = sleep_for / step;
 
-    // var i: usize = 0;
-    // while (i < loop_count) : (i += 1) std.time.sleep(step);
+    for (0..times) |_| {
+        std.time.sleep(step);
 
-    std.time.sleep(sleep_for);
+        // Upon wakeup, check to see if this particular sleep was longer than expected
+        // if so we should exit early, but probably not skip a whole frame period
+        ns_late = timer.read() -| wake_time;
+        if (ns_late > frame_period) return null;
+    }
 
     return null;
 }
@@ -160,3 +186,71 @@ fn sleep(timer: *Timer, wake_time: u64) ?u64 {
 fn spinLoop(timer: *Timer, wake_time: u64) void {
     while (true) if (timer.read() > wake_time) break;
 }
+
+pub const EmuThing = struct {
+    const Self = @This();
+    const Interface = @import("gdbstub").Emulator;
+    const Allocator = std.mem.Allocator;
+
+    cpu: *Arm7tdmi,
+    scheduler: *Scheduler,
+
+    pub fn init(cpu: *Arm7tdmi, scheduler: *Scheduler) Self {
+        return .{ .cpu = cpu, .scheduler = scheduler };
+    }
+
+    pub fn interface(self: *Self, allocator: Allocator) Interface {
+        return Interface.init(allocator, self);
+    }
+
+    pub fn read(self: *const Self, addr: u32) u8 {
+        return self.cpu.bus.dbgRead(u8, addr);
+    }
+
+    pub fn write(self: *Self, addr: u32, value: u8) void {
+        self.cpu.bus.dbgWrite(u8, addr, value);
+    }
+
+    pub fn registers(self: *const Self) *[16]u32 {
+        return &self.cpu.r;
+    }
+
+    pub fn cpsr(self: *const Self) u32 {
+        return self.cpu.cpsr.raw;
+    }
+
+    pub fn step(self: *Self) void {
+        const cpu = self.cpu;
+        const sched = self.scheduler;
+
+        // Is true when we have executed one (1) instruction
+        var did_step: bool = false;
+
+        // TODO: How can I make it easier to keep this in lock-step with runFrame?
+        while (!did_step) {
+            if (!cpu.stepDmaTransfer()) {
+                if (cpu.isHalted()) {
+                    // Fast-forward to next Event
+                    sched.tick = sched.queue.peek().?.tick;
+                } else {
+                    cpu.step();
+                    did_step = true;
+                }
+            }
+
+            if (sched.tick >= sched.nextTimestamp()) sched.handleEvent(cpu);
+        }
+    }
+};
+
+pub fn reset(cpu: *Arm7tdmi) void {
+    // @breakpoint();
+    cpu.sched.reset(); // Yes this is order sensitive, see the PPU reset for why
+    cpu.bus.reset();
+    cpu.reset();
+}
+
+pub fn replaceGamepak(cpu: *Arm7tdmi, file_path: []const u8) !void {
+    try cpu.bus.replaceGamepak(file_path);
+    reset(cpu);
+}

src/core/ppu/Oam.zig

@@ -0,0 +1,44 @@
+const std = @import("std");
+
+const Allocator = std.mem.Allocator;
+
+const buf_len = 0x400;
+const Self = @This();
+
+buf: []u8,
+allocator: Allocator,
+
+pub fn read(self: *const Self, comptime T: type, address: usize) T {
+    const addr = address & 0x3FF;
+
+    return switch (T) {
+        u32, u16, u8 => std.mem.readIntSliceLittle(T, self.buf[addr..][0..@sizeOf(T)]),
+        else => @compileError("OAM: Unsupported read width"),
+    };
+}
+
+pub fn write(self: *Self, comptime T: type, address: usize, value: T) void {
+    const addr = address & 0x3FF;
+
+    switch (T) {
+        u32, u16 => std.mem.writeIntSliceLittle(T, self.buf[addr..][0..@sizeOf(T)], value),
+        u8 => return, // 8-bit writes are explicitly ignored
+        else => @compileError("OAM: Unsupported write width"),
+    }
+}
+
+pub fn init(allocator: Allocator) !Self {
+    const buf = try allocator.alloc(u8, buf_len);
+    std.mem.set(u8, buf, 0);
+
+    return Self{ .buf = buf, .allocator = allocator };
+}
+
+pub fn reset(self: *Self) void {
+    std.mem.set(u8, self.buf, 0);
+}
+
+pub fn deinit(self: *Self) void {
+    self.allocator.free(self.buf);
+    self.* = undefined;
+}

src/core/ppu/Palette.zig

@@ -0,0 +1,51 @@
+const std = @import("std");
+
+const Allocator = std.mem.Allocator;
+
+const buf_len = 0x400;
+const Self = @This();
+
+buf: []u8,
+allocator: Allocator,
+
+pub fn read(self: *const Self, comptime T: type, address: usize) T {
+    const addr = address & 0x3FF;
+
+    return switch (T) {
+        u32, u16, u8 => std.mem.readIntSliceLittle(T, self.buf[addr..][0..@sizeOf(T)]),
+        else => @compileError("PALRAM: Unsupported read width"),
+    };
+}
+
+pub fn write(self: *Self, comptime T: type, address: usize, value: T) void {
+    const addr = address & 0x3FF;
+
+    switch (T) {
+        u32, u16 => std.mem.writeIntSliceLittle(T, self.buf[addr..][0..@sizeOf(T)], value),
+        u8 => {
+            const align_addr = addr & ~@as(u32, 1); // Aligned to Halfword boundary
+            std.mem.writeIntSliceLittle(u16, self.buf[align_addr..][0..@sizeOf(u16)], @as(u16, value) * 0x101);
+        },
+        else => @compileError("PALRAM: Unsupported write width"),
+    }
+}
+
+pub fn init(allocator: Allocator) !Self {
+    const buf = try allocator.alloc(u8, buf_len);
+    std.mem.set(u8, buf, 0);
+
+    return Self{ .buf = buf, .allocator = allocator };
+}
+
+pub fn reset(self: *Self) void {
+    std.mem.set(u8, self.buf, 0);
+}
+
+pub fn deinit(self: *Self) void {
+    self.allocator.free(self.buf);
+    self.* = undefined;
+}
+
+pub inline fn backdrop(self: *const Self) u16 {
+    return std.mem.readIntNative(u16, self.buf[0..2]);
+}

src/core/ppu/Vram.zig

@@ -0,0 +1,64 @@
+const std = @import("std");
+const io = @import("../bus/io.zig");
+
+const Allocator = std.mem.Allocator;
+
+const buf_len = 0x18000;
+const Self = @This();
+
+buf: []u8,
+allocator: Allocator,
+
+pub fn read(self: *const Self, comptime T: type, address: usize) T {
+    const addr = Self.mirror(address);
+
+    return switch (T) {
+        u32, u16, u8 => std.mem.readIntSliceLittle(T, self.buf[addr..][0..@sizeOf(T)]),
+        else => @compileError("VRAM: Unsupported read width"),
+    };
+}
+
+pub fn write(self: *Self, comptime T: type, dispcnt: io.DisplayControl, address: usize, value: T) void {
+    const mode: u3 = dispcnt.bg_mode.read();
+    const idx = Self.mirror(address);
+
+    switch (T) {
+        u32, u16 => std.mem.writeIntSliceLittle(T, self.buf[idx..][0..@sizeOf(T)], value),
+        u8 => {
+            // Ignore write if it falls within the boundaries of OBJ VRAM
+            switch (mode) {
+                0, 1, 2 => if (0x0001_0000 <= idx) return,
+                else => if (0x0001_4000 <= idx) return,
+            }
+
+            const align_idx = idx & ~@as(u32, 1); // Aligned to a halfword boundary
+            std.mem.writeIntSliceLittle(u16, self.buf[align_idx..][0..@sizeOf(u16)], @as(u16, value) * 0x101);
+        },
+        else => @compileError("VRAM: Unsupported write width"),
+    }
+}
+
+pub fn init(allocator: Allocator) !Self {
+    const buf = try allocator.alloc(u8, buf_len);
+    std.mem.set(u8, buf, 0);
+
+    return Self{ .buf = buf, .allocator = allocator };
+}
+
+pub fn reset(self: *Self) void {
+    std.mem.set(u8, self.buf, 0);
+}
+
+pub fn deinit(self: *Self) void {
+    self.allocator.free(self.buf);
+    self.* = undefined;
+}
+
+pub fn mirror(address: usize) usize {
+    // Mirrored in steps of 128K (64K + 32K + 32K) (abcc)
+    const addr = address & 0x1FFFF;
+
+    // If the address is within 96K we don't do anything,
+    // otherwise we want to mirror the last 32K (addresses between 64K and 96K)
+    return if (addr < buf_len) addr else 0x10000 + (addr & 0x7FFF);
+}

src/core/scheduler.zig

@@ -1,6 +1,5 @@
 const std = @import("std");
 
-const Bus = @import("Bus.zig");
 const Arm7tdmi = @import("cpu.zig").Arm7tdmi;
 const Clock = @import("bus/gpio.zig").Clock;
 
@@ -12,11 +11,11 @@ const log = std.log.scoped(.Scheduler);
 pub const Scheduler = struct {
     const Self = @This();
 
-    tick: u64,
+    tick: u64 = 0,
     queue: PriorityQueue(Event, void, lessThan),
 
     pub fn init(allocator: Allocator) Self {
-        var sched = Self{ .tick = 0, .queue = PriorityQueue(Event, void, lessThan).init(allocator, {}) };
+        var sched = Self{ .queue = PriorityQueue(Event, void, lessThan).init(allocator, {}) };
         sched.queue.add(.{ .kind = .HeatDeath, .tick = std.math.maxInt(u64) }) catch unreachable;
 
         return sched;
@@ -27,69 +26,71 @@ pub const Scheduler = struct {
         self.* = undefined;
     }
 
+    pub fn reset(self: *Self) void {
+        // `std.PriorityQueue` provides no reset function, so we will just create a new one
+        const allocator = self.queue.allocator;
+        self.queue.deinit();
+
+        var new_queue = PriorityQueue(Event, void, lessThan).init(allocator, {});
+        new_queue.add(.{ .kind = .HeatDeath, .tick = std.math.maxInt(u64) }) catch unreachable;
+
+        self.* = .{ .queue = new_queue };
+    }
+
     pub inline fn now(self: *const Self) u64 {
         return self.tick;
     }
 
     pub fn handleEvent(self: *Self, cpu: *Arm7tdmi) void {
-        if (self.queue.removeOrNull()) |event| {
-            const late = self.tick - event.tick;
+        const event = self.queue.remove();
+        const late = self.tick - event.tick;
 
-            switch (event.kind) {
-                .HeatDeath => {
-                    log.err("u64 overflow. This *actually* should never happen.", .{});
-                    unreachable;
-                },
-                .Draw => {
-                    // The end of a VDraw
-                    cpu.bus.ppu.drawScanline();
-                    cpu.bus.ppu.onHdrawEnd(cpu, late);
-                },
-                .TimerOverflow => |id| {
-                    switch (id) {
-                        0 => cpu.bus.tim[0].onTimerExpire(cpu, late),
-                        1 => cpu.bus.tim[1].onTimerExpire(cpu, late),
-                        2 => cpu.bus.tim[2].onTimerExpire(cpu, late),
-                        3 => cpu.bus.tim[3].onTimerExpire(cpu, late),
-                    }
-                },
-                .ApuChannel => |id| {
-                    switch (id) {
-                        0 => cpu.bus.apu.ch1.onToneSweepEvent(late),
-                        1 => cpu.bus.apu.ch2.onToneEvent(late),
-                        2 => cpu.bus.apu.ch3.onWaveEvent(late),
-                        3 => cpu.bus.apu.ch4.onNoiseEvent(late),
-                    }
-                },
-                .RealTimeClock => {
-                    const device = &cpu.bus.pak.gpio.device;
-                    if (device.kind != .Rtc or device.ptr == null) return;
+        switch (event.kind) {
+            .HeatDeath => {
+                log.err("u64 overflow. This *actually* should never happen.", .{});
+                unreachable;
+            },
+            .Draw => {
+                // The end of a VDraw
+                cpu.bus.ppu.drawScanline();
+                cpu.bus.ppu.onHdrawEnd(cpu, late);
+            },
+            .TimerOverflow => |id| {
+                switch (id) {
+                    inline 0...3 => |idx| cpu.bus.tim[idx].onTimerExpire(cpu, late),
+                }
+            },
+            .ApuChannel => |id| {
+                switch (id) {
+                    0 => cpu.bus.apu.ch1.onToneSweepEvent(late),
+                    1 => cpu.bus.apu.ch2.onToneEvent(late),
+                    2 => cpu.bus.apu.ch3.onWaveEvent(late),
+                    3 => cpu.bus.apu.ch4.onNoiseEvent(late),
+                }
+            },
+            .RealTimeClock => {
+                const device = &cpu.bus.pak.gpio.device;
+                if (device.kind != .Rtc or device.ptr == null) return;
 
-                    const clock = @ptrCast(*Clock, @alignCast(@alignOf(*Clock), device.ptr.?));
-                    clock.onClockUpdate(late);
-                },
-                .FrameSequencer => cpu.bus.apu.onSequencerTick(late),
-                .SampleAudio => cpu.bus.apu.sampleAudio(late),
-                .HBlank => cpu.bus.ppu.onHblankEnd(cpu, late), // The end of a HBlank
-                .VBlank => cpu.bus.ppu.onHdrawEnd(cpu, late), // The end of a VBlank
-            }
+                const clock = @ptrCast(*Clock, @alignCast(@alignOf(*Clock), device.ptr.?));
+                clock.onClockUpdate(late);
+            },
+            .FrameSequencer => cpu.bus.apu.onSequencerTick(late),
+            .SampleAudio => cpu.bus.apu.sampleAudio(late),
+            .HBlank => cpu.bus.ppu.onHblankEnd(cpu, late), // The end of a HBlank
+            .VBlank => cpu.bus.ppu.onHdrawEnd(cpu, late), // The end of a VBlank
         }
     }
 
     /// Removes the **first** scheduled event of type `needle`
     pub fn removeScheduledEvent(self: *Self, needle: EventKind) void {
-        var it = self.queue.iterator();
-
-        var i: usize = 0;
-        while (it.next()) |event| : (i += 1) {
+        for (self.queue.items, 0..) |event, i| {
             if (std.meta.eql(event.kind, needle)) {
 
-                // This invalidates the iterator
+                // invalidates the slice we're iterating over
                 _ = self.queue.removeIndex(i);
 
-                // Since removing something from the PQ invalidates the iterator,
-                // this implementation can safely only remove the first instance of
-                // a Scheduled Event. Exit Early
+                log.debug("Removed {?}@{}", .{ event.kind, event.tick });
                 break;
             }
         }

src/imgui.zig

@@ -0,0 +1,307 @@
+//! Namespace for dealing with ZBA's immediate-mode GUI
+//! Currently, ZBA uses zgui from https://github.com/michal-z/zig-gamedev
+//! which provides Zig bindings for https://github.com/ocornut/imgui under the hood
+
+const std = @import("std");
+const zgui = @import("zgui");
+const gl = @import("gl");
+const nfd = @import("nfd");
+const config = @import("config.zig");
+const emu = @import("core/emu.zig");
+
+const Gui = @import("platform.zig").Gui;
+const Arm7tdmi = @import("core/cpu.zig").Arm7tdmi;
+const RingBuffer = @import("zba-util").RingBuffer;
+
+const Allocator = std.mem.Allocator;
+const GLuint = gl.GLuint;
+
+const gba_width = @import("core/ppu.zig").width;
+const gba_height = @import("core/ppu.zig").height;
+
+const log = std.log.scoped(.Imgui);
+
+// two seconds worth of fps values into the past
+const histogram_len = 0x80;
+
+/// Immediate-Mode GUI State
+pub const State = struct {
+    title: [12:0]u8,
+
+    fps_hist: RingBuffer(u32),
+    should_quit: bool = false,
+
+    /// if zba is initialized with a ROM already provided, this initializer should be called
+    /// with `title_opt` being non-null
+    pub fn init(allocator: Allocator, title_opt: ?*const [12]u8) !@This() {
+        const history = try allocator.alloc(u32, histogram_len);
+
+        const title: [12:0]u8 = if (title_opt) |t| t.* ++ [_:0]u8{} else "[No Title]\x00\x00".*;
+        return .{ .title = title, .fps_hist = RingBuffer(u32).init(history) };
+    }
+
+    pub fn deinit(self: *@This(), allocator: Allocator) void {
+        allocator.free(self.fps_hist.buf);
+        self.* = undefined;
+    }
+};
+
+pub fn draw(state: *State, tex_id: GLuint, cpu: *Arm7tdmi) void {
+    const win_scale = config.config().host.win_scale;
+
+    {
+        _ = zgui.beginMainMenuBar();
+        defer zgui.endMainMenuBar();
+
+        if (zgui.beginMenu("File", true)) {
+            defer zgui.endMenu();
+
+            if (zgui.menuItem("Quit", .{})) state.should_quit = true;
+
+            if (zgui.menuItem("Insert ROM", .{})) blk: {
+                const maybe_path = nfd.openFileDialog("gba", null) catch |e| {
+                    log.err("failed to open file dialog: {}", .{e});
+                    break :blk;
+                };
+
+                const file_path = maybe_path orelse {
+                    log.warn("did not receive a file path", .{});
+                    break :blk;
+                };
+                defer nfd.freePath(file_path);
+
+                log.info("user chose: \"{s}\"", .{file_path});
+                emu.replaceGamepak(cpu, file_path) catch |e| {
+                    log.err("failed to replace GamePak: {}", .{e});
+                    break :blk;
+                };
+
+                state.title = cpu.bus.pak.title ++ [_:0]u8{};
+            }
+        }
+
+        if (zgui.beginMenu("Emulation", true)) {
+            defer zgui.endMenu();
+
+            if (zgui.menuItem("Restart", .{})) {
+                emu.reset(cpu);
+            }
+        }
+    }
+
+    {
+        const w = @intToFloat(f32, gba_width * win_scale);
+        const h = @intToFloat(f32, gba_height * win_scale);
+
+        const window_title = std.mem.sliceTo(&state.title, 0);
+        _ = zgui.begin(window_title, .{ .flags = .{ .no_resize = true, .always_auto_resize = true } });
+        defer zgui.end();
+
+        zgui.image(@intToPtr(*anyopaque, tex_id), .{ .w = w, .h = h, .uv0 = .{ 0, 1 }, .uv1 = .{ 1, 0 } });
+    }
+
+    {
+        _ = zgui.begin("Information", .{});
+        defer zgui.end();
+
+        for (0..8) |i| {
+            zgui.text("R{}: 0x{X:0>8}", .{ i, cpu.r[i] });
+
+            zgui.sameLine(.{});
+
+            const padding = if (8 + i < 10) " " else "";
+            zgui.text("{s}R{}: 0x{X:0>8}", .{ padding, 8 + i, cpu.r[8 + i] });
+        }
+
+        zgui.separator();
+
+        widgets.psr("CPSR", cpu.cpsr);
+        widgets.psr("SPSR", cpu.spsr);
+
+        zgui.separator();
+
+        widgets.interrupts(" IE", cpu.bus.io.ie);
+        widgets.interrupts("IRQ", cpu.bus.io.irq);
+    }
+
+    {
+        _ = zgui.begin("Performance", .{});
+        defer zgui.end();
+
+        const tmp = blk: {
+            var buf: [histogram_len]u32 = undefined;
+            const len = state.fps_hist.copy(&buf);
+
+            break :blk .{ buf, len };
+        };
+        const values = tmp[0];
+        const len = tmp[1];
+
+        if (len == values.len) _ = state.fps_hist.pop();
+
+        const sorted = blk: {
+            var buf: @TypeOf(values) = undefined;
+
+            std.mem.copy(u32, buf[0..len], values[0..len]);
+            std.sort.sort(u32, buf[0..len], {}, std.sort.asc(u32));
+
+            break :blk buf;
+        };
+
+        const y_max = 2 * if (len != 0) @intToFloat(f64, sorted[len - 1]) else emu.frame_rate;
+        const x_max = @intToFloat(f64, values.len);
+
+        const y_args = .{ .flags = .{ .no_grid_lines = true } };
+        const x_args = .{ .flags = .{ .no_grid_lines = true, .no_tick_labels = true, .no_tick_marks = true } };
+
+        if (zgui.plot.beginPlot("Emulation FPS", .{ .w = 0.0, .flags = .{ .no_title = true, .no_frame = true } })) {
+            defer zgui.plot.endPlot();
+
+            zgui.plot.setupLegend(.{ .north = true, .east = true }, .{});
+            zgui.plot.setupAxis(.x1, x_args);
+            zgui.plot.setupAxis(.y1, y_args);
+            zgui.plot.setupAxisLimits(.y1, .{ .min = 0.0, .max = y_max, .cond = .always });
+            zgui.plot.setupAxisLimits(.x1, .{ .min = 0.0, .max = x_max, .cond = .always });
+            zgui.plot.setupFinish();
+
+            zgui.plot.plotLineValues("FPS", u32, .{ .v = values[0..len] });
+        }
+
+        const stats: struct { u32, u32, u32 } = blk: {
+            if (len == 0) break :blk .{ 0, 0, 0 };
+
+            const average = average: {
+                var sum: u32 = 0;
+                for (sorted[0..len]) |value| sum += value;
+
+                break :average @intCast(u32, sum / len);
+            };
+            const median = sorted[len / 2];
+            const low = sorted[len / 100]; // 1% Low
+
+            break :blk .{ average, median, low };
+        };
+
+        zgui.text("Average: {:0>3} fps", .{stats[0]});
+        zgui.text(" Median: {:0>3} fps", .{stats[1]});
+        zgui.text(" 1% Low: {:0>3} fps", .{stats[2]});
+    }
+
+    {
+        _ = zgui.begin("Scheduler", .{});
+        defer zgui.end();
+
+        const scheduler = cpu.sched;
+
+        zgui.text("tick: {X:0>16}", .{scheduler.tick});
+        zgui.separator();
+
+        const Event = std.meta.Child(@TypeOf(scheduler.queue.items));
+
+        var items: [20]Event = undefined;
+        const len = scheduler.queue.len;
+
+        std.mem.copy(Event, &items, scheduler.queue.items);
+        std.sort.sort(Event, items[0..len], {}, widgets.eventDesc(Event));
+
+        for (items[0..len]) |event| {
+            zgui.text("{X:0>16} | {?}", .{ event.tick, event.kind });
+        }
+    }
+
+    // {
+    //     zgui.showDemoWindow(null);
+    // }
+}
+
+const widgets = struct {
+    fn interrupts(comptime label: []const u8, int: anytype) void {
+        const h = 15.0;
+        const w = 9.0 * 2 + 3.5;
+        const ww = 9.0 * 3;
+
+        {
+            zgui.text(label ++ ":", .{});
+
+            zgui.sameLine(.{});
+            _ = zgui.selectable("VBL", .{ .w = w, .h = h, .selected = int.vblank.read() });
+
+            zgui.sameLine(.{});
+            _ = zgui.selectable("HBL", .{ .w = w, .h = h, .selected = int.hblank.read() });
+
+            zgui.sameLine(.{});
+            _ = zgui.selectable("VCT", .{ .w = w, .h = h, .selected = int.coincidence.read() });
+
+            {
+                zgui.sameLine(.{});
+                _ = zgui.selectable("TIM0", .{ .w = ww, .h = h, .selected = int.tim0.read() });
+
+                zgui.sameLine(.{});
+                _ = zgui.selectable("TIM1", .{ .w = ww, .h = h, .selected = int.tim1.read() });
+
+                zgui.sameLine(.{});
+                _ = zgui.selectable("TIM2", .{ .w = ww, .h = h, .selected = int.tim2.read() });
+
+                zgui.sameLine(.{});
+                _ = zgui.selectable("TIM3", .{ .w = ww, .h = h, .selected = int.tim3.read() });
+            }
+
+            zgui.sameLine(.{});
+            _ = zgui.selectable("SRL", .{ .w = w, .h = h, .selected = int.serial.read() });
+
+            {
+                zgui.sameLine(.{});
+                _ = zgui.selectable("DMA0", .{ .w = ww, .h = h, .selected = int.dma0.read() });
+
+                zgui.sameLine(.{});
+                _ = zgui.selectable("DMA1", .{ .w = ww, .h = h, .selected = int.dma1.read() });
+
+                zgui.sameLine(.{});
+                _ = zgui.selectable("DMA2", .{ .w = ww, .h = h, .selected = int.dma2.read() });
+
+                zgui.sameLine(.{});
+                _ = zgui.selectable("DMA3", .{ .w = ww, .h = h, .selected = int.dma3.read() });
+            }
+
+            zgui.sameLine(.{});
+            _ = zgui.selectable("KPD", .{ .w = w, .h = h, .selected = int.keypad.read() });
+
+            zgui.sameLine(.{});
+            _ = zgui.selectable("GPK", .{ .w = w, .h = h, .selected = int.game_pak.read() });
+        }
+    }
+
+    fn psr(comptime label: []const u8, register: anytype) void {
+        const Mode = @import("core/cpu.zig").Mode;
+
+        const maybe_mode = std.meta.intToEnum(Mode, register.mode.read()) catch null;
+        const mode = if (maybe_mode) |mode| mode.toString() else "???";
+        const w = 9.0;
+        const h = 15.0;
+
+        zgui.text(label ++ ": 0x{X:0>8}", .{register.raw});
+
+        zgui.sameLine(.{});
+        _ = zgui.selectable("N", .{ .w = w, .h = h, .selected = register.n.read() });
+
+        zgui.sameLine(.{});
+        _ = zgui.selectable("Z", .{ .w = w, .h = h, .selected = register.z.read() });
+
+        zgui.sameLine(.{});
+        _ = zgui.selectable("C", .{ .w = w, .h = h, .selected = register.c.read() });
+
+        zgui.sameLine(.{});
+        _ = zgui.selectable("V", .{ .w = w, .h = h, .selected = register.v.read() });
+
+        zgui.sameLine(.{});
+        zgui.text("{s}", .{mode});
+    }
+
+    fn eventDesc(comptime T: type) fn (void, T, T) bool {
+        return struct {
+            fn inner(_: void, left: T, right: T) bool {
+                return left.tick > right.tick;
+            }
+        }.inner;
+    }
+};

src/main.zig

@@ -4,17 +4,18 @@ const known_folders = @import("known_folders");
 const clap = @import("clap");
 
 const config = @import("config.zig");
+const emu = @import("core/emu.zig");
 
+const TwoWayChannel = @import("zba-util").TwoWayChannel;
 const Gui = @import("platform.zig").Gui;
 const Bus = @import("core/Bus.zig");
 const Arm7tdmi = @import("core/cpu.zig").Arm7tdmi;
 const Scheduler = @import("core/scheduler.zig").Scheduler;
 const FilePaths = @import("util.zig").FilePaths;
-
+const FpsTracker = @import("util.zig").FpsTracker;
 const Allocator = std.mem.Allocator;
+
 const log = std.log.scoped(.Cli);
-const width = @import("core/ppu.zig").width;
-const height = @import("core/ppu.zig").height;
 pub const log_level = if (builtin.mode != .Debug) .info else std.log.default_level;
 
 // CLI Arguments + Help Text
@@ -22,41 +23,57 @@ const params = clap.parseParamsComptime(
     \\-h, --help            Display this help and exit.
     \\-s, --skip            Skip BIOS.
     \\-b, --bios <str>      Optional path to a GBA BIOS ROM.
+    \\ --gdb                Run ZBA from the context of a GDB Server
     \\<str>                 Path to the GBA GamePak ROM.
     \\
 );
 
-pub fn main() anyerror!void {
+pub fn main() void {
     // Main Allocator for ZBA
     var gpa = std.heap.GeneralPurposeAllocator(.{}){};
     defer std.debug.assert(!gpa.deinit());
 
     const allocator = gpa.allocator();
 
-    // Determine the Data Directory (stores saves, config file, etc.)
+    // Determine the Data Directory (stores saves)
     const data_path = blk: {
         const result = known_folders.getPath(allocator, .data);
-        const option = result catch |e| exitln("interrupted while attempting to find a data directory: {}", .{e});
-        break :blk option orelse exitln("no valid data directory could be found", .{});
+        const option = result catch |e| exitln("interrupted while determining the data folder: {}", .{e});
+        const path = option orelse exitln("no valid data folder found", .{});
+        ensureDataDirsExist(path) catch |e| exitln("failed to create folders under \"{s}\": {}", .{ path, e });
+
+        break :blk path;
     };
     defer allocator.free(data_path);
 
+    // Determine the Config Directory
+    const config_path = blk: {
+        const result = known_folders.getPath(allocator, .roaming_configuration);
+        const option = result catch |e| exitln("interreupted while determining the config folder: {}", .{e});
+        const path = option orelse exitln("no valid config folder found", .{});
+        ensureConfigDirExists(path) catch |e| exitln("failed to create required folder \"{s}\": {}", .{ path, e });
+
+        break :blk path;
+    };
+    defer allocator.free(config_path);
+
     // Parse CLI
     const result = clap.parse(clap.Help, &params, clap.parsers.default, .{}) catch |e| exitln("failed to parse cli: {}", .{e});
     defer result.deinit();
 
     // TODO: Move config file to XDG Config directory?
-    const config_path = configFilePath(allocator, data_path) catch |e| exitln("failed to determine the config file path for ZBA: {}", .{e});
-    defer allocator.free(config_path);
+    const cfg_file_path = configFilePath(allocator, config_path) catch |e| exitln("failed to ready config file for access: {}", .{e});
+    defer allocator.free(cfg_file_path);
 
-    config.load(allocator, config_path) catch |e| exitln("failed to read config file: {}", .{e});
+    config.load(allocator, cfg_file_path) catch |e| exitln("failed to load config file: {}", .{e});
 
     const paths = handleArguments(allocator, data_path, &result) catch |e| exitln("failed to handle cli arguments: {}", .{e});
     defer if (paths.save) |path| allocator.free(path);
 
-    const log_file = if (config.config().debug.cpu_trace) blk: {
-        break :blk std.fs.cwd().createFile("zba.log", .{}) catch |e| exitln("failed to create trace log file: {}", .{e});
-    } else null;
+    const log_file = switch (config.config().debug.cpu_trace) {
+        true => std.fs.cwd().createFile("zba.log", .{}) catch |e| exitln("failed to create trace log file: {}", .{e}),
+        false => null,
+    };
     defer if (log_file) |file| file.close();
 
     // TODO: Take Emulator Init Code out of main.zig
@@ -73,20 +90,63 @@ pub fn main() anyerror!void {
         cpu.fastBoot();
     }
 
-    var gui = Gui.init(&bus.pak.title, &bus.apu, width, height);
+    const title_ptr = if (paths.rom != null) &bus.pak.title else null;
+
+    // TODO: Just copy the title instead of grabbing a pointer to it
+    var gui = Gui.init(allocator, &bus.apu, title_ptr) catch |e| exitln("failed to init gui: {}", .{e});
     defer gui.deinit();
 
-    gui.run(&cpu, &scheduler) catch |e| exitln("failed to run gui thread: {}", .{e});
+    var quit = std.atomic.Atomic(bool).init(false);
+
+    var items: [0x100]u8 = undefined;
+    var channel = TwoWayChannel.init(&items);
+
+    if (result.args.gdb) {
+        const Server = @import("gdbstub").Server;
+        const EmuThing = @import("core/emu.zig").EmuThing;
+
+        var wrapper = EmuThing.init(&cpu, &scheduler);
+        var emulator = wrapper.interface(allocator);
+        defer emulator.deinit();
+
+        log.info("Ready to connect", .{});
+
+        var server = Server.init(emulator) catch |e| exitln("failed to init gdb server: {}", .{e});
+        defer server.deinit(allocator);
+
+        log.info("Starting GDB Server Thread", .{});
+
+        const thread = std.Thread.spawn(.{}, Server.run, .{ &server, allocator, &quit }) catch |e| exitln("gdb server thread crashed: {}", .{e});
+        defer thread.join();
+
+        gui.run(.{
+            .cpu = &cpu,
+            .scheduler = &scheduler,
+            .channel = &channel,
+        }) catch |e| exitln("main thread panicked: {}", .{e});
+    } else {
+        var tracker = FpsTracker.init();
+
+        const thread = std.Thread.spawn(.{}, emu.run, .{ &cpu, &scheduler, &tracker, &channel }) catch |e| exitln("emu thread panicked: {}", .{e});
+        defer thread.join();
+
+        gui.run(.{
+            .cpu = &cpu,
+            .scheduler = &scheduler,
+            .channel = &channel,
+            .tracker = &tracker,
+        }) catch |e| exitln("main thread panicked: {}", .{e});
+    }
 }
 
-pub fn handleArguments(allocator: Allocator, data_path: []const u8, result: *const clap.Result(clap.Help, &params, clap.parsers.default)) !FilePaths {
+fn handleArguments(allocator: Allocator, data_path: []const u8, result: *const clap.Result(clap.Help, &params, clap.parsers.default)) !FilePaths {
     const rom_path = romPath(result);
-    log.info("ROM path: {s}", .{rom_path});
+    log.info("ROM path: {?s}", .{rom_path});
 
     const bios_path = result.args.bios;
     if (bios_path) |path| log.info("BIOS path: {s}", .{path}) else log.warn("No BIOS provided", .{});
 
-    const save_path = try savePath(allocator, data_path);
+    const save_path = try std.fs.path.join(allocator, &[_][]const u8{ data_path, "zba", "save" });
     log.info("Save path: {s}", .{save_path});
 
     return .{
@@ -96,38 +156,44 @@ pub fn handleArguments(allocator: Allocator, data_path: []const u8, result: *con
     };
 }
 
-fn configFilePath(allocator: Allocator, data_path: []const u8) ![]const u8 {
-    const path = try std.fs.path.join(allocator, &[_][]const u8{ data_path, "zba", "config.toml" });
+fn configFilePath(allocator: Allocator, config_path: []const u8) ![]const u8 {
+    const path = try std.fs.path.join(allocator, &[_][]const u8{ config_path, "zba", "config.toml" });
     errdefer allocator.free(path);
 
     // We try to create the file exclusively, meaning that we err out if the file already exists.
     // All we care about is a file being there so we can just ignore that error in particular and
     // continue down the happy pathj
-    std.fs.accessAbsolute(path, .{}) catch {
-        const file_handle = try std.fs.createFileAbsolute(path, .{});
-        defer file_handle.close();
+    std.fs.accessAbsolute(path, .{}) catch |e| {
+        if (e != error.FileNotFound) return e;
 
-        // TODO: Write Default valeus to config file
+        const config_file = std.fs.createFileAbsolute(path, .{}) catch |err| exitln("failed to create \"{s}\": {}", .{ path, err });
+        defer config_file.close();
+
+        try config_file.writeAll(@embedFile("../example.toml"));
     };
 
     return path;
 }
 
-fn savePath(allocator: Allocator, data_path: []const u8) ![]const u8 {
+fn ensureDataDirsExist(data_path: []const u8) !void {
     var dir = try std.fs.openDirAbsolute(data_path, .{});
     defer dir.close();
 
-    // Will either make the path recursively, or just exit early since it already exists
-    try dir.makePath("zba" ++ [_]u8{std.fs.path.sep} ++ "save");
-
-    // FIXME: Do we have to allocate? :sad:
-    return try std.fs.path.join(allocator, &[_][]const u8{ data_path, "zba", "save" });
+    // Will recursively create directories
+    try dir.makePath("zba" ++ std.fs.path.sep_str ++ "save");
 }
 
-fn romPath(result: *const clap.Result(clap.Help, &params, clap.parsers.default)) []const u8 {
+fn ensureConfigDirExists(config_path: []const u8) !void {
+    var dir = try std.fs.openDirAbsolute(config_path, .{});
+    defer dir.close();
+
+    try dir.makePath("zba");
+}
+
+fn romPath(result: *const clap.Result(clap.Help, &params, clap.parsers.default)) ?[]const u8 {
     return switch (result.positionals.len) {
+        0 => null,
         1 => result.positionals[0],
-        0 => exitln("ZBA requires a path to a GamePak ROM", .{}),
         else => exitln("ZBA received too many positional arguments.", .{}),
     };
 }

src/platform.zig

@@ -1,25 +1,36 @@
 const std = @import("std");
 const SDL = @import("sdl2");
 const gl = @import("gl");
+const zgui = @import("zgui");
+
 const emu = @import("core/emu.zig");
 const config = @import("config.zig");
+const imgui = @import("imgui.zig");
 
 const Apu = @import("core/apu.zig").Apu;
 const Arm7tdmi = @import("core/cpu.zig").Arm7tdmi;
 const Scheduler = @import("core/scheduler.zig").Scheduler;
 const FpsTracker = @import("util.zig").FpsTracker;
+const TwoWayChannel = @import("zba-util").TwoWayChannel;
 
-const span = @import("util.zig").span;
-
-const pitch = @import("core/ppu.zig").framebuf_pitch;
 const gba_width = @import("core/ppu.zig").width;
 const gba_height = @import("core/ppu.zig").height;
 
-const default_title: []const u8 = "ZBA";
+const GLuint = gl.GLuint;
+const GLsizei = gl.GLsizei;
+const SDL_GLContext = *anyopaque;
+const Allocator = std.mem.Allocator;
+
+const width = 1280;
+const height = 720;
+
+pub const sample_rate = 1 << 15;
+pub const sample_format = SDL.AUDIO_U16;
+
+const window_title = "ZBA";
 
 pub const Gui = struct {
     const Self = @This();
-    const SDL_GLContext = *anyopaque; // SDL.SDL_GLContext is a ?*anyopaque
     const log = std.log.scoped(.Gui);
 
     // zig fmt: off
@@ -39,47 +50,88 @@ pub const Gui = struct {
 
     window: *SDL.SDL_Window,
     ctx: SDL_GLContext,
-    title: []const u8,
     audio: Audio,
 
+    state: imgui.State,
+
+    allocator: Allocator,
     program_id: gl.GLuint,
 
-    pub fn init(title: *const [12]u8, apu: *Apu, width: i32, height: i32) Self {
+    pub fn init(allocator: Allocator, apu: *Apu, title_opt: ?*const [12]u8) !Self {
         if (SDL.SDL_Init(SDL.SDL_INIT_VIDEO | SDL.SDL_INIT_EVENTS | SDL.SDL_INIT_AUDIO) < 0) panic();
         if (SDL.SDL_GL_SetAttribute(SDL.SDL_GL_CONTEXT_PROFILE_MASK, SDL.SDL_GL_CONTEXT_PROFILE_CORE) < 0) panic();
         if (SDL.SDL_GL_SetAttribute(SDL.SDL_GL_CONTEXT_MAJOR_VERSION, 3) < 0) panic();
         if (SDL.SDL_GL_SetAttribute(SDL.SDL_GL_CONTEXT_MAJOR_VERSION, 3) < 0) panic();
 
-        const win_scale = @intCast(c_int, config.config().host.win_scale);
-
         const window = SDL.SDL_CreateWindow(
-            default_title.ptr,
+            window_title,
             SDL.SDL_WINDOWPOS_CENTERED,
             SDL.SDL_WINDOWPOS_CENTERED,
-            @as(c_int, width * win_scale),
-            @as(c_int, height * win_scale),
+            width,
+            height,
             SDL.SDL_WINDOW_OPENGL | SDL.SDL_WINDOW_SHOWN,
         ) orelse panic();
 
         const ctx = SDL.SDL_GL_CreateContext(window) orelse panic();
         if (SDL.SDL_GL_MakeCurrent(window, ctx) < 0) panic();
 
-        gl.load(ctx, Self.glGetProcAddress) catch @panic("gl.load failed");
-        if (config.config().host.vsync) if (SDL.SDL_GL_SetSwapInterval(1) < 0) panic();
+        gl.load(ctx, Self.glGetProcAddress) catch {};
+        if (SDL.SDL_GL_SetSwapInterval(@boolToInt(config.config().host.vsync)) < 0) panic();
 
-        const program_id = compileShaders();
+        zgui.init(allocator);
+        zgui.plot.init();
+        zgui.backend.init(window, ctx, "#version 330 core");
+
+        // zgui.io.setIniFilename(null);
 
         return Self{
             .window = window,
-            .title = span(title),
             .ctx = ctx,
-            .program_id = program_id,
+            .program_id = try compileShaders(),
             .audio = Audio.init(apu),
+
+            .allocator = allocator,
+            .state = try imgui.State.init(allocator, title_opt),
         };
     }
 
-    fn compileShaders() gl.GLuint {
-        // TODO: Panic on Shader Compiler Failure + Error Message
+    pub fn deinit(self: *Self) void {
+        self.audio.deinit();
+        self.state.deinit(self.allocator);
+
+        zgui.backend.deinit();
+        zgui.plot.deinit();
+        zgui.deinit();
+
+        gl.deleteProgram(self.program_id);
+        SDL.SDL_GL_DeleteContext(self.ctx);
+        SDL.SDL_DestroyWindow(self.window);
+        SDL.SDL_Quit();
+
+        self.* = undefined;
+    }
+
+    fn drawGbaTexture(self: *const Self, obj_ids: struct { GLuint, GLuint, GLuint }, tex_id: GLuint, buf: []const u8) void {
+        gl.bindTexture(gl.TEXTURE_2D, tex_id);
+        defer gl.bindTexture(gl.TEXTURE_2D, 0);
+
+        gl.texSubImage2D(gl.TEXTURE_2D, 0, 0, 0, gba_width, gba_height, gl.RGBA, gl.UNSIGNED_INT_8_8_8_8, buf.ptr);
+
+        // Bind VAO, EBO. VBO not bound
+        gl.bindVertexArray(obj_ids[0]); // VAO
+        defer gl.bindVertexArray(0);
+
+        gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, obj_ids[2]); // EBO
+        defer gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, 0);
+
+        // Use compiled frag + vertex shader
+        gl.useProgram(self.program_id);
+        defer gl.useProgram(0);
+
+        gl.drawElements(gl.TRIANGLES, 6, gl.UNSIGNED_INT, null);
+    }
+
+    fn compileShaders() !GLuint {
         const vert_shader = @embedFile("shader/pixelbuf.vert");
         const frag_shader = @embedFile("shader/pixelbuf.frag");
 
@@ -89,12 +141,16 @@ pub const Gui = struct {
         gl.shaderSource(vs, 1, &[_][*c]const u8{vert_shader}, 0);
         gl.compileShader(vs);
 
+        if (!shader.didCompile(vs)) return error.VertexCompileError;
+
         const fs = gl.createShader(gl.FRAGMENT_SHADER);
         defer gl.deleteShader(fs);
 
         gl.shaderSource(fs, 1, &[_][*c]const u8{frag_shader}, 0);
         gl.compileShader(fs);
 
+        if (!shader.didCompile(fs)) return error.FragmentCompileError;
+
         const program = gl.createProgram();
         gl.attachShader(program, vs);
         gl.attachShader(program, fs);
@@ -104,24 +160,29 @@ pub const Gui = struct {
     }
 
     // Returns the VAO ID since it's used in run()
-    fn generateBuffers() [3]c_uint {
-        var vao_id: c_uint = undefined;
-        var vbo_id: c_uint = undefined;
-        var ebo_id: c_uint = undefined;
+    fn genBufferObjects() struct { GLuint, GLuint, GLuint } {
+        var vao_id: GLuint = undefined;
+        var vbo_id: GLuint = undefined;
+        var ebo_id: GLuint = undefined;
+
         gl.genVertexArrays(1, &vao_id);
         gl.genBuffers(1, &vbo_id);
         gl.genBuffers(1, &ebo_id);
 
         gl.bindVertexArray(vao_id);
+        defer gl.bindVertexArray(0);
 
         gl.bindBuffer(gl.ARRAY_BUFFER, vbo_id);
-        gl.bufferData(gl.ARRAY_BUFFER, @sizeOf(@TypeOf(vertices)), &vertices, gl.STATIC_DRAW);
+        defer gl.bindBuffer(gl.ARRAY_BUFFER, 0);
 
         gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, ebo_id);
+        defer gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, 0);
+
+        gl.bufferData(gl.ARRAY_BUFFER, @sizeOf(@TypeOf(vertices)), &vertices, gl.STATIC_DRAW);
         gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, @sizeOf(@TypeOf(indices)), &indices, gl.STATIC_DRAW);
 
         // Position
-        gl.vertexAttribPointer(0, 3, gl.FLOAT, gl.FALSE, 8 * @sizeOf(f32), @intToPtr(?*anyopaque, 0)); // lmao
+        gl.vertexAttribPointer(0, 3, gl.FLOAT, gl.FALSE, 8 * @sizeOf(f32), null); // lmao
         gl.enableVertexAttribArray(0);
         // Colour
         gl.vertexAttribPointer(1, 3, gl.FLOAT, gl.FALSE, 8 * @sizeOf(f32), @intToPtr(?*anyopaque, (3 * @sizeOf(f32))));
@@ -133,115 +194,176 @@ pub const Gui = struct {
         return .{ vao_id, vbo_id, ebo_id };
     }
 
-    fn generateTexture(buf: []const u8) c_uint {
-        var tex_id: c_uint = undefined;
+    fn genGbaTexture(buf: []const u8) GLuint {
+        var tex_id: GLuint = undefined;
         gl.genTextures(1, &tex_id);
-        gl.bindTexture(gl.TEXTURE_2D, tex_id);
 
-        // gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
-        // gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
+        gl.bindTexture(gl.TEXTURE_2D, tex_id);
+        defer gl.bindTexture(gl.TEXTURE_2D, 0);
 
         gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
         gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
 
         gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gba_width, gba_height, 0, gl.RGBA, gl.UNSIGNED_INT_8_8_8_8, buf.ptr);
-        // gl.generateMipmap(gl.TEXTURE_2D); // TODO: Remove?
 
         return tex_id;
     }
 
-    pub fn run(self: *Self, cpu: *Arm7tdmi, scheduler: *Scheduler) !void {
-        var quit = std.atomic.Atomic(bool).init(false);
-        var tracker = FpsTracker.init();
+    fn genOutTexture() GLuint {
+        var tex_id: GLuint = undefined;
+        gl.genTextures(1, &tex_id);
 
-        const thread = try std.Thread.spawn(.{}, emu.run, .{ &quit, scheduler, cpu, &tracker });
-        defer thread.join();
+        gl.bindTexture(gl.TEXTURE_2D, tex_id);
+        defer gl.bindTexture(gl.TEXTURE_2D, 0);
 
-        var title_buf: [0x100]u8 = [_]u8{0} ** 0x100;
+        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
+        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
 
-        const vao_id = Self.generateBuffers()[0];
-        _ = Self.generateTexture(cpu.bus.ppu.framebuf.get(.Renderer));
+        gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gba_width, gba_height, 0, gl.RGBA, gl.UNSIGNED_INT_8_8_8_8, null);
+
+        return tex_id;
+    }
+
+    fn genFrameBufObject(tex_id: c_uint) !GLuint {
+        var fbo_id: GLuint = undefined;
+        gl.genFramebuffers(1, &fbo_id);
+
+        gl.bindFramebuffer(gl.FRAMEBUFFER, fbo_id);
+        defer gl.bindFramebuffer(gl.FRAMEBUFFER, 0);
+
+        gl.framebufferTexture(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, tex_id, 0);
+
+        const draw_buffers: [1]GLuint = .{gl.COLOR_ATTACHMENT0};
+        gl.drawBuffers(1, &draw_buffers);
+
+        if (gl.checkFramebufferStatus(gl.FRAMEBUFFER) != gl.FRAMEBUFFER_COMPLETE)
+            return error.FrameBufferObejctInitFailed;
+
+        return fbo_id;
+    }
+
+    const RunOptions = struct {
+        channel: *TwoWayChannel,
+        tracker: ?*FpsTracker = null,
+        cpu: *Arm7tdmi,
+        scheduler: *Scheduler,
+    };
+
+    pub fn run(self: *Self, opt: RunOptions) !void {
+        const cpu = opt.cpu;
+        const tracker = opt.tracker;
+        const channel = opt.channel;
+
+        const obj_ids = Self.genBufferObjects();
+        defer gl.deleteBuffers(3, @as(*const [3]c_uint, &obj_ids));
+
+        const emu_tex = Self.genGbaTexture(cpu.bus.ppu.framebuf.get(.Renderer));
+        const out_tex = Self.genOutTexture();
+        defer gl.deleteTextures(2, &[_]c_uint{ emu_tex, out_tex });
+
+        const fbo_id = try Self.genFrameBufObject(out_tex);
+        defer gl.deleteFramebuffers(1, &fbo_id);
 
         emu_loop: while (true) {
+            // `quit` from RunOptions may be modified by the GDBSTUB thread,
+            // so we want to recognize that it may change to `true` and exit the GUI thread
+            if (channel.gui.pop()) |event| switch (event) {
+                .Quit => break :emu_loop,
+                .Paused => @panic("TODO: We want to peek (and then pop if it's .Quit), not always pop"),
+            };
+
+            // Outside of `SDL.SDL_QUIT` below, the DearImgui UI might signal that the program
+            // should exit, in which case we should also handle this
+            if (self.state.should_quit) break :emu_loop;
+
             var event: SDL.SDL_Event = undefined;
             while (SDL.SDL_PollEvent(&event) != 0) {
+                _ = zgui.backend.processEvent(&event);
+
                 switch (event.type) {
                     SDL.SDL_QUIT => break :emu_loop,
                     SDL.SDL_KEYDOWN => {
-                        const io = &cpu.bus.io;
                         const key_code = event.key.keysym.sym;
+                        var keyinput = cpu.bus.io.keyinput.load(.Monotonic);
 
                         switch (key_code) {
-                            SDL.SDLK_UP => io.keyinput.up.unset(),
-                            SDL.SDLK_DOWN => io.keyinput.down.unset(),
-                            SDL.SDLK_LEFT => io.keyinput.left.unset(),
-                            SDL.SDLK_RIGHT => io.keyinput.right.unset(),
-                            SDL.SDLK_x => io.keyinput.a.unset(),
-                            SDL.SDLK_z => io.keyinput.b.unset(),
-                            SDL.SDLK_a => io.keyinput.shoulder_l.unset(),
-                            SDL.SDLK_s => io.keyinput.shoulder_r.unset(),
-                            SDL.SDLK_RETURN => io.keyinput.start.unset(),
-                            SDL.SDLK_RSHIFT => io.keyinput.select.unset(),
+                            SDL.SDLK_UP => keyinput.up.unset(),
+                            SDL.SDLK_DOWN => keyinput.down.unset(),
+                            SDL.SDLK_LEFT => keyinput.left.unset(),
+                            SDL.SDLK_RIGHT => keyinput.right.unset(),
+                            SDL.SDLK_x => keyinput.a.unset(),
+                            SDL.SDLK_z => keyinput.b.unset(),
+                            SDL.SDLK_a => keyinput.shoulder_l.unset(),
+                            SDL.SDLK_s => keyinput.shoulder_r.unset(),
+                            SDL.SDLK_RETURN => keyinput.start.unset(),
+                            SDL.SDLK_RSHIFT => keyinput.select.unset(),
                             else => {},
                         }
+
+                        cpu.bus.io.keyinput.store(keyinput.raw, .Monotonic);
                     },
                     SDL.SDL_KEYUP => {
-                        const io = &cpu.bus.io;
                         const key_code = event.key.keysym.sym;
+                        var keyinput = cpu.bus.io.keyinput.load(.Monotonic);
 
                         switch (key_code) {
-                            SDL.SDLK_UP => io.keyinput.up.set(),
-                            SDL.SDLK_DOWN => io.keyinput.down.set(),
-                            SDL.SDLK_LEFT => io.keyinput.left.set(),
-                            SDL.SDLK_RIGHT => io.keyinput.right.set(),
-                            SDL.SDLK_x => io.keyinput.a.set(),
-                            SDL.SDLK_z => io.keyinput.b.set(),
-                            SDL.SDLK_a => io.keyinput.shoulder_l.set(),
-                            SDL.SDLK_s => io.keyinput.shoulder_r.set(),
-                            SDL.SDLK_RETURN => io.keyinput.start.set(),
-                            SDL.SDLK_RSHIFT => io.keyinput.select.set(),
-                            SDL.SDLK_i => log.err("Sample Count: {}", .{@intCast(u32, SDL.SDL_AudioStreamAvailable(cpu.bus.apu.stream)) / (2 * @sizeOf(u16))}),
-                            SDL.SDLK_j => log.err("Scheduler Capacity: {} | Scheduler Event Count: {}", .{ scheduler.queue.capacity(), scheduler.queue.count() }),
-                            SDL.SDLK_k => {
-                                // Dump IWRAM to file
-                                log.info("PC: 0x{X:0>8}", .{cpu.r[15]});
-                                log.info("LR: 0x{X:0>8}", .{cpu.r[14]});
-                                // const iwram_file = try std.fs.cwd().createFile("iwram.bin", .{});
-                                // defer iwram_file.close();
-
-                                // try iwram_file.writeAll(cpu.bus.iwram.buf);
-                            },
+                            SDL.SDLK_UP => keyinput.up.set(),
+                            SDL.SDLK_DOWN => keyinput.down.set(),
+                            SDL.SDLK_LEFT => keyinput.left.set(),
+                            SDL.SDLK_RIGHT => keyinput.right.set(),
+                            SDL.SDLK_x => keyinput.a.set(),
+                            SDL.SDLK_z => keyinput.b.set(),
+                            SDL.SDLK_a => keyinput.shoulder_l.set(),
+                            SDL.SDLK_s => keyinput.shoulder_r.set(),
+                            SDL.SDLK_RETURN => keyinput.start.set(),
+                            SDL.SDLK_RSHIFT => keyinput.select.set(),
                             else => {},
                         }
+
+                        cpu.bus.io.keyinput.store(keyinput.raw, .Monotonic);
                     },
                     else => {},
                 }
             }
 
-            // Emulator has an internal Double Buffer
-            const framebuf = cpu.bus.ppu.framebuf.get(.Renderer);
-            gl.texSubImage2D(gl.TEXTURE_2D, 0, 0, 0, gba_width, gba_height, gl.RGBA, gl.UNSIGNED_INT_8_8_8_8, framebuf.ptr);
+            {
+                channel.emu.push(.Pause);
+                defer channel.emu.push(.Resume);
+
+                // Spin Loop until we know that the emu is paused
+                wait: while (true) switch (channel.gui.pop() orelse continue) {
+                    .Paused => break :wait,
+                    else => |any| std.debug.panic("[Gui/Channel]: Unhandled Event: {}", .{any}),
+                };
+
+                // Add FPS count to the histogram
+                if (tracker) |t| self.state.fps_hist.push(t.value()) catch {};
+
+                // Draw GBA Screen to Texture
+                {
+                    gl.bindFramebuffer(gl.FRAMEBUFFER, fbo_id);
+                    defer gl.bindFramebuffer(gl.FRAMEBUFFER, 0);
+
+                    const buf = cpu.bus.ppu.framebuf.get(.Renderer);
+                    gl.viewport(0, 0, gba_width, gba_height);
+                    self.drawGbaTexture(obj_ids, emu_tex, buf);
+                }
+
+                // Background Colour
+                const size = zgui.io.getDisplaySize();
+                gl.viewport(0, 0, @floatToInt(c_int, size[0]), @floatToInt(c_int, size[1]));
+                gl.clearColor(0, 0, 0, 1.0);
+                gl.clear(gl.COLOR_BUFFER_BIT);
+
+                zgui.backend.newFrame(width, height);
+                imgui.draw(&self.state, out_tex, cpu);
+                zgui.backend.draw();
+            }
 
-            gl.useProgram(self.program_id);
-            gl.bindVertexArray(vao_id);
-            gl.drawElements(gl.TRIANGLES, 6, gl.UNSIGNED_INT, null);
             SDL.SDL_GL_SwapWindow(self.window);
-
-            const dyn_title = std.fmt.bufPrint(&title_buf, "ZBA | {s} [Emu: {}fps] ", .{ self.title, tracker.value() }) catch unreachable;
-            SDL.SDL_SetWindowTitle(self.window, dyn_title.ptr);
         }
 
-        quit.store(true, .SeqCst); // Terminate Emulator Thread
-    }
-
-    pub fn deinit(self: *Self) void {
-        self.audio.deinit();
-        // TODO: Buffer deletions
-        gl.deleteProgram(self.program_id);
-        SDL.SDL_GL_DeleteContext(self.ctx);
-        SDL.SDL_DestroyWindow(self.window);
-        SDL.SDL_Quit();
-        self.* = undefined;
+        channel.emu.push(.Quit);
     }
 
     fn glGetProcAddress(ctx: SDL.SDL_GLContext, proc: [:0]const u8) ?*anyopaque {
@@ -253,7 +375,6 @@ pub const Gui = struct {
 const Audio = struct {
     const Self = @This();
     const log = std.log.scoped(.PlatformAudio);
-    const sample_rate = @import("core/apu.zig").host_sample_rate;
 
     device: SDL.SDL_AudioDeviceID,
 
@@ -261,16 +382,22 @@ const Audio = struct {
         var have: SDL.SDL_AudioSpec = undefined;
         var want: SDL.SDL_AudioSpec = std.mem.zeroes(SDL.SDL_AudioSpec);
         want.freq = sample_rate;
-        want.format = SDL.AUDIO_U16;
+        want.format = sample_format;
         want.channels = 2;
         want.samples = 0x100;
         want.callback = Self.callback;
         want.userdata = apu;
 
+        std.debug.assert(sample_format == SDL.AUDIO_U16);
+        log.info("Host Sample Rate: {}Hz, Host Format: SDL.AUDIO_U16", .{sample_rate});
+
         const device = SDL.SDL_OpenAudioDevice(null, 0, &want, &have, 0);
         if (device == 0) panic();
 
-        SDL.SDL_PauseAudioDevice(device, 0); // Unpause Audio
+        if (!config.config().host.mute) {
+            SDL.SDL_PauseAudioDevice(device, 0); // Unpause Audio
+            log.info("Unpaused Device", .{});
+        }
 
         return .{ .device = device };
     }
@@ -281,18 +408,32 @@ const Audio = struct {
     }
 
     export fn callback(userdata: ?*anyopaque, stream: [*c]u8, len: c_int) void {
-        const apu = @ptrCast(*Apu, @alignCast(@alignOf(*Apu), userdata));
+        const T = *Apu;
+        const apu = @ptrCast(T, @alignCast(@alignOf(T), userdata));
 
-        // TODO: Find a better way to mute this
-        if (!config.config().host.mute) {
-            _ = SDL.SDL_AudioStreamGet(apu.stream, stream, len);
-        } else {
-            // FIXME: I don't think this hack to remove DC Offset is acceptable :thinking:
-            std.mem.set(u8, stream[0..@intCast(usize, len)], 0x40);
-        }
+        _ = SDL.SDL_AudioStreamGet(apu.stream, stream, len);
+    }
+};
 
-        // If we don't write anything, play silence otherwise garbage will be played
-        // if (written == 0) std.mem.set(u8, stream[0..@intCast(usize, len)], 0x40);
+const shader = struct {
+    const Kind = enum { vertex, fragment };
+    const log = std.log.scoped(.Shader);
+
+    fn didCompile(id: gl.GLuint) bool {
+        var success: gl.GLint = undefined;
+        gl.getShaderiv(id, gl.COMPILE_STATUS, &success);
+
+        if (success == 0) err(id);
+
+        return success == 1;
+    }
+
+    fn err(id: gl.GLuint) void {
+        const buf_len = 512;
+        var error_msg: [buf_len]u8 = undefined;
+
+        gl.getShaderInfoLog(id, buf_len, 0, &error_msg);
+        log.err("{s}", .{std.mem.sliceTo(&error_msg, 0)});
     }
 };
 

src/util.zig

@@ -5,26 +5,7 @@ const config = @import("config.zig");
 const Log2Int = std.math.Log2Int;
 const Arm7tdmi = @import("core/cpu.zig").Arm7tdmi;
 
-// Sign-Extend value of type `T` to type `U`
-pub fn sext(comptime T: type, comptime U: type, value: T) T {
-    // U must have less bits than T
-    comptime std.debug.assert(@typeInfo(U).Int.bits <= @typeInfo(T).Int.bits);
-
-    const iT = std.meta.Int(.signed, @typeInfo(T).Int.bits);
-    const ExtU = if (@typeInfo(U).Int.signedness == .unsigned) T else iT;
-    const shift = @intCast(Log2Int(T), @typeInfo(T).Int.bits - @typeInfo(U).Int.bits);
-
-    return @bitCast(T, @bitCast(iT, @as(ExtU, @truncate(U, value)) << shift) >> shift);
-}
-
-/// See https://godbolt.org/z/W3en9Eche
-pub inline fn rotr(comptime T: type, x: T, r: anytype) T {
-    if (@typeInfo(T).Int.signedness == .signed)
-        @compileError("cannot rotate signed integer");
-
-    const ar = @intCast(Log2Int(T), @mod(r, @typeInfo(T).Int.bits));
-    return x >> ar | x << (1 +% ~ar);
-}
+const Allocator = std.mem.Allocator;
 
 pub const FpsTracker = struct {
     const Self = @This();
@@ -47,7 +28,7 @@ pub const FpsTracker = struct {
 
     pub fn value(self: *Self) u32 {
         if (self.timer.read() >= std.time.ns_per_s) {
-            self.fps = self.count.swap(0, .SeqCst);
+            self.fps = self.count.swap(0, .Monotonic);
             self.timer.reset();
         }
 
@@ -55,68 +36,6 @@ pub const FpsTracker = struct {
     }
 };
 
-pub fn intToBytes(comptime T: type, value: anytype) [@sizeOf(T)]u8 {
-    comptime std.debug.assert(@typeInfo(T) == .Int);
-
-    var result: [@sizeOf(T)]u8 = undefined;
-
-    var i: Log2Int(T) = 0;
-    while (i < result.len) : (i += 1) result[i] = @truncate(u8, value >> i * @bitSizeOf(u8));
-
-    return result;
-}
-
-/// The Title from the GBA Cartridge is an Uppercase ASCII string which is
-/// null-padded to 12 bytes
-///
-/// This function returns a slice of the ASCII string without the null terminator(s)
-/// (essentially, a proper Zig/Rust/Any modern language String)
-pub fn span(title: *const [12]u8) []const u8 {
-    const end = std.mem.indexOfScalar(u8, title, '\x00');
-    return title[0 .. end orelse title.len];
-}
-
-test "span" {
-    var example: *const [12]u8 = "POKEMON_EMER";
-    try std.testing.expectEqualSlices(u8, "POKEMON_EMER", span(example));
-
-    example = "POKEMON_EME\x00";
-    try std.testing.expectEqualSlices(u8, "POKEMON_EME", span(example));
-
-    example = "POKEMON_EM\x00\x00";
-    try std.testing.expectEqualSlices(u8, "POKEMON_EM", span(example));
-
-    example = "POKEMON_E\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "POKEMON_E", span(example));
-
-    example = "POKEMON_\x00\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "POKEMON_", span(example));
-
-    example = "POKEMON\x00\x00\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "POKEMON", span(example));
-
-    example = "POKEMO\x00\x00\x00\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "POKEMO", span(example));
-
-    example = "POKEM\x00\x00\x00\x00\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "POKEM", span(example));
-
-    example = "POKE\x00\x00\x00\x00\x00\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "POKE", span(example));
-
-    example = "POK\x00\x00\x00\x00\x00\x00\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "POK", span(example));
-
-    example = "PO\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "PO", span(example));
-
-    example = "P\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "P", span(example));
-
-    example = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00";
-    try std.testing.expectEqualSlices(u8, "", span(example));
-}
-
 /// Creates a copy of a title with all Filesystem-invalid characters replaced
 ///
 /// e.g. POKEPIN R/S to POKEPIN R_S
@@ -131,7 +50,7 @@ pub fn escape(title: [12]u8) [12]u8 {
 }
 
 pub const FilePaths = struct {
-    rom: []const u8,
+    rom: ?[]const u8,
     bios: ?[]const u8,
     save: ?[]const u8,
 };
@@ -143,7 +62,9 @@ pub const io = struct {
             return 0;
         }
 
-        pub fn undef(comptime T: type, log: anytype, comptime format: []const u8, args: anytype) ?T {
+        pub fn undef(comptime T: type, comptime log: anytype, comptime format: []const u8, args: anytype) ?T {
+            @setCold(true);
+
             const unhandled_io = config.config().debug.unhandled_io;
 
             log.warn(format, args);
@@ -151,6 +72,13 @@ pub const io = struct {
 
             return null;
         }
+
+        pub fn err(comptime T: type, comptime log: anytype, comptime format: []const u8, args: anytype) ?T {
+            @setCold(true);
+
+            log.err(format, args);
+            return null;
+        }
     };
 
     pub const write = struct {
@@ -165,6 +93,7 @@ pub const io = struct {
 
 pub const Logger = struct {
     const Self = @This();
+    const FmtArgTuple = std.meta.Tuple(&.{ u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32 });
 
     buf: std.io.BufferedWriter(4096 << 2, std.fs.File.Writer),
 
@@ -187,7 +116,7 @@ pub const Logger = struct {
         if (cpu.cpsr.t.read()) {
             if (opcode >> 11 == 0x1E) {
                 // Instruction 1 of a BL Opcode, print in ARM mode
-                const low = cpu.bus.dbgRead(u16, cpu.r[15]);
+                const low = cpu.bus.dbgRead(u16, cpu.r[15] - 2);
                 const bl_opcode = @as(u32, opcode) << 16 | low;
 
                 self.print(arm_fmt, Self.fmtArgs(cpu, bl_opcode)) catch @panic("failed to write to log file");
@@ -223,8 +152,6 @@ pub const Logger = struct {
     }
 };
 
-const FmtArgTuple = std.meta.Tuple(&.{ u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32 });
-
 pub const audio = struct {
     const _io = @import("core/bus/io.zig");
 
@@ -274,22 +201,37 @@ pub const audio = struct {
     };
 };
 
-/// Sets the high bits of an integer to a value
-pub inline fn setHi(comptime T: type, left: T, right: HalfInt(T)) T {
-    return switch (T) {
-        u32 => (left & 0xFFFF_0000) | right,
-        u16 => (left & 0xFF00) | right,
-        u8 => (left & 0xF0) | right,
-        else => @compileError("unsupported type"),
+/// Sets a quarter (8) of the bits of the u32 `left` to the value of u8 `right`
+pub inline fn setQuart(left: u32, addr: u8, right: u8) u32 {
+    const offset = @truncate(u2, addr);
+
+    return switch (offset) {
+        0b00 => (left & 0xFFFF_FF00) | right,
+        0b01 => (left & 0xFFFF_00FF) | @as(u32, right) << 8,
+        0b10 => (left & 0xFF00_FFFF) | @as(u32, right) << 16,
+        0b11 => (left & 0x00FF_FFFF) | @as(u32, right) << 24,
     };
 }
 
-/// sets the low bits of an integer to a value
-pub inline fn setLo(comptime T: type, left: T, right: HalfInt(T)) T {
+/// Calculates the correct shift offset for an aligned/unaligned u8 read
+///
+/// TODO: Support u16 reads of u32 values?
+pub inline fn getHalf(byte: u8) u4 {
+    return @truncate(u4, byte & 1) << 3;
+}
+
+pub inline fn setHalf(comptime T: type, left: T, addr: u8, right: HalfInt(T)) T {
+    const offset = @truncate(u1, addr >> if (T == u32) 1 else 0);
+
     return switch (T) {
-        u32 => (left & 0x0000_FFFF) | @as(u32, right) << 16,
-        u16 => (left & 0x00FF) | @as(u16, right) << 8,
-        u8 => (left & 0x0F) | @as(u8, right) << 4,
+        u32 => switch (offset) {
+            0b0 => (left & 0xFFFF_0000) | right,
+            0b1 => (left & 0x0000_FFFF) | @as(u32, right) << 16,
+        },
+        u16 => switch (offset) {
+            0b0 => (left & 0xFF00) | right,
+            0b1 => (left & 0x00FF) | @as(u16, right) << 8,
+        },
         else => @compileError("unsupported type"),
     };
 }
@@ -302,3 +244,48 @@ fn HalfInt(comptime T: type) type {
 
     return std.meta.Int(type_info.Int.signedness, type_info.Int.bits >> 1);
 }
+
+/// Double Buffering Implementation
+pub const FrameBuffer = struct {
+    const Self = @This();
+
+    layers: [2][]u8,
+    buf: []u8,
+    current: u1 = 0,
+
+    allocator: Allocator,
+
+    // TODO: Rename
+    const Device = enum { Emulator, Renderer };
+
+    pub fn init(allocator: Allocator, comptime len: comptime_int) !Self {
+        const buf = try allocator.alloc(u8, len * 2);
+        std.mem.set(u8, buf, 0);
+
+        return .{
+            // Front and Back Framebuffers
+            .layers = [_][]u8{ buf[0..][0..len], buf[len..][0..len] },
+            .buf = buf,
+
+            .allocator = allocator,
+        };
+    }
+
+    pub fn reset(self: *Self) void {
+        std.mem.set(u8, self.buf, 0);
+        self.current = 0;
+    }
+
+    pub fn deinit(self: *Self) void {
+        self.allocator.free(self.buf);
+        self.* = undefined;
+    }
+
+    pub fn swap(self: *Self) void {
+        self.current = ~self.current;
+    }
+
+    pub fn get(self: *Self, comptime dev: Device) []u8 {
+        return self.layers[if (dev == .Emulator) self.current else ~self.current];
+    }
+};