fix: 8-bit writes to WIN PPU registers

Advance Wars depends on these registers similar to Mario Kart's 8-bit
writes to Affine Background registers:

.gitmodules

@@ -7,3 +7,6 @@
 [submodule "lib/known-folders"]
 	path = lib/known-folders
 	url = https://github.com/ziglibs/known-folders
+[submodule "lib/zig-datetime"]
+	path = lib/zig-datetime
+	url = https://github.com/frmdstryr/zig-datetime

README.md

@@ -1,5 +1,5 @@
 # ZBA (working title)
-An in-progress Gameboy Advance Emulator written in Zig ⚡!
+An in-progress Game Boy Advance Emulator written in Zig ⚡!
 
 ## Tests 
 - [ ] [jsmolka's GBA Test Collection](https://github.com/jsmolka/gba-tests)
@@ -10,8 +10,7 @@ An in-progress Gameboy Advance Emulator written in Zig ⚡!
     - [x] `bios.gba`
     - [ ] `nes.gba`
 - [ ] [DenSinH's GBA ROMs](https://github.com/DenSinH/GBARoms)
-    - [x] `eeprom-test`
+    - [x] `eeprom-test` and `flash-test`
-    - [x] `flash-test`
     - [x] `midikey2freq`
     - [ ] `swi-tests-random`
 - [ ] [destoer's GBA Tests](https://github.com/destoer/gba_tests)
@@ -36,14 +35,14 @@ An in-progress Gameboy Advance Emulator written in Zig ⚡!
 * [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.2978+803376708](https://github.com/ziglang/zig/tree/803376708)
+Most recently built on Zig [0.10.0-dev.3900+ab4b26d8a](https://github.com/ziglang/zig/tree/ab4b26d8a)
 
 ### 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)
-* [`bitfields.zig`](https://github.com/FlorenceOS/Florence/blob/f6044db788d35d43d66c1d7e58ef1e3c79f10d6f/lib/util/bitfields.zig)
+* [`bitfields.zig`](https://github.com/FlorenceOS/Florence/blob/aaa5a9e568197ad24780ec9adb421217530d4466/lib/util/bitfields.zig)
 
 `bitfields.zig` from [FlorenceOS](https://github.com/FlorenceOS) is included under `lib/util/bitfield.zig`.
 

build.zig

@@ -13,10 +13,12 @@ pub fn build(b: *std.build.Builder) void {
     const mode = b.standardReleaseOptions();
 
     const exe = b.addExecutable("zba", "src/main.zig");
-    
     // Known Folders (%APPDATA%, XDG, etc.)
     exe.addPackagePath("known_folders", "lib/known-folders/known-folders.zig");
 
+    // DateTime Library
+    exe.addPackagePath("datetime", "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");

lib/SDL.zig

@@ -1 +1 @@
-Subproject commit d66925011971fbe221fc2a7f7cb4cd8c181d9ba3
+Subproject commit 76ec54bf1d13170f1a9998063eecf8087856541a

lib/zig-datetime

@@ -0,0 +1 @@
+Subproject commit 5ec1c36cf3791b3c6c5b330357bdb6feb93979ba

src/core/Bus.zig

@@ -51,7 +51,7 @@ sched: *Scheduler,
 
 pub fn init(self: *Self, allocator: Allocator, sched: *Scheduler, cpu: *Arm7tdmi, paths: FilePaths) !void {
     self.* = .{
-        .pak = try GamePak.init(allocator, paths.rom, paths.save),
+        .pak = try GamePak.init(allocator, cpu, paths.rom, paths.save),
         .bios = try Bios.init(allocator, paths.bios),
         .ppu = try Ppu.init(allocator, sched),
         .apu = Apu.init(sched),
@@ -88,7 +88,7 @@ pub fn dbgRead(self: *const Self, comptime T: type, address: u32) T {
         },
         0x02 => self.ewram.read(T, aligned_addr),
         0x03 => self.iwram.read(T, aligned_addr),
-        0x04 => io.read(self, T, aligned_addr),
+        0x04 => self.readIo(T, address),
 
         // Internal Display Memory
         0x05 => self.ppu.palette.read(T, aligned_addr),
@@ -113,31 +113,46 @@ pub fn dbgRead(self: *const Self, comptime T: type, address: u32) T {
     };
 }
 
+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.readOpenBus(T, unaligned_address);
+}
+
 fn readOpenBus(self: *const Self, comptime T: type, address: u32) T {
     const r15 = self.cpu.?.r[15];
 
-    const word = if (self.cpu.?.cpsr.t.read()) blk: {
+    const word = blk: {
+        // If u32 Open Bus, read recently fetched opcode (PC + 8)
+        if (!self.cpu.?.cpsr.t.read()) break :blk self.dbgRead(u32, r15 + 4);
         const page = @truncate(u8, r15 >> 24);
 
         switch (page) {
             // EWRAM, PALRAM, VRAM, and Game ROM (16-bit)
             0x02, 0x05, 0x06, 0x08...0x0D => {
+                // (PC + 4)
                 const halfword = self.dbgRead(u16, r15 + 2);
+
                 break :blk @as(u32, halfword) << 16 | halfword;
             },
             // BIOS or OAM (32-bit)
             0x00, 0x07 => {
+                // Aligned: (PC + 6) | (PC + 4)
+                // Unaligned: (PC + 4) | (PC + 2)
                 const offset: u32 = if (address & 3 == 0b00) 2 else 0;
-                break :blk @as(u32, self.dbgRead(u16, (r15 + 2) + offset)) << 16 | self.dbgRead(u16, r15 + offset);
+
+                break :blk @as(u32, self.dbgRead(u16, r15 + 2 + offset)) << 16 | self.dbgRead(u16, r15 + offset);
             },
             // IWRAM (16-bit but special)
             0x03 => {
+                // Aligned: (PC + 2) | (PC + 4)
+                // Unaligned: (PC + 4) | (PC + 2)
                 const offset: u32 = if (address & 3 == 0b00) 2 else 0;
-                break :blk @as(u32, self.dbgRead(u16, (r15 + 2) - offset)) << 16 | self.dbgRead(u16, r15 + offset);
+
+                break :blk @as(u32, self.dbgRead(u16, r15 + 2 - offset)) << 16 | self.dbgRead(u16, r15 + offset);
             },
             else => unreachable,
         }
-    } else self.dbgRead(u32, r15 + 4);
+    };
 
     return @truncate(T, rotr(u32, word, 8 * (address & 3)));
 }
@@ -158,7 +173,7 @@ pub fn read(self: *Self, comptime T: type, address: u32) T {
         },
         0x02 => self.ewram.read(T, aligned_addr),
         0x03 => self.iwram.read(T, aligned_addr),
-        0x04 => io.read(self, T, aligned_addr),
+        0x04 => self.readIo(T, address),
 
         // Internal Display Memory
         0x05 => self.ppu.palette.read(T, aligned_addr),

src/core/apu.zig

@@ -1,6 +1,7 @@
 const std = @import("std");
 const SDL = @import("sdl2");
 const io = @import("bus/io.zig");
+const util = @import("util.zig");
 
 const Arm7tdmi = @import("cpu.zig").Arm7tdmi;
 const Scheduler = @import("scheduler.zig").Scheduler;
@@ -9,13 +10,11 @@ const SoundFifo = std.fifo.LinearFifo(u8, .{ .Static = 0x20 });
 const AudioDeviceId = SDL.SDL_AudioDeviceID;
 
 const intToBytes = @import("util.zig").intToBytes;
-const readUndefined = @import("util.zig").readUndefined;
-const writeUndefined = @import("util.zig").writeUndefined;
 const log = std.log.scoped(.APU);
 
 pub const host_sample_rate = 1 << 15;
 
-pub fn read(comptime T: type, apu: *const Apu, addr: u32) T {
+pub fn read(comptime T: type, apu: *const Apu, addr: u32) ?T {
     const byte = @truncate(u8, addr);
 
     return switch (T) {
@@ -38,7 +37,7 @@ pub fn read(comptime T: type, apu: *const Apu, addr: u32) T {
             0x84 => apu.getSoundCntX(),
             0x88 => apu.bias.raw, // SOUNDBIAS
             0x90...0x9F => apu.ch3.wave_dev.read(T, apu.ch3.select, addr),
-            else => readUndefined(log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         },
         u8 => switch (byte) {
             0x60 => apu.ch1.getSoundCntL(), // NR10
@@ -52,9 +51,9 @@ pub fn read(comptime T: type, apu: *const Apu, addr: u32) T {
             0x81 => @truncate(u8, apu.psg_cnt.raw >> 8), // NR51
             0x84 => apu.getSoundCntX(),
             0x89 => @truncate(u8, apu.bias.raw >> 8), // SOUNDBIAS_H
-            else => readUndefined(log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         },
-        u32 => readUndefined(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"),
     };
 }
@@ -78,7 +77,7 @@ pub fn write(comptime T: type, apu: *Apu, addr: u32, value: T) void {
             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 => writeUndefined(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
+            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.setSoundCntL(@truncate(u8, value)), // SOUND1CNT_L
@@ -101,7 +100,7 @@ pub fn write(comptime T: type, apu: *Apu, addr: u32, value: T) void {
             0x88 => apu.bias.raw = value, // SOUNDBIAS
             // WAVE_RAM
             0x90...0x9F => apu.ch3.wave_dev.write(T, apu.ch3.select, addr, value),
-            else => writeUndefined(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
+            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.setSoundCntL(value),
@@ -133,7 +132,7 @@ pub fn write(comptime T: type, apu: *Apu, addr: u32, value: T) void {
             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 => writeUndefined(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, addr }),
+            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"),
     }

src/core/bus/GamePak.zig

@@ -1,7 +1,13 @@
 const std = @import("std");
+const DateTime = @import("datetime").datetime.Datetime;
 
+const Arm7tdmi = @import("../cpu.zig").Arm7tdmi;
+const Bit = @import("bitfield").Bit;
+const Bitfield = @import("bitfield").Bitfield;
 const Backup = @import("backup.zig").Backup;
 const Allocator = std.mem.Allocator;
+
+const force_rtc = @import("../emu.zig").force_rtc;
 const log = std.log.scoped(.GamePak);
 
 const Self = @This();
@@ -10,31 +16,48 @@ title: [12]u8,
 buf: []u8,
 allocator: Allocator,
 backup: Backup,
+gpio: *Gpio,
 
-pub fn init(allocator: Allocator, rom_path: []const u8, save_path: ?[]const u8) !Self {
+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();
 
     const file_buf = try file.readToEndAlloc(allocator, try file.getEndPos());
-    const title = parseTitle(file_buf);
+    const title = file_buf[0xA0..0xAC].*;
-    const kind = Backup.guessKind(file_buf) orelse .None;
+    const kind = Backup.guessKind(file_buf);
+    const device = if (force_rtc) .Rtc else guessDevice(file_buf);
 
-    const pak = Self{
+    logHeader(file_buf, &title);
+
+    return .{
         .buf = file_buf,
         .allocator = allocator,
         .title = title,
         .backup = try Backup.init(allocator, kind, title, save_path),
+        .gpio = try Gpio.init(allocator, cpu, device),
     };
-    pak.parseHeader();
-
-    return pak;
 }
 
-fn parseHeader(self: *const Self) void {
+/// Searches the ROM to see if it can determine whether the ROM it's searching uses
-    const title = parseTitle(self.buf);
+/// any GPIO device, like a RTC for example.
-    const code = self.buf[0xAC..0xB0];
+fn guessDevice(buf: []const u8) Gpio.Device.Kind {
-    const maker = self.buf[0xB0..0xB2];
+    // Try to Guess if ROM uses RTC
-    const version = self.buf[0xBC];
+    const needle = "RTC_V"; // I was told SIIRTC_V, though Pokemen Firered (USA) is a false negative
+
+    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});
@@ -42,10 +65,6 @@ fn parseHeader(self: *const Self) void {
     if (lookupMaker(maker)) |c| log.info("Maker: {s}", .{c}) else log.info("Maker Code: {s}", .{maker});
 }
 
-fn parseTitle(buf: []u8) [12]u8 {
-    return buf[0xA0..0xAC].*;
-}
-
 fn lookupMaker(slice: *const [2]u8) ?[]const u8 {
     const id = @as(u16, slice[1]) << 8 | @as(u16, slice[0]);
     return switch (id) {
@@ -60,6 +79,8 @@ inline fn isLarge(self: *const Self) bool {
 
 pub fn deinit(self: *Self) void {
     self.backup.deinit();
+    self.gpio.deinit(self.allocator);
+    self.allocator.destroy(self.gpio);
     self.allocator.free(self.buf);
     self.* = undefined;
 }
@@ -83,6 +104,35 @@ pub fn read(self: *Self, comptime T: type, address: u32) T {
         }
     }
 
+    if (self.gpio.cnt == 1) {
+        // GPIO Can be read from
+        // We assume that this will only be true when a ROM actually does want something from GPIO
+
+        switch (T) {
+            u32 => switch (address) {
+                // TODO: 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),
+                else => {},
+            },
+            u8 => switch (address) {
+                0x0800_00C4 => return self.gpio.read(.Data),
+                0x0800_00C6 => return self.gpio.read(.Direction),
+                0x0800_00C8 => return self.gpio.read(.Control),
+                else => {},
+            },
+            else => @compileError("GamePak[GPIO]: Unsupported read width"),
+        }
+    }
+
     return switch (T) {
         u32 => (@as(T, self.get(addr + 3)) << 24) | (@as(T, self.get(addr + 2)) << 16) | (@as(T, self.get(addr + 1)) << 8) | (@as(T, self.get(addr))),
         u16 => (@as(T, self.get(addr + 1)) << 8) | @as(T, self.get(addr)),
@@ -141,17 +191,23 @@ pub fn write(self: *Self, comptime T: type, word_count: u16, address: u32, value
 
     switch (T) {
         u32 => switch (address) {
-            0x0800_00C4 => log.debug("Wrote {} 0x{X:} to I/O Port Data and Direction", .{ T, value }),
+            0x0800_00C4 => {
-            0x0800_00C6 => log.debug("Wrote {} 0x{X:} to I/O Port Direction and Control", .{ T, value }),
+                self.gpio.write(.Data, @truncate(u4, value));
-            else => {},
+                self.gpio.write(.Direction, @truncate(u4, value >> 16));
+            },
+            0x0800_00C6 => {
+                self.gpio.write(.Direction, @truncate(u4, value));
+                self.gpio.write(.Control, @truncate(u1, value >> 16));
+            },
+            else => log.err("Wrote {} 0x{X:0>8} to 0x{X:0>8}, Unhandled", .{ T, value, address }),
         },
         u16 => switch (address) {
-            0x0800_00C4 => log.debug("Wrote {} 0x{X:} to I/O Port Data", .{ T, value }),
+            0x0800_00C4 => self.gpio.write(.Data, @truncate(u4, value)),
-            0x0800_00C6 => log.debug("Wrote {} 0x{X:} to I/O Port Direction", .{ T, value }),
+            0x0800_00C6 => self.gpio.write(.Direction, @truncate(u4, value)),
-            0x0800_00C8 => log.debug("Wrote {} 0x{X:} to I/O Port Control", .{ T, value }),
+            0x0800_00C8 => self.gpio.write(.Control, @truncate(u1, value)),
-            else => {},
+            else => log.err("Wrote {} 0x{X:0>4} to 0x{X:0>8}, Unhandled", .{ T, value, address }),
         },
-        u8 => log.debug("Wrote {} 0x{X:} to 0x{X:0>8}, Ignored.", .{ T, value, address }),
+        u8 => log.debug("Wrote {} 0x{X:0>2} to 0x{X:0>8}, Ignored.", .{ T, value, address }),
         else => @compileError("GamePak: Unsupported write width"),
     }
 }
@@ -183,3 +239,463 @@ test "OOB Access" {
     std.debug.assert(pak.get(4) == 0x02); // 0x0002
     std.debug.assert(pak.get(5) == 0x00);
 }
+
+/// GPIO Register Implementation
+const Gpio = struct {
+    const This = @This();
+
+    data: u4,
+    direction: u4,
+    cnt: u1,
+
+    device: Device,
+
+    const Device = struct {
+        ptr: ?*anyopaque,
+        kind: Kind, // TODO: Make comptime known?
+
+        const Kind = enum { Rtc, None };
+
+        fn step(self: *Device, value: u4) u4 {
+            return switch (self.kind) {
+                .Rtc => blk: {
+                    const clock = @ptrCast(*Clock, @alignCast(@alignOf(*Clock), self.ptr.?));
+                    break :blk clock.step(Clock.Data{ .raw = value });
+                },
+                .None => value,
+            };
+        }
+
+        fn init(kind: Kind, ptr: ?*anyopaque) Device {
+            return .{ .kind = kind, .ptr = ptr };
+        }
+    };
+
+    const Register = enum {
+        Data,
+        Direction,
+        Control,
+    };
+
+    fn init(allocator: Allocator, cpu: *Arm7tdmi, kind: Device.Kind) !*This {
+        log.info("Device: {}", .{kind});
+
+        const self = try allocator.create(This);
+        self.* = .{
+            .data = 0b0000,
+            .direction = 0b1111, // TODO: What is GPIO DIrection set to by default?
+            .cnt = 0b0,
+
+            .device = switch (kind) {
+                .Rtc => blk: {
+                    const clock = try allocator.create(Clock);
+                    clock.init(cpu, self);
+
+                    break :blk Device{ .kind = kind, .ptr = clock };
+                },
+                .None => Device{ .kind = kind, .ptr = null },
+            },
+        };
+
+        return self;
+    }
+
+    fn deinit(self: *This, allocator: Allocator) void {
+        switch (self.device.kind) {
+            .Rtc => {
+                allocator.destroy(@ptrCast(*Clock, @alignCast(@alignOf(*Clock), self.device.ptr.?)));
+            },
+            .None => {},
+        }
+
+        self.* = undefined;
+    }
+
+    fn write(self: *This, comptime reg: Register, value: if (reg == .Control) u1 else u4) void {
+        switch (reg) {
+            .Data => {
+                const masked_value = value & self.direction;
+
+                // The value which is actually stored in the GPIO register
+                // might be modified by the device implementing the GPIO interface e.g. RTC reads
+                self.data = self.device.step(masked_value);
+            },
+            .Direction => self.direction = value,
+            .Control => self.cnt = value,
+        }
+    }
+
+    fn read(self: *const This, comptime reg: Register) if (reg == .Control) u1 else u4 {
+        if (self.cnt == 0) return 0;
+
+        return switch (reg) {
+            .Data => self.data & ~self.direction,
+            .Direction => self.direction,
+            .Control => self.cnt,
+        };
+    }
+};
+
+/// GBA Real Time Clock
+pub const Clock = struct {
+    const This = @This();
+
+    writer: Writer,
+    reader: Reader,
+    state: State,
+    cnt: Control,
+
+    year: u8,
+    month: u5,
+    day: u6,
+    weekday: u3,
+    hour: u6,
+    minute: u7,
+    second: u7,
+
+    cpu: *Arm7tdmi,
+    gpio: *const Gpio,
+
+    const Register = enum {
+        Control,
+        DateTime,
+        Time,
+    };
+
+    const State = union(enum) {
+        Idle,
+        Command,
+        Write: Register,
+        Read: Register,
+    };
+
+    const Reader = struct {
+        i: u4,
+        count: u8,
+
+        /// Reads a bit from RTC registers. Which bit it reads is dependent on
+        ///
+        /// 1. The RTC State Machine, whitch tells us which register we're accessing
+        /// 2. A `count`, which keeps track of which byte is currently being read
+        /// 3. An index, which keeps track of which bit of the byte determined by `count` is being read
+        fn read(self: *Reader, clock: *const Clock, register: Register) u1 {
+            const idx = @intCast(u3, self.i);
+            defer self.i += 1;
+
+            // FIXME: What do I do about the unused bits?
+            return switch (register) {
+                .Control => @truncate(u1, switch (self.count) {
+                    0 => clock.cnt.raw >> idx,
+                    else => std.debug.panic("Tried to read from byte #{} of {} (hint: there's only 1 byte)", .{ self.count, register }),
+                }),
+                .DateTime => @truncate(u1, switch (self.count) {
+                    // Date
+                    0 => clock.year >> idx,
+                    1 => @as(u8, clock.month) >> idx,
+                    2 => @as(u8, clock.day) >> idx,
+                    3 => @as(u8, clock.weekday) >> idx,
+
+                    // Time
+                    4 => @as(u8, clock.hour) >> idx,
+                    5 => @as(u8, clock.minute) >> idx,
+                    6 => @as(u8, clock.second) >> idx,
+                    else => std.debug.panic("Tried to read from byte #{} of {} (hint: there's only 7 bytes)", .{ self.count, register }),
+                }),
+                .Time => @truncate(u1, switch (self.count) {
+                    0 => @as(u8, clock.hour) >> idx,
+                    1 => @as(u8, clock.minute) >> idx,
+                    2 => @as(u8, clock.second) >> idx,
+                    else => std.debug.panic("Tried to read from byte #{} of {} (hint: there's only 3 bytes)", .{ self.count, register }),
+                }),
+            };
+        }
+
+        /// Is true when a Reader has read a u8's worth of bits
+        fn finished(self: *const Reader) bool {
+            return self.i >= 8;
+        }
+
+        /// Resets the index used to shift bits out of RTC registers
+        /// and `count`, which is used to keep track of which byte we're reading
+        /// is incremeneted
+        fn lap(self: *Reader) void {
+            self.i = 0;
+            self.count += 1;
+        }
+
+        /// Resets the state of a `Reader` in preparation for a future
+        /// read command
+        fn reset(self: *Reader) void {
+            self.i = 0;
+            self.count = 0;
+        }
+    };
+
+    const Writer = struct {
+        buf: u8,
+        i: u4,
+
+        /// The Number of bytes written since last reset
+        count: u8,
+
+        /// Append a bit to the internal bit buffer (aka an integer)
+        fn push(self: *Writer, value: u1) void {
+            const idx = @intCast(u3, self.i);
+            self.buf = (self.buf & ~(@as(u8, 1) << idx)) | @as(u8, value) << idx;
+            self.i += 1;
+        }
+
+        /// Takes the contents of the internal buffer and writes it to an RTC register
+        /// Where it writes to is dependent on:
+        ///
+        /// 1. The RTC State Machine, whitch tells us which register we're accessing
+        /// 2. A `count`, which keeps track of which byte is currently being read
+        fn write(self: *const Writer, clock: *Clock, register: Register) void {
+            // FIXME: What do do about unused bits?
+            switch (register) {
+                .Control => switch (self.count) {
+                    0 => clock.cnt.raw = (clock.cnt.raw & 0x80) | (self.buf & 0x7F), // Bit 7 read-only
+                    else => std.debug.panic("Tried to write to byte #{} of {} (hint: there's only 1 byte)", .{ self.count, register }),
+                },
+                .DateTime, .Time => log.debug("RTC: Ignoring {} write", .{register}),
+            }
+        }
+
+        /// Is true when 8 bits have been shifted into the internal buffer
+        fn finished(self: *const Writer) bool {
+            return self.i >= 8;
+        }
+
+        /// Resets the internal buffer
+        /// resets the index used to shift bits into the internal buffer
+        /// increments `count` (which keeps track of byte offsets) by one
+        fn lap(self: *Writer) void {
+            self.buf = 0;
+            self.i = 0;
+            self.count += 1;
+        }
+
+        /// Resets `Writer` to a clean state in preparation for a future write command
+        fn reset(self: *Writer) void {
+            self.buf = 0;
+            self.i = 0;
+            self.count = 0;
+        }
+    };
+
+    const Data = extern union {
+        sck: Bit(u8, 0),
+        sio: Bit(u8, 1),
+        cs: Bit(u8, 2),
+        raw: u8,
+    };
+
+    const Control = extern union {
+        /// Unknown, value should be preserved though
+        unk: Bit(u8, 1),
+        /// Per-minute IRQ
+        /// If set, fire a Gamepak IRQ every 30s,
+        irq: Bit(u8, 3),
+        /// 12/24 Hour Bit
+        /// If set, 12h mode
+        /// If cleared, 24h mode
+        mode: Bit(u8, 6),
+        /// Read-Only, bit cleared on read
+        /// If is set, means that there has been a failure / time has been lost
+        off: Bit(u8, 7),
+        raw: u8,
+    };
+
+    fn init(ptr: *This, cpu: *Arm7tdmi, gpio: *const Gpio) void {
+        ptr.* = .{
+            .writer = .{ .buf = 0, .i = 0, .count = 0 },
+            .reader = .{ .i = 0, .count = 0 },
+            .state = .Idle,
+            .cnt = .{ .raw = 0 },
+            .year = 0x01,
+            .month = 0x6,
+            .day = 0x13,
+            .weekday = 0x3,
+            .hour = 0x23,
+            .minute = 0x59,
+            .second = 0x59,
+            .cpu = cpu,
+            .gpio = gpio, // Can't use Arm7tdmi ptr b/c not initialized yet
+        };
+
+        cpu.sched.push(.RealTimeClock, 1 << 24); // Every Second
+    }
+
+    pub fn updateTime(self: *This, late: u64) void {
+        self.cpu.sched.push(.RealTimeClock, (1 << 24) -| late); // Reschedule
+
+        const now = DateTime.now();
+        self.year = toBcd(u8, @intCast(u8, now.date.year - 2000));
+        self.month = toBcd(u5, now.date.month);
+        self.day = toBcd(u6, now.date.day);
+        self.weekday = toBcd(u3, (now.date.weekday() + 1) % 7); // API is Monday = 0, Sunday = 6. We want Sunday = 0, Saturday = 6
+        self.hour = toBcd(u6, now.time.hour);
+        self.minute = toBcd(u7, now.time.minute);
+        self.second = toBcd(u7, now.time.second);
+    }
+
+    fn step(self: *This, value: Data) u4 {
+        const cache: Data = .{ .raw = self.gpio.data };
+
+        return switch (self.state) {
+            .Idle => blk: {
+                // FIXME: Maybe check incoming value to see if SCK is also high?
+                if (cache.sck.read()) {
+                    if (!cache.cs.read() and value.cs.read()) {
+                        log.debug("RTC: Entering Command Mode", .{});
+                        self.state = .Command;
+                    }
+                }
+
+                break :blk @truncate(u4, value.raw);
+            },
+            .Command => blk: {
+                if (!value.cs.read()) log.err("RTC: Expected CS to be set during {}, however CS was cleared", .{self.state});
+
+                // If SCK rises, sample SIO
+                if (!cache.sck.read() and value.sck.read()) {
+                    self.writer.push(@boolToInt(value.sio.read()));
+
+                    if (self.writer.finished()) {
+                        self.state = self.processCommand(self.writer.buf);
+                        self.writer.reset();
+
+                        log.debug("RTC: Switching to {}", .{self.state});
+                    }
+                }
+
+                break :blk @truncate(u4, value.raw);
+            },
+            .Write => |register| blk: {
+                if (!value.cs.read()) log.err("RTC: Expected CS to be set during {}, however CS was cleared", .{self.state});
+
+                // If SCK rises, sample SIO
+                if (!cache.sck.read() and value.sck.read()) {
+                    self.writer.push(@boolToInt(value.sio.read()));
+
+                    const register_width: u32 = switch (register) {
+                        .Control => 1,
+                        .DateTime => 7,
+                        .Time => 3,
+                    };
+
+                    if (self.writer.finished()) {
+                        self.writer.write(self, register); // write inner buffer to RTC register
+                        self.writer.lap();
+
+                        if (self.writer.count == register_width) {
+                            self.writer.reset();
+                            self.state = .Idle;
+                        }
+                    }
+                }
+
+                break :blk @truncate(u4, value.raw);
+            },
+            .Read => |register| blk: {
+                if (!value.cs.read()) log.err("RTC: Expected CS to be set during {}, however CS was cleared", .{self.state});
+                var ret = value;
+
+                // if SCK rises, sample SIO
+                if (!cache.sck.read() and value.sck.read()) {
+                    ret.sio.write(self.reader.read(self, register) == 0b1);
+
+                    const register_width: u32 = switch (register) {
+                        .Control => 1,
+                        .DateTime => 7,
+                        .Time => 3,
+                    };
+
+                    if (self.reader.finished()) {
+                        self.reader.lap();
+
+                        if (self.reader.count == register_width) {
+                            self.reader.reset();
+                            self.state = .Idle;
+                        }
+                    }
+                }
+
+                break :blk @truncate(u4, ret.raw);
+            },
+        };
+    }
+
+    fn reset(self: *This) void {
+        // mGBA and NBA only zero the control register. We will do the same
+        log.debug("RTC: Reset (control register was zeroed)", .{});
+
+        self.cnt.raw = 0;
+    }
+
+    fn irq(self: *This) void {
+        // TODO: Confirm that this is the right behaviour
+        log.debug("RTC: Force GamePak IRQ", .{});
+
+        self.cpu.bus.io.irq.game_pak.set();
+        self.cpu.handleInterrupt();
+    }
+
+    fn processCommand(self: *This, raw_command: u8) State {
+        const command = blk: {
+            // If High Nybble is 0x6, no need to switch the endianness
+            if (raw_command >> 4 & 0xF == 0x6) break :blk raw_command;
+
+            // Turns out reversing the order of bits isn't trivial at all
+            // https://stackoverflow.com/questions/2602823/in-c-c-whats-the-simplest-way-to-reverse-the-order-of-bits-in-a-byte
+            var ret = raw_command;
+            ret = (ret & 0xF0) >> 4 | (ret & 0x0F) << 4;
+            ret = (ret & 0xCC) >> 2 | (ret & 0x33) << 2;
+            ret = (ret & 0xAA) >> 1 | (ret & 0x55) << 1;
+
+            break :blk ret;
+        };
+        log.debug("RTC: Handling Command 0x{X:0>2} [0b{b:0>8}]", .{ command, command });
+
+        const is_write = command & 1 == 0;
+        const rtc_register = @truncate(u3, command >> 1 & 0x7);
+
+        if (is_write) {
+            return switch (rtc_register) {
+                0 => blk: {
+                    self.reset();
+                    break :blk .Idle;
+                },
+                1 => .{ .Write = .Control },
+                2 => .{ .Write = .DateTime },
+                3 => .{ .Write = .Time },
+                6 => blk: {
+                    self.irq();
+                    break :blk .Idle;
+                },
+                4, 5, 7 => .Idle,
+            };
+        } else {
+            return switch (rtc_register) {
+                1 => .{ .Read = .Control },
+                2 => .{ .Read = .DateTime },
+                3 => .{ .Read = .Time },
+                0, 4, 5, 6, 7 => .Idle, // Do Nothing
+            };
+        }
+    }
+};
+
+fn toBcd(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);
+}

src/core/bus/backup.zig

@@ -61,7 +61,7 @@ pub const Backup = struct {
         return backup;
     }
 
-    pub fn guessKind(rom: []const u8) ?Kind {
+    pub fn guessKind(rom: []const u8) Kind {
         for (backup_kinds) |needle| {
             const needle_len = needle.str.len;
 
@@ -71,7 +71,7 @@ pub const Backup = struct {
             }
         }
 
-        return null;
+        return .None;
     }
 
     pub fn deinit(self: *Self) void {

src/core/bus/dma.zig

@@ -1,11 +1,10 @@
 const std = @import("std");
+const util = @import("../util.zig");
 
 const DmaControl = @import("io.zig").DmaControl;
 const Bus = @import("../Bus.zig");
 const Arm7tdmi = @import("../cpu.zig").Arm7tdmi;
 
-const readUndefined = @import("../util.zig").readUndefined;
-const writeUndefined = @import("../util.zig").writeUndefined;
 pub const DmaTuple = std.meta.Tuple(&[_]type{ DmaController(0), DmaController(1), DmaController(2), DmaController(3) });
 const log = std.log.scoped(.DmaTransfer);
 
@@ -13,7 +12,7 @@ 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 {
+pub fn read(comptime T: type, dma: *const DmaTuple, addr: u32) ?T {
     const byte = @truncate(u8, addr);
 
     return switch (T) {
@@ -22,16 +21,16 @@ pub fn read(comptime T: type, dma: *const DmaTuple, addr: u32) T {
             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 => readUndefined(log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+            else => util.io.read.undef(T, log, "Tried to perform a {} read to 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 => readUndefined(log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         },
-        u8 => readUndefined(log, "Tried to perform a {} read to 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"),
     };
 }
@@ -53,7 +52,7 @@ pub fn write(comptime T: type, dma: *DmaTuple, addr: u32, value: T) void {
             0xD4 => dma.*[3].setSad(value),
             0xD8 => dma.*[3].setDad(value),
             0xDC => dma.*[3].setCnt(value),
-            else => writeUndefined(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
+            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].setSad(setU32L(dma.*[0].sad, value)),
@@ -83,9 +82,9 @@ pub fn write(comptime T: type, dma: *DmaTuple, addr: u32, value: T) void {
             0xDA => dma.*[3].setDad(setU32H(dma.*[3].dad, value)),
             0xDC => dma.*[3].setCntL(value),
             0xDE => dma.*[3].setCntH(value),
-            else => writeUndefined(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
+            else => util.io.write.undef(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
-        u8 => writeUndefined(log, "Tried to write 0x{X:0>2}{} 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 }),
         else => @compileError("DMA: Unsupported write width"),
     }
 }

src/core/bus/io.zig

@@ -1,5 +1,9 @@
 const std = @import("std");
 const builtin = @import("builtin");
+const timer = @import("timer.zig");
+const dma = @import("dma.zig");
+const apu = @import("../apu.zig");
+const util = @import("../util.zig");
 
 const Bit = @import("bitfield").Bit;
 const Bitfield = @import("bitfield").Bitfield;
@@ -7,12 +11,6 @@ const Bus = @import("../Bus.zig");
 const DmaController = @import("dma.zig").DmaController;
 const Scheduler = @import("../scheduler.zig").Scheduler;
 
-const timer = @import("timer.zig");
-const dma = @import("dma.zig");
-const apu = @import("../apu.zig");
-
-const readUndefined = @import("../util.zig").readUndefined;
-const writeUndefined = @import("../util.zig").writeUndefined;
 const log = std.log.scoped(.@"I/O");
 
 pub const Io = struct {
@@ -43,7 +41,7 @@ pub const Io = struct {
     }
 };
 
-pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
+pub fn read(bus: *const Bus, comptime T: type, address: u32) ?T {
     return switch (T) {
         u32 => switch (address) {
             // Display
@@ -58,18 +56,18 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
             0x0400_0100...0x0400_010C => timer.read(T, &bus.tim, address),
 
             // Serial Communication 1
-            0x0400_0128 => readTodo("Read {} from SIOCNT and SIOMLT_SEND", .{T}),
+            0x0400_0128 => util.io.read.todo(log, "Read {} from SIOCNT and SIOMLT_SEND", .{T}),
 
             // Keypad Input
-            0x0400_0130 => readTodo("Read {} from KEYINPUT", .{T}),
+            0x0400_0130 => util.io.read.todo(log, "Read {} from KEYINPUT", .{T}),
 
             // Serial Communication 2
-            0x0400_0150 => readTodo("Read {} from JOY_RECV", .{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_0208 => @boolToInt(bus.io.ime),
-            else => readUndefined(log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, address }),
+            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, address }),
         },
         u16 => switch (address) {
             // Display
@@ -80,8 +78,10 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
             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 => readTodo("Read {} from MOSAIC", .{T}),
+            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,
 
             // Sound
             0x0400_0060...0x0400_009E => apu.read(T, &bus.apu, address),
@@ -93,20 +93,20 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
             0x0400_0100...0x0400_010E => timer.read(T, &bus.tim, address),
 
             // Serial Communication 1
-            0x0400_0128 => readTodo("Read {} from SIOCNT", .{T}),
+            0x0400_0128 => util.io.read.todo(log, "Read {} from SIOCNT", .{T}),
 
             // Keypad Input
             0x0400_0130 => bus.io.keyinput.raw,
 
             // Serial Communication 2
-            0x0400_0134 => readTodo("Read {} from RCNT", .{T}),
+            0x0400_0134 => util.io.read.todo(log, "Read {} from RCNT", .{T}),
 
             // Interrupts
             0x0400_0200 => bus.io.ie.raw,
             0x0400_0202 => bus.io.irq.raw,
-            0x0400_0204 => readTodo("Read {} from WAITCNT", .{T}),
+            0x0400_0204 => util.io.read.todo(log, "Read {} from WAITCNT", .{T}),
             0x0400_0208 => @boolToInt(bus.io.ime),
-            else => readUndefined(log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, address }),
+            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, address }),
         },
         u8 => return switch (address) {
             // Display
@@ -123,18 +123,18 @@ pub fn read(bus: *const Bus, comptime T: type, address: u32) T {
             0x0400_0060...0x0400_00A7 => apu.read(T, &bus.apu, address),
 
             // Serial Communication 1
-            0x0400_0128 => readTodo("Read {} from SIOCNT_L", .{T}),
+            0x0400_0128 => util.io.read.todo(log, "Read {} from SIOCNT_L", .{T}),
 
             // Keypad Input
-            0x0400_0130 => readTodo("read {} from KEYINPUT_L", .{T}),
+            0x0400_0130 => util.io.read.todo(log, "read {} from KEYINPUT_L", .{T}),
 
             // Serial Communication 2
-            0x0400_0135 => readTodo("Read {} from RCNT_H", .{T}),
+            0x0400_0135 => util.io.read.todo(log, "Read {} from RCNT_H", .{T}),
 
             // Interrupts
             0x0400_0200 => @truncate(T, bus.io.ie.raw),
             0x0400_0300 => @enumToInt(bus.io.postflg),
-            else => readUndefined(log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, address }),
+            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"),
     };
@@ -210,7 +210,7 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             0x0400_0204 => log.debug("Wrote 0x{X:0>8} to WAITCNT", .{value}),
             0x0400_0208 => bus.io.ime = value & 1 == 1,
             0x0400_020C...0x0400_021C => {}, // Unused
-            else => writeUndefined(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, address }),
+            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
@@ -292,7 +292,7 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             0x0400_0204 => log.debug("Wrote 0x{X:0>4} to WAITCNT", .{value}),
             0x0400_0208 => bus.io.ime = value & 1 == 1,
             0x0400_0206, 0x0400_020A => {}, // Not Used
-            else => writeUndefined(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, address }),
+            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
@@ -302,6 +302,14 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             0x0400_0009 => bus.ppu.bg[0].cnt.raw = (@as(u16, value) << 8) | (bus.ppu.bg[0].cnt.raw & 0xFF),
             0x0400_000A => bus.ppu.bg[1].cnt.raw = (bus.ppu.bg[1].cnt.raw & 0xFF00) | value,
             0x0400_000B => bus.ppu.bg[1].cnt.raw = (@as(u16, value) << 8) | (bus.ppu.bg[1].cnt.raw & 0xFF),
+            0x0400_0040 => bus.ppu.win.h[0].set(.Lo, value),
+            0x0400_0041 => bus.ppu.win.h[0].set(.Hi, value),
+            0x0400_0042 => bus.ppu.win.h[1].set(.Lo, value),
+            0x0400_0043 => bus.ppu.win.h[1].set(.Hi, value),
+            0x0400_0044 => bus.ppu.win.v[0].set(.Lo, value),
+            0x0400_0045 => bus.ppu.win.v[0].set(.Hi, value),
+            0x0400_0046 => bus.ppu.win.v[1].set(.Lo, value),
+            0x0400_0047 => bus.ppu.win.v[1].set(.Hi, value),
             0x0400_0048 => bus.ppu.win.setInL(value),
             0x0400_0049 => bus.ppu.win.setInH(value),
             0x0400_004A => bus.ppu.win.setOutL(value),
@@ -325,17 +333,12 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             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 }),
-            else => writeUndefined(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, address }),
+            else => util.io.write.undef(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, address }),
         },
         else => @compileError("I/O: Unsupported write width"),
     };
 }
 
-fn readTodo(comptime format: []const u8, args: anytype) u8 {
-    log.debug(format, args);
-    return 0;
-}
-
 /// Read / Write
 pub const PostFlag = enum(u1) {
     FirstBoot = 0,
@@ -464,37 +467,57 @@ pub const BldY = extern union {
     raw: u16,
 };
 
+const u8WriteKind = enum { Hi, Lo };
+
 /// Write-only
 pub const WinH = extern union {
+    const Self = @This();
+
     x2: Bitfield(u16, 0, 8),
     x1: Bitfield(u16, 8, 8),
     raw: u16,
+
+    pub fn set(self: *Self, comptime K: u8WriteKind, value: u8) void {
+        self.raw = switch (K) {
+            .Hi => (@as(u16, value) << 8) | self.raw & 0xFF,
+            .Lo => (self.raw & 0xFF00) | value,
+        };
+    }
 };
 
 /// Write-only
 pub const WinV = extern union {
+    const Self = @This();
+
     y2: Bitfield(u16, 0, 8),
     y1: Bitfield(u16, 8, 8),
     raw: u16,
+
+    pub fn set(self: *Self, comptime K: u8WriteKind, value: u8) void {
+        self.raw = switch (K) {
+            .Hi => (@as(u16, value) << 8) | self.raw & 0xFF,
+            .Lo => (self.raw & 0xFF00) | value,
+        };
+    }
 };
 
 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,
 };
 

src/core/bus/timer.zig

@@ -1,4 +1,5 @@
 const std = @import("std");
+const util = @import("../util.zig");
 
 const TimerControl = @import("io.zig").TimerControl;
 const Io = @import("io.zig").Io;
@@ -6,8 +7,6 @@ const Scheduler = @import("../scheduler.zig").Scheduler;
 const Event = @import("../scheduler.zig").Event;
 const Arm7tdmi = @import("../cpu.zig").Arm7tdmi;
 
-const readUndefined = @import("../util.zig").readUndefined;
-const writeUndefined = @import("../util.zig").writeUndefined;
 pub const TimerTuple = std.meta.Tuple(&[_]type{ Timer(0), Timer(1), Timer(2), Timer(3) });
 const log = std.log.scoped(.Timer);
 
@@ -15,7 +14,7 @@ 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 {
+pub fn read(comptime T: type, tim: *const TimerTuple, addr: u32) ?T {
     const nybble = @truncate(u4, addr);
 
     return switch (T) {
@@ -24,7 +23,7 @@ pub fn read(comptime T: type, tim: *const TimerTuple, addr: u32) T {
             0x4 => @as(T, tim.*[1].cnt.raw) << 16 | tim.*[1].getCntL(),
             0x8 => @as(T, tim.*[2].cnt.raw) << 16 | tim.*[2].getCntL(),
             0xC => @as(T, tim.*[3].cnt.raw) << 16 | tim.*[3].getCntL(),
-            else => readUndefined(log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         },
         u16 => switch (nybble) {
             0x0 => tim.*[0].getCntL(),
@@ -35,9 +34,9 @@ pub fn read(comptime T: type, tim: *const TimerTuple, addr: u32) T {
             0xA => tim.*[2].cnt.raw,
             0xC => tim.*[3].getCntL(),
             0xE => tim.*[3].cnt.raw,
-            else => readUndefined(log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
+            else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         },
-        u8 => readUndefined(log, "Tried to perform a {} read to 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("TIM: Unsupported read width"),
     };
 }
@@ -51,7 +50,7 @@ pub fn write(comptime T: type, tim: *TimerTuple, addr: u32, value: T) void {
             0x4 => tim.*[1].setCnt(value),
             0x8 => tim.*[2].setCnt(value),
             0xC => tim.*[3].setCnt(value),
-            else => writeUndefined(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
+            else => util.io.write.undef(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
         u16 => switch (nybble) {
             0x0 => tim.*[0].setCntL(value),
@@ -62,9 +61,9 @@ pub fn write(comptime T: type, tim: *TimerTuple, addr: u32, value: T) void {
             0xA => tim.*[2].setCntH(value),
             0xC => tim.*[3].setCntL(value),
             0xE => tim.*[3].setCntH(value),
-            else => writeUndefined(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
+            else => util.io.write.undef(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
-        u8 => writeUndefined(log, "Tried to write 0x{X:0>2}{} 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 }),
         else => @compileError("TIM: Unsupported write width"),
     };
 }

src/core/emu.zig

@@ -13,10 +13,12 @@ const Atomic = std.atomic.Atomic;
 const Allocator = std.mem.Allocator;
 
 // TODO: Move these to a TOML File
-const sync_audio = true; // Enable Audio Sync
+const sync_audio = false; // Enable Audio Sync
 const sync_video: RunKind = .LimitedFPS; // Configure Video Sync
 pub const win_scale = 3; // 1x, 2x, 3x, etc. Window Scaling
 pub const cpu_logging = false; // Enable detailed CPU logging
+pub const allow_unhandled_io = true; // Only relevant in Debug Builds
+pub const force_rtc = false;
 
 // 228 Lines which consist of 308 dots (which are 4 cycles long)
 const cycles_per_frame: u64 = 228 * (308 * 4); //280896

src/core/ppu.zig

@@ -277,16 +277,17 @@ pub const Ppu = struct {
             aff_x += self.aff_bg[n - 2].pa;
             aff_y += self.aff_bg[n - 2].pc;
 
-            if (!shouldDrawBackground(n, self.bldcnt, &self.scanline, i)) continue;
+            const x = @bitCast(u32, ix);
+            const y = @bitCast(u32, iy);
+
+            const win_bounds = self.windowBounds(@truncate(u9, x), @truncate(u8, y));
+            if (!shouldDrawBackground(self, n, win_bounds, i)) continue;
 
             if (self.bg[n].cnt.display_overflow.read()) {
                 ix = if (ix > px_width) @rem(ix, px_width) else if (ix < 0) px_width + @rem(ix, px_width) else ix;
                 iy = if (iy > px_height) @rem(iy, px_height) else if (iy < 0) px_height + @rem(iy, px_height) else iy;
             } else if (ix > px_width or iy > px_height or ix < 0 or iy < 0) continue;
 
-            const x = @bitCast(u32, ix);
-            const y = @bitCast(u32, iy);
-
             const tile_id: u32 = self.vram.read(u8, screen_base + ((y / 8) * @bitCast(u32, tile_width) + (x / 8)));
             const row = y & 7;
             const col = x & 7;
@@ -296,7 +297,7 @@ pub const Ppu = struct {
 
             if (pal_id != 0) {
                 const bgr555 = self.palette.read(u16, pal_id * 2);
-                copyToBackgroundBuffer(n, self.bldcnt, &self.scanline, i, bgr555);
+                self.copyToBackgroundBuffer(n, win_bounds, i, bgr555);
             }
         }
 
@@ -305,7 +306,7 @@ pub const Ppu = struct {
         self.aff_bg[n - 2].y_latch.? += self.aff_bg[n - 2].pd; // PD is added to BGxY
     }
 
-    fn drawBackround(self: *Self, comptime n: u2) void {
+    fn drawBackground(self: *Self, comptime n: u2) void {
         // A Tile in a charblock is a byte, while a Screen Entry is a halfword
 
         const char_base = 0x4000 * @as(u32, self.bg[n].cnt.char_base.read());
@@ -325,10 +326,11 @@ pub const Ppu = struct {
 
         var i: u32 = 0;
         while (i < width) : (i += 1) {
-            if (!shouldDrawBackground(n, self.bldcnt, &self.scanline, i)) continue;
-
             const x = hofs + i;
 
+            const win_bounds = self.windowBounds(@truncate(u9, x), @truncate(u8, y));
+            if (!shouldDrawBackground(self, n, win_bounds, i)) continue;
+
             // Grab the Screen Entry from VRAM
             const entry_addr = screen_base + tilemapOffset(size, x, y);
             const entry = @bitCast(ScreenEntry, self.vram.read(u16, entry_addr));
@@ -353,7 +355,7 @@ pub const Ppu = struct {
 
             if (pal_id != 0) {
                 const bgr555 = self.palette.read(u16, pal_id * 2);
-                copyToBackgroundBuffer(n, self.bldcnt, &self.scanline, i, bgr555);
+                self.copyToBackgroundBuffer(n, win_bounds, i, bgr555);
             }
         }
     }
@@ -379,10 +381,10 @@ pub const Ppu = struct {
                 var layer: usize = 0;
                 while (layer < 4) : (layer += 1) {
                     self.drawSprites(@truncate(u2, layer));
-                    if (layer == self.bg[0].cnt.priority.read() and bg_enable & 1 == 1) self.drawBackround(0);
+                    if (layer == self.bg[0].cnt.priority.read() and bg_enable & 1 == 1) self.drawBackground(0);
-                    if (layer == self.bg[1].cnt.priority.read() and bg_enable >> 1 & 1 == 1) self.drawBackround(1);
+                    if (layer == self.bg[1].cnt.priority.read() and bg_enable >> 1 & 1 == 1) self.drawBackground(1);
-                    if (layer == self.bg[2].cnt.priority.read() and bg_enable >> 2 & 1 == 1) self.drawBackround(2);
+                    if (layer == self.bg[2].cnt.priority.read() and bg_enable >> 2 & 1 == 1) self.drawBackground(2);
-                    if (layer == self.bg[3].cnt.priority.read() and bg_enable >> 3 & 1 == 1) self.drawBackround(3);
+                    if (layer == self.bg[3].cnt.priority.read() and bg_enable >> 3 & 1 == 1) self.drawBackground(3);
                 }
 
                 // Copy Drawn Scanline to Frame Buffer
@@ -407,8 +409,8 @@ pub const Ppu = struct {
                 var layer: usize = 0;
                 while (layer < 4) : (layer += 1) {
                     self.drawSprites(@truncate(u2, layer));
-                    if (layer == self.bg[0].cnt.priority.read() and bg_enable & 1 == 1) self.drawBackround(0);
+                    if (layer == self.bg[0].cnt.priority.read() and bg_enable & 1 == 1) self.drawBackground(0);
-                    if (layer == self.bg[1].cnt.priority.read() and bg_enable >> 1 & 1 == 1) self.drawBackround(1);
+                    if (layer == self.bg[1].cnt.priority.read() and bg_enable >> 1 & 1 == 1) self.drawBackground(1);
                     if (layer == self.bg[2].cnt.priority.read() and bg_enable >> 2 & 1 == 1) self.drawAffineBackground(2);
                 }
 
@@ -533,6 +535,93 @@ pub const Ppu = struct {
         return self.palette.getBackdrop();
     }
 
+    fn copyToBackgroundBuffer(self: *Self, comptime n: u2, bounds: ?WindowBounds, i: usize, bgr555: u16) void {
+        if (self.bldcnt.mode.read() != 0b00) {
+            // Standard Alpha Blending
+            const a_layers = self.bldcnt.layer_a.read();
+            const is_blend_enabled = (a_layers >> n) & 1 == 1;
+
+            // If Alpha Blending is enabled and we've found an eligible layer for
+            // Pixel A, store the pixel in the bottom pixel buffer
+
+            const win_part = if (bounds) |win| blk: {
+                // Window Enabled
+                break :blk switch (win) {
+                    .win0 => self.win.in.w0_bld.read(),
+                    .win1 => self.win.in.w1_bld.read(),
+                    .out => self.win.out.out_bld.read(),
+                };
+            } else true;
+
+            if (win_part and is_blend_enabled) {
+                self.scanline.btm()[i] = bgr555;
+                return;
+            }
+        }
+
+        self.scanline.top()[i] = bgr555;
+    }
+
+    const WindowBounds = enum { win0, win1, out };
+
+    fn windowBounds(self: *Self, x: u9, y: u8) ?WindowBounds {
+        const win0 = self.dispcnt.win_enable.read() & 1 == 1;
+        const win1 = (self.dispcnt.win_enable.read() >> 1) & 1 == 1;
+        const winObj = self.dispcnt.obj_win_enable.read();
+
+        if (!(win0 or win1 or winObj)) return null;
+
+        if (win0 and self.win.inRange(0, x, y)) return .win0;
+        if (win1 and self.win.inRange(1, x, y)) return .win1;
+
+        return .out;
+    }
+
+    fn shouldDrawBackground(self: *Self, comptime n: u2, bounds: ?WindowBounds, i: usize) bool {
+        // If a pixel has been drawn on the top layer, it's because:
+        // 1. The pixel is to be blended with a pixel on the bottom layer
+        // 2. The pixel is not to be blended at all
+        // Also, if we find a pixel on the top layer we don't need to bother with this I think?
+        if (self.scanline.top()[i] != null) return false;
+
+        if (bounds) |win| {
+            switch (win) {
+                .win0 => if ((self.win.in.w0_bg.read() >> n) & 1 == 0) return false,
+                .win1 => if ((self.win.in.w1_bg.read() >> n) & 1 == 0) return false,
+                .out => if ((self.win.out.out_bg.read() >> n) & 1 == 0) return false,
+            }
+        }
+
+        if (self.scanline.btm()[i] != null) {
+            // The pixel found in the bottom layer is:
+            // 1. From a higher priority background
+            // 2. From a background that is marked for blending (Pixel A)
+
+            // If Alpha Blending isn't enabled, then we've already found a higher prio
+            // pixel, we can return early
+            if (self.bldcnt.mode.read() != 0b01) return false;
+
+            const b_layers = self.bldcnt.layer_b.read();
+
+            const win_part = if (bounds) |win| blk: {
+                // Window Enabled
+                break :blk switch (win) {
+                    .win0 => self.win.in.w0_bld.read(),
+                    .win1 => self.win.in.w1_bld.read(),
+                    .out => self.win.out.out_bld.read(),
+                };
+            } else true;
+
+            // If the Background is not marked for blending, we've already found
+            // a higher priority pixel, move on.
+
+            const is_blend_enabled = win_part and ((b_layers >> n) & 1 == 1);
+            if (!is_blend_enabled) return false;
+        }
+
+        return true;
+    }
+
     // TODO: Comment this + get a better understanding
     fn tilemapOffset(size: u2, x: u32, y: u32) u32 {
         // Current Row: (y % PIXEL_COUNT) / 8
@@ -794,6 +883,25 @@ const Window = struct {
         };
     }
 
+    fn inRange(self: *const Self, comptime id: u1, x: u9, y: u8) bool {
+        const h = self.h[id];
+        const v = self.v[id];
+
+        const y1 = v.y1.read();
+        const y2 = if (y1 > v.y2.read()) 160 else std.math.min(160, v.y2.read());
+
+        if (y1 <= y and y < y2) {
+            // Within Y bounds
+            const x1 = h.x1.read();
+            const x2 = if (x1 > h.x2.read()) 240 else std.math.min(240, h.x2.read());
+
+            // Within X Bounds
+            return x1 <= x and x < x2;
+        }
+
+        return false;
+    }
+
     pub fn setH(self: *Self, value: u32) void {
         self.h[0].raw = @truncate(u16, value);
         self.h[1].raw = @truncate(u16, value >> 16);
@@ -1135,37 +1243,6 @@ fn alphaBlend(top: u16, btm: u16, bldalpha: io.BldAlpha) u16 {
     return (bld_b << 10) | (bld_g << 5) | bld_r;
 }
 
-fn shouldDrawBackground(comptime n: u2, bldcnt: io.BldCnt, scanline: *Scanline, i: usize) bool {
-    // If a pixel has been drawn on the top layer, it's because
-    // Either the pixel is to be blended with a pixel on the bottom layer
-    // or the pixel is not to be blended at all
-    // Consequentially, if we find a pixel on the top layer, there's no need
-    // to render anything I think?
-    if (scanline.top()[i] != null) return false;
-
-    if (scanline.btm()[i] != null) {
-        // The Pixel found in the Bottom layer is
-        // 1. From a higher priority
-        // 2. From a Backround that is marked for Blending (Pixel A)
-        //
-        // We now have to confirm whether this current Background can be used
-        // as Pixel B or not.
-
-        // If Alpha Blending isn't enabled, we've aready found a higher
-        // priority pixel to render. Move on
-        if (bldcnt.mode.read() != 0b01) return false;
-
-        const b_layers = bldcnt.layer_b.read();
-        const is_blend_enabled = (b_layers >> n) & 1 == 1;
-
-        // If the Background is not marked for blending, we've already found
-        // a higher priority pixel, move on.
-        if (!is_blend_enabled) return false;
-    }
-
-    return true;
-}
-
 fn shouldDrawSprite(bldcnt: io.BldCnt, scanline: *Scanline, x: u9) bool {
     if (scanline.top()[x] != null) return false;
 
@@ -1180,23 +1257,6 @@ fn shouldDrawSprite(bldcnt: io.BldCnt, scanline: *Scanline, x: u9) bool {
     return true;
 }
 
-fn copyToBackgroundBuffer(comptime n: u2, bldcnt: io.BldCnt, scanline: *Scanline, i: usize, bgr555: u16) void {
-    if (bldcnt.mode.read() != 0b00) {
-        // Standard Alpha Blending
-        const a_layers = bldcnt.layer_a.read();
-        const is_blend_enabled = (a_layers >> n) & 1 == 1;
-
-        // If Alpha Blending is enabled and we've found an eligible layer for
-        // Pixel A, store the pixel in the bottom pixel buffer
-        if (is_blend_enabled) {
-            scanline.btm()[i] = bgr555;
-            return;
-        }
-    }
-
-    scanline.top()[i] = bgr555;
-}
-
 fn copyToSpriteBuffer(bldcnt: io.BldCnt, scanline: *Scanline, x: u9, bgr555: u16) void {
     if (bldcnt.mode.read() != 0b00) {
         // Alpha Blending

src/core/scheduler.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 
 const Bus = @import("Bus.zig");
 const Arm7tdmi = @import("cpu.zig").Arm7tdmi;
+const Clock = @import("bus/GamePak.zig").Clock;
 
 const Order = std.math.Order;
 const PriorityQueue = std.PriorityQueue;
@@ -60,6 +61,13 @@ pub const Scheduler = struct {
                         3 => cpu.bus.apu.ch4.channelTimerOverflow(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.updateTime(late);
+                },
                 .FrameSequencer => cpu.bus.apu.tickFrameSequencer(late),
                 .SampleAudio => cpu.bus.apu.sampleAudio(late),
                 .HBlank => cpu.bus.ppu.handleHBlankEnd(cpu, late), // The end of a HBlank
@@ -118,4 +126,5 @@ pub const EventKind = union(enum) {
     SampleAudio,
     FrameSequencer,
     ApuChannel: u2,
+    RealTimeClock,
 };

src/core/util.zig

@@ -3,6 +3,8 @@ const builtin = @import("builtin");
 const Log2Int = std.math.Log2Int;
 const Arm7tdmi = @import("cpu.zig").Arm7tdmi;
 
+const allow_unhandled_io = @import("emu.zig").allow_unhandled_io;
+
 // 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
@@ -102,6 +104,28 @@ pub const FilePaths = struct {
     save: ?[]const u8,
 };
 
+pub const io = struct {
+    pub const read = struct {
+        pub fn todo(comptime log: anytype, comptime format: []const u8, args: anytype) u8 {
+            log.debug(format, args);
+            return 0;
+        }
+
+        pub fn undef(comptime T: type, log: anytype, comptime format: []const u8, args: anytype) ?T {
+            log.warn(format, args);
+            if (builtin.mode == .Debug and !allow_unhandled_io) std.debug.panic("TODO: Implement I/O Register", .{});
+
+            return null;
+        }
+    };
+
+    pub const write = struct {
+        pub fn undef(log: anytype, comptime format: []const u8, args: anytype) void {
+            log.warn(format, args);
+            if (builtin.mode == .Debug and !allow_unhandled_io) std.debug.panic("TODO: Implement I/O Register", .{});
+        }
+    };
+};
 pub fn readUndefined(log: anytype, comptime format: []const u8, args: anytype) u8 {
     log.warn(format, args);
     if (builtin.mode == .Debug) std.debug.panic("TODO: Implement I/O Register", .{});