chore: shorten `orelse @panic` to `.?`

u4's are no longer supported in extern unions :\

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`.
 

lib/SDL.zig

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

src/core/Bus.zig

@@ -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

@@ -1,5 +1,7 @@
 const std = @import("std");
 
+const Bit = @import("bitfield").Bit;
+const Bitfield = @import("bitfield").Bitfield;
 const Backup = @import("backup.zig").Backup;
 const Allocator = std.mem.Allocator;
 const log = std.log.scoped(.GamePak);
@@ -10,6 +12,7 @@ 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 {
     const file = try std.fs.cwd().openFile(rom_path, .{});
@@ -19,17 +22,35 @@ pub fn init(allocator: Allocator, rom_path: []const u8, save_path: ?[]const u8)
     const title = parseTitle(file_buf);
     const kind = Backup.guessKind(file_buf) orelse .None;
 
-    const pak = Self{
+    var pak = Self{
         .buf = file_buf,
         .allocator = allocator,
         .title = title,
         .backup = try Backup.init(allocator, kind, title, save_path),
+        .gpio = Gpio.init(allocator, .Rtc),
     };
     pak.parseHeader();
 
     return pak;
 }
 
+/// Configures any GPIO Device that may be enabled
+///
+/// Fundamentally, this just passes a pointer to the initialized GPIO struct to whatever heap allocated GPIO Device struct
+/// we happen to be using
+///
+/// WARNIG: As far as I know, this method must be called in main() or else we'll have a dangling pointer issue
+/// Despite using the General Purpose Allocator, Zig doesn't prevent me from doing this :sadface:
+pub fn setupGpio(self: *Self) void {
+    switch (self.gpio.device.kind) {
+        .Rtc => {
+            const clock = @ptrCast(*Clock, @alignCast(@alignOf(*Clock), self.gpio.device.ptr.?));
+            Clock.init(clock, &self.gpio);
+        },
+        .None => {},
+    }
+}
+
 fn parseHeader(self: *const Self) void {
     const title = parseTitle(self.buf);
     const code = self.buf[0xAC..0xB0];
@@ -60,6 +81,7 @@ inline fn isLarge(self: *const Self) bool {
 
 pub fn deinit(self: *Self) void {
     self.backup.deinit();
+    self.gpio.deinit(self.allocator);
     self.allocator.free(self.buf);
     self.* = undefined;
 }
@@ -83,6 +105,35 @@ 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 +192,23 @@ 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 +240,341 @@ 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,
+        // TODO: Maybe make this comptime known? Removes some if statements
+        kind: Kind,
+
+        const Kind = enum {
+            Rtc,
+            None,
+        };
+
+        fn step(self: *Device, value: u4) void {
+            switch (self.kind) {
+                .Rtc => {
+                    const clock = @ptrCast(*Clock, @alignCast(@alignOf(*Clock), self.ptr.?));
+
+                    clock.step(Clock.GpioData{ .raw = value });
+                },
+                .None => {},
+            }
+        }
+
+        fn init(kind: Kind, ptr: ?*anyopaque) Device {
+            return .{ .kind = kind, .ptr = ptr };
+        }
+    };
+
+    const Register = enum {
+        Data,
+        Direction,
+        Control,
+    };
+
+    fn init(allocator: Allocator, kind: Device.Kind) This {
+        return .{
+            .data = 0b0000,
+            .direction = 0b1111, // TODO: What is GPIO Direction set to by default?
+            .cnt = 0b0,
+
+            .device = switch (kind) {
+                .Rtc => blk: {
+                    const ptr = allocator.create(Clock) catch @panic("Failed to allocate RTC struct on heap");
+                    break :blk Device{ .kind = kind, .ptr = ptr };
+                },
+                .None => Device{ .kind = kind, .ptr = null },
+            },
+        };
+    }
+
+    fn deinit(self: This, allocator: Allocator) void {
+        switch (self.device.kind) {
+            .Rtc => {
+                allocator.destroy(@ptrCast(*Clock, @alignCast(@alignOf(*Clock), self.device.ptr.?)));
+            },
+            .None => {},
+        }
+    }
+
+    fn write(self: *This, comptime reg: Register, value: if (reg == .Control) u1 else u4) void {
+        log.debug("RTC: Wrote 0b{b:0>4} to {}", .{ value, reg });
+
+        // if (reg == .Data)
+        // log.err("original: 0b{b:0>4} masked: 0b{b:0>4} result: 0b{b:0>4}", .{ self.data, value & self.direction, self.data | (value & self.direction) });
+
+        switch (reg) {
+            .Data => {
+                const masked_value = value & self.direction;
+
+                self.device.step(masked_value);
+                self.data = 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
+const Clock = struct {
+    const This = @This();
+
+    cmd: Command,
+    writer: Writer,
+    state: State,
+    cnt: Control,
+
+    year: u8,
+    month: u5,
+    day: u6,
+    day_of_week: u3,
+    hour: u6,
+    minute: u7,
+    second: u7,
+
+    gpio: *const Gpio,
+
+    const Register = enum {
+        Control,
+        DateTime,
+        Time,
+    };
+
+    const State = union(enum) {
+        Idle,
+        CommandInput,
+        Write: Register,
+        Read: Register,
+    };
+
+    const Writer = struct {
+        buf: u8,
+        i: u4,
+
+        /// The Number of bytes written to since last reset
+        count: u8,
+
+        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;
+        }
+
+        fn lap(self: *Writer) void {
+            self.buf = 0;
+            self.i = 0;
+            self.count += 1;
+        }
+
+        fn reset(self: *Writer) void {
+            self.buf = 0;
+            self.i = 0;
+            self.count = 0;
+        }
+
+        fn isFinished(self: *const Writer) bool {
+            return self.i >= 8;
+        }
+
+        fn getCount(self: *const Writer) u8 {
+            return self.count;
+        }
+
+        fn getValue(self: *const Writer) u8 {
+            return self.buf;
+        }
+    };
+
+    const Command = struct {
+        buf: u8,
+        i: u4,
+
+        fn push(self: *Command, value: u1) void {
+            const idx = @intCast(u3, self.i);
+            self.buf = (self.buf & ~(@as(u8, 1) << idx)) | @as(u8, value) << idx;
+            self.i += 1;
+        }
+
+        fn reset(self: *Command) void {
+            self.buf = 0;
+            self.i = 0;
+        }
+
+        fn isFinished(self: *const Command) bool {
+            return self.i >= 8;
+        }
+
+        fn getCommand(self: *const Command) u8 {
+            // If high Nybble does not contain 0x6, reverse the order of the nybbles.
+            // For some reason RTC commands can be LSB or MSB which is funny
+            return if (self.buf >> 4 & 0xF == 0x6) self.buf else (self.buf & 0xF) << 4 | (self.buf >> 4 & 0xF);
+        }
+
+        fn handleCommand(self: *const Command, rtc: *Clock) State {
+            log.info("RTC: Failed to handle Command 0b{b:0>8} aka 0x{X:0>2}", .{ self.buf, self.buf });
+            const command = self.getCommand();
+
+            const is_write = command & 1 == 0;
+            const rtc_register = @intCast(u3, command >> 1 & 0x7); // TODO: Make Truncate
+
+            if (is_write) {
+                return switch (rtc_register) {
+                    0 => blk: {
+                        rtc.reset();
+                        break :blk .Idle;
+                    },
+                    1 => .{ .Write = .Control },
+                    2 => .{ .Write = .DateTime },
+                    3 => .{ .Write = .Time },
+                    6 => blk: {
+                        rtc.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
+                };
+            }
+        }
+    };
+
+    const GpioData = 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, gpio: *const Gpio) void {
+        ptr.* = .{
+            .cmd = .{ .buf = 0, .i = 0 },
+            .writer = .{ .buf = 0, .i = 0, .count = 0 },
+            .state = .Idle,
+            .cnt = .{ .raw = 0 },
+            .year = 0,
+            .month = 0,
+            .day = 0,
+            .day_of_week = 0,
+            .hour = 0,
+            .minute = 0,
+            .second = 0,
+            .gpio = gpio,
+        };
+    }
+
+    fn attachGpio(self: *This, gpio: *const Gpio) void {
+        self.gpio = gpio;
+    }
+
+    fn step(self: *This, value: GpioData) void {
+        const cache: GpioData = .{ .raw = self.gpio.data };
+
+        switch (self.state) {
+            .Idle => {
+                // If SCK is high and CS rises, then prepare for Command
+                // 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.err("RTC: Entering Command Mode", .{});
+                        self.state = .CommandInput;
+                        self.cmd.reset();
+                    }
+                }
+            },
+            .CommandInput => {
+                if (!value.cs.read()) log.err("RTC: Expected CS to be set during {}, however CS was cleared", .{self.state});
+
+                if (!cache.sck.read() and value.sck.read()) {
+                    // If SCK rises, sample SIO
+                    log.debug("RTC: Sampled 0b{b:0>1} from SIO", .{@boolToInt(value.sio.read())});
+                    self.cmd.push(@boolToInt(value.sio.read()));
+
+                    if (self.cmd.isFinished()) {
+                        self.state = self.cmd.handleCommand(self);
+                    }
+                }
+            },
+            State{ .Write = .Control } => {
+                if (!value.cs.read()) log.err("RTC: Expected CS to be set during {}, however CS was cleared", .{self.state});
+
+                if (!cache.sck.read() and value.sck.read()) {
+                    // If SCK rises, sample SIO
+
+                    log.debug("RTC: Sampled 0b{b:0>1} from SIO", .{@boolToInt(value.sio.read())});
+                    self.writer.push(@boolToInt(value.sio.read()));
+
+                    if (self.writer.isFinished()) {
+                        self.writer.lap();
+                        self.cnt.raw = self.writer.getValue();
+
+                        // FIXME: Move this to a constant or something
+                        if (self.writer.getCount() == 1) {
+                            self.writer.reset();
+                            self.state = .Idle;
+                        }
+                    }
+                }
+            },
+            else => {
+                // TODO: Implement Read/Writes for Date/Time and Time and Control
+                log.err("RTC: Ignored request to handle {} command", .{self.state});
+                self.state = .Idle;
+            },
+        }
+    }
+
+    fn reset(self: *This) void {
+        // mGBA and NBA only zero the control register
+        // we'll do the same
+        self.cnt.raw = 0;
+        log.info("RTC: Reset executed (control register was zeroed)", .{});
+    }
+
+    fn irq(_: *const This) void {
+        // TODO: Force GamePak IRQ
+        log.err("RTC: TODO: Force GamePak IRQ", .{});
+    }
+};

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
@@ -80,7 +78,7 @@ 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,
 
             // Sound
@@ -93,20 +91,20 @@ 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 +121,18 @@ 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 +208,7 @@ 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 +290,7 @@ 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
@@ -325,17 +323,12 @@ 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,

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 +13,11 @@ 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
 
 // 228 Lines which consist of 308 dots (which are 4 cycles long)
 const cycles_per_frame: u64 = 228 * (308 * 4); //280896

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", .{});

src/main.zig

@@ -52,6 +52,7 @@ pub fn main() anyerror!void {
     if (paths.bios == null) cpu.fastBoot();
 
     try bus.init(allocator, &scheduler, &cpu, paths);
+    bus.pak.setupGpio(); // FIXME: Can I not call this in main()?
     defer bus.deinit();
 
     var gui = Gui.init(bus.pak.title, width, height);