chore(emu): refactor code

Also took the chance to rework parts of the logic that determines
ZBA's save path

lib/zig-clap

@@ -1 +1 @@
-Subproject commit 4f4196fc3bc95c4bd3b12ce2e4a5f1050742cd3c
+Subproject commit e5d09c4b2d121025ad7195b2de704451e6306807

src/core/apu/Noise.zig

@@ -0,0 +1,142 @@
+const io = @import("../bus/io.zig");
+const util = @import("../../util.zig");
+
+const Scheduler = @import("../scheduler.zig").Scheduler;
+const FrameSequencer = @import("../apu.zig").FrameSequencer;
+const Tick = @import("../apu.zig").Apu.Tick;
+const Envelope = @import("device/Envelope.zig");
+const Length = @import("device/Length.zig");
+const Lfsr = @import("signal/Lfsr.zig");
+
+const Self = @This();
+
+/// Write-only
+/// NR41
+len: u6,
+/// NR42
+envelope: io.Envelope,
+/// NR43
+poly: io.PolyCounter,
+/// NR44
+cnt: io.NoiseControl,
+
+/// Length Functionarlity
+len_dev: Length,
+
+/// Envelope Functionality
+env_dev: Envelope,
+
+// Linear Feedback Shift Register
+lfsr: Lfsr,
+
+enabled: bool,
+sample: i8,
+
+pub fn init(sched: *Scheduler) Self {
+    return .{
+        .len = 0,
+        .envelope = .{ .raw = 0 },
+        .poly = .{ .raw = 0 },
+        .cnt = .{ .raw = 0 },
+        .enabled = false,
+
+        .len_dev = Length.create(),
+        .env_dev = Envelope.create(),
+        .lfsr = Lfsr.create(sched),
+
+        .sample = 0,
+    };
+}
+
+pub fn reset(self: *Self) void {
+    self.len = 0;
+    self.envelope.raw = 0;
+    self.poly.raw = 0;
+    self.cnt.raw = 0;
+
+    self.sample = 0;
+    self.enabled = false;
+}
+
+pub fn tick(self: *Self, comptime kind: Tick) void {
+    switch (kind) {
+        .Length => self.len_dev.tick(self.cnt.length_enable.read(), &self.enabled),
+        .Envelope => self.env_dev.tick(self.envelope),
+        .Sweep => @compileError("Channel 4 does not implement Sweep"),
+    }
+}
+
+/// NR41, NR42
+pub fn sound4CntL(self: *const Self) u16 {
+    return @as(u16, self.envelope.raw) << 8;
+}
+
+/// NR41, NR42
+pub fn setSound4CntL(self: *Self, value: u16) void {
+    self.setNr41(@truncate(u8, value));
+    self.setNr42(@truncate(u8, value >> 8));
+}
+
+/// NR41
+pub fn setNr41(self: *Self, len: u8) void {
+    self.len = @truncate(u6, len);
+    self.len_dev.timer = @as(u7, 64) - @truncate(u6, len);
+}
+
+/// NR42
+pub fn setNr42(self: *Self, value: u8) void {
+    self.envelope.raw = value;
+    if (!self.isDacEnabled()) self.enabled = false;
+}
+
+/// NR43, NR44
+pub fn sound4CntH(self: *const Self) u16 {
+    return @as(u16, self.poly.raw & 0x40) << 8 | self.cnt.raw;
+}
+
+/// NR43, NR44
+pub fn setSound4CntH(self: *Self, fs: *const FrameSequencer, value: u16) void {
+    self.poly.raw = @truncate(u8, value);
+    self.setNr44(fs, @truncate(u8, value >> 8));
+}
+
+/// NR44
+pub fn setNr44(self: *Self, fs: *const FrameSequencer, byte: u8) void {
+    var new: io.NoiseControl = .{ .raw = byte };
+
+    if (new.trigger.read()) {
+        self.enabled = true;
+
+        if (self.len_dev.timer == 0) {
+            self.len_dev.timer =
+                if (!fs.isLengthNext() and new.length_enable.read()) 63 else 64;
+        }
+
+        // Update The Frequency Timer
+        self.lfsr.reload(self.poly);
+        self.lfsr.shift = 0x7FFF;
+
+        // Update Envelope and Volume
+        self.env_dev.timer = self.envelope.period.read();
+        if (fs.isEnvelopeNext() and self.env_dev.timer != 0b111) self.env_dev.timer += 1;
+
+        self.env_dev.vol = self.envelope.init_vol.read();
+
+        self.enabled = self.isDacEnabled();
+    }
+
+    util.audio.length.ch4.update(self, fs, new);
+    self.cnt = new;
+}
+
+pub fn onNoiseEvent(self: *Self, late: u64) void {
+    self.lfsr.onLfsrTimerExpire(self.poly, late);
+
+    self.sample = 0;
+    if (!self.isDacEnabled()) return;
+    self.sample = if (self.enabled) self.lfsr.sample() * @as(i8, self.env_dev.vol) else 0;
+}
+
+fn isDacEnabled(self: *const Self) bool {
+    return self.envelope.raw & 0xF8 != 0x00;
+}

src/core/apu/Tone.zig

@@ -0,0 +1,138 @@
+const io = @import("../bus/io.zig");
+const util = @import("../../util.zig");
+
+const Scheduler = @import("../scheduler.zig").Scheduler;
+const FrameSequencer = @import("../apu.zig").FrameSequencer;
+const Tick = @import("../apu.zig").Apu.Tick;
+const Length = @import("device/Length.zig");
+const Envelope = @import("device/Envelope.zig");
+const Square = @import("signal/Square.zig");
+
+const Self = @This();
+
+/// NR21
+duty: io.Duty,
+/// NR22
+envelope: io.Envelope,
+/// NR23, NR24
+freq: io.Frequency,
+
+/// Length Functionarlity
+len_dev: Length,
+/// Envelope Functionality
+env_dev: Envelope,
+/// FrequencyTimer Functionality
+square: Square,
+
+enabled: bool,
+sample: i8,
+
+pub fn init(sched: *Scheduler) Self {
+    return .{
+        .duty = .{ .raw = 0 },
+        .envelope = .{ .raw = 0 },
+        .freq = .{ .raw = 0 },
+        .enabled = false,
+
+        .square = Square.init(sched),
+        .len_dev = Length.create(),
+        .env_dev = Envelope.create(),
+
+        .sample = 0,
+    };
+}
+
+pub fn reset(self: *Self) void {
+    self.duty.raw = 0;
+    self.envelope.raw = 0;
+    self.freq.raw = 0;
+
+    self.sample = 0;
+    self.enabled = false;
+}
+
+pub fn tick(self: *Self, comptime kind: Tick) void {
+    switch (kind) {
+        .Length => self.len_dev.tick(self.freq.length_enable.read(), &self.enabled),
+        .Envelope => self.env_dev.tick(self.envelope),
+        .Sweep => @compileError("Channel 2 does not implement Sweep"),
+    }
+}
+
+pub fn onToneEvent(self: *Self, late: u64) void {
+    self.square.onSquareTimerExpire(Self, self.freq, late);
+
+    self.sample = 0;
+    if (!self.isDacEnabled()) return;
+    self.sample = if (self.enabled) self.square.sample(self.duty) * @as(i8, self.env_dev.vol) else 0;
+}
+
+/// NR21, NR22
+pub fn sound2CntL(self: *const Self) u16 {
+    return @as(u16, self.envelope.raw) << 8 | (self.duty.raw & 0xC0);
+}
+
+/// NR21, NR22
+pub fn setSound2CntL(self: *Self, value: u16) void {
+    self.setNr21(@truncate(u8, value));
+    self.setNr22(@truncate(u8, value >> 8));
+}
+
+/// NR21
+pub fn setNr21(self: *Self, value: u8) void {
+    self.duty.raw = value;
+    self.len_dev.timer = @as(u7, 64) - @truncate(u6, value);
+}
+
+/// NR22
+pub fn setNr22(self: *Self, value: u8) void {
+    self.envelope.raw = value;
+    if (!self.isDacEnabled()) self.enabled = false;
+}
+
+/// NR23, NR24
+pub fn sound2CntH(self: *const Self) u16 {
+    return self.freq.raw & 0x4000;
+}
+
+/// NR23, NR24
+pub fn setSound2CntH(self: *Self, fs: *const FrameSequencer, value: u16) void {
+    self.setNr23(@truncate(u8, value));
+    self.setNr24(fs, @truncate(u8, value >> 8));
+}
+
+/// NR23
+pub fn setNr23(self: *Self, byte: u8) void {
+    self.freq.raw = (self.freq.raw & 0xFF00) | byte;
+}
+
+/// NR24
+pub fn setNr24(self: *Self, fs: *const FrameSequencer, byte: u8) void {
+    var new: io.Frequency = .{ .raw = (@as(u16, byte) << 8) | (self.freq.raw & 0xFF) };
+
+    if (new.trigger.read()) {
+        self.enabled = true;
+
+        if (self.len_dev.timer == 0) {
+            self.len_dev.timer =
+                if (!fs.isLengthNext() and new.length_enable.read()) 63 else 64;
+        }
+
+        self.square.reload(Self, self.freq.frequency.read());
+
+        // Reload Envelope period and timer
+        self.env_dev.timer = self.envelope.period.read();
+        if (fs.isEnvelopeNext() and self.env_dev.timer != 0b111) self.env_dev.timer += 1;
+
+        self.env_dev.vol = self.envelope.init_vol.read();
+
+        self.enabled = self.isDacEnabled();
+    }
+
+    util.audio.length.update(Self, self, fs, new);
+    self.freq = new;
+}
+
+fn isDacEnabled(self: *const Self) bool {
+    return self.envelope.raw & 0xF8 != 0;
+}

src/core/apu/ToneSweep.zig

@@ -0,0 +1,184 @@
+const io = @import("../bus/io.zig");
+const util = @import("../../util.zig");
+
+const Scheduler = @import("../scheduler.zig").Scheduler;
+const FrameSequencer = @import("../apu.zig").FrameSequencer;
+const Length = @import("device/Length.zig");
+const Envelope = @import("device/Envelope.zig");
+const Sweep = @import("device/Sweep.zig");
+const Square = @import("signal/Square.zig");
+
+const Tick = @import("../apu.zig").Apu.Tick;
+
+const Self = @This();
+
+/// NR10
+sweep: io.Sweep,
+/// NR11
+duty: io.Duty,
+/// NR12
+envelope: io.Envelope,
+/// NR13, NR14
+freq: io.Frequency,
+
+/// Length Functionality
+len_dev: Length,
+/// Sweep Functionality
+sweep_dev: Sweep,
+/// Envelope Functionality
+env_dev: Envelope,
+/// Frequency Timer Functionality
+square: Square,
+enabled: bool,
+
+sample: i8,
+
+pub fn init(sched: *Scheduler) Self {
+    return .{
+        .sweep = .{ .raw = 0 },
+        .duty = .{ .raw = 0 },
+        .envelope = .{ .raw = 0 },
+        .freq = .{ .raw = 0 },
+        .sample = 0,
+        .enabled = false,
+
+        .square = Square.init(sched),
+        .len_dev = Length.create(),
+        .sweep_dev = Sweep.create(),
+        .env_dev = Envelope.create(),
+    };
+}
+
+pub fn reset(self: *Self) void {
+    self.sweep.raw = 0;
+    self.sweep_dev.calc_performed = false;
+
+    self.duty.raw = 0;
+    self.envelope.raw = 0;
+    self.freq.raw = 0;
+
+    self.sample = 0;
+    self.enabled = false;
+}
+
+pub fn tick(self: *Self, comptime kind: Tick) void {
+    switch (kind) {
+        .Length => self.len_dev.tick(self.freq.length_enable.read(), &self.enabled),
+        .Envelope => self.env_dev.tick(self.envelope),
+        .Sweep => self.sweep_dev.tick(self),
+    }
+}
+
+pub fn onToneSweepEvent(self: *Self, late: u64) void {
+    self.square.onSquareTimerExpire(Self, self.freq, late);
+
+    self.sample = 0;
+    if (!self.isDacEnabled()) return;
+    self.sample = if (self.enabled) self.square.sample(self.duty) * @as(i8, self.env_dev.vol) else 0;
+}
+
+/// NR10, NR11, NR12
+pub fn setSound1Cnt(self: *Self, value: u32) void {
+    self.setSound1CntL(@truncate(u8, value));
+    self.setSound1CntH(@truncate(u16, value >> 16));
+}
+
+/// NR10
+pub fn sound1CntL(self: *const Self) u8 {
+    return self.sweep.raw & 0x7F;
+}
+
+/// NR10
+pub fn setSound1CntL(self: *Self, value: u8) void {
+    const new = io.Sweep{ .raw = value };
+
+    if (self.sweep.direction.read() and !new.direction.read()) {
+        // Sweep Negate bit has been cleared
+        // If At least 1 Sweep Calculation has been made since
+        // the last trigger, the channel is immediately disabled
+
+        if (self.sweep_dev.calc_performed) self.enabled = false;
+    }
+
+    self.sweep.raw = value;
+}
+
+/// NR11, NR12
+pub fn sound1CntH(self: *const Self) u16 {
+    return @as(u16, self.envelope.raw) << 8 | (self.duty.raw & 0xC0);
+}
+
+/// NR11, NR12
+pub fn setSound1CntH(self: *Self, value: u16) void {
+    self.setNr11(@truncate(u8, value));
+    self.setNr12(@truncate(u8, value >> 8));
+}
+
+/// NR11
+pub fn setNr11(self: *Self, value: u8) void {
+    self.duty.raw = value;
+    self.len_dev.timer = @as(u7, 64) - @truncate(u6, value);
+}
+
+/// NR12
+pub fn setNr12(self: *Self, value: u8) void {
+    self.envelope.raw = value;
+    if (!self.isDacEnabled()) self.enabled = false;
+}
+
+/// NR13, NR14
+pub fn sound1CntX(self: *const Self) u16 {
+    return self.freq.raw & 0x4000;
+}
+
+/// NR13, NR14
+pub fn setSound1CntX(self: *Self, fs: *const FrameSequencer, value: u16) void {
+    self.setNr13(@truncate(u8, value));
+    self.setNr14(fs, @truncate(u8, value >> 8));
+}
+
+/// NR13
+pub fn setNr13(self: *Self, byte: u8) void {
+    self.freq.raw = (self.freq.raw & 0xFF00) | byte;
+}
+
+/// NR14
+pub fn setNr14(self: *Self, fs: *const FrameSequencer, byte: u8) void {
+    var new: io.Frequency = .{ .raw = (@as(u16, byte) << 8) | (self.freq.raw & 0xFF) };
+
+    if (new.trigger.read()) {
+        self.enabled = true;
+
+        if (self.len_dev.timer == 0) {
+            self.len_dev.timer =
+                if (!fs.isLengthNext() and new.length_enable.read()) 63 else 64;
+        }
+
+        self.square.reload(Self, self.freq.frequency.read());
+
+        // Reload Envelope period and timer
+        self.env_dev.timer = self.envelope.period.read();
+        if (fs.isEnvelopeNext() and self.env_dev.timer != 0b111) self.env_dev.timer += 1;
+
+        self.env_dev.vol = self.envelope.init_vol.read();
+
+        // Sweep Trigger Behaviour
+        const sw_period = self.sweep.period.read();
+        const sw_shift = self.sweep.shift.read();
+
+        self.sweep_dev.calc_performed = false;
+        self.sweep_dev.shadow = self.freq.frequency.read();
+        self.sweep_dev.timer = if (sw_period == 0) 8 else sw_period;
+        self.sweep_dev.enabled = sw_period != 0 or sw_shift != 0;
+        if (sw_shift != 0) _ = self.sweep_dev.calculate(self.sweep, &self.enabled);
+
+        self.enabled = self.isDacEnabled();
+    }
+
+    util.audio.length.update(Self, self, fs, new);
+    self.freq = new;
+}
+
+fn isDacEnabled(self: *const Self) bool {
+    return self.envelope.raw & 0xF8 != 0;
+}

src/core/apu/Wave.zig

@@ -0,0 +1,132 @@
+const io = @import("../bus/io.zig");
+const util = @import("../../util.zig");
+
+const Scheduler = @import("../scheduler.zig").Scheduler;
+const FrameSequencer = @import("../apu.zig").FrameSequencer;
+const Tick = @import("../apu.zig").Apu.Tick;
+
+const Length = @import("device/Length.zig");
+const Wave = @import("signal/Wave.zig");
+
+const Self = @This();
+
+/// Write-only
+/// NR30
+select: io.WaveSelect,
+/// NR31
+length: u8,
+/// NR32
+vol: io.WaveVolume,
+/// NR33, NR34
+freq: io.Frequency,
+
+/// Length Functionarlity
+len_dev: Length,
+wave_dev: Wave,
+
+enabled: bool,
+sample: i8,
+
+pub fn init(sched: *Scheduler) Self {
+    return .{
+        .select = .{ .raw = 0 },
+        .vol = .{ .raw = 0 },
+        .freq = .{ .raw = 0 },
+        .length = 0,
+
+        .len_dev = Length.create(),
+        .wave_dev = Wave.init(sched),
+        .enabled = false,
+        .sample = 0,
+    };
+}
+
+pub fn reset(self: *Self) void {
+    self.select.raw = 0;
+    self.length = 0;
+    self.vol.raw = 0;
+    self.freq.raw = 0;
+
+    self.sample = 0;
+    self.enabled = false;
+}
+
+pub fn tick(self: *Self, comptime kind: Tick) void {
+    switch (kind) {
+        .Length => self.len_dev.tick(self.freq.length_enable.read(), &self.enabled),
+        .Envelope => @compileError("Channel 3 does not implement Envelope"),
+        .Sweep => @compileError("Channel 3 does not implement Sweep"),
+    }
+}
+
+/// NR30, NR31, NR32
+pub fn setSound3Cnt(self: *Self, value: u32) void {
+    self.setSound3CntL(@truncate(u8, value));
+    self.setSound3CntH(@truncate(u16, value >> 16));
+}
+
+/// NR30
+pub fn setSound3CntL(self: *Self, value: u8) void {
+    self.select.raw = value;
+    if (!self.select.enabled.read()) self.enabled = false;
+}
+
+/// NR31, NR32
+pub fn sound3CntH(self: *const Self) u16 {
+    return @as(u16, self.length & 0xE0) << 8;
+}
+
+/// NR31, NR32
+pub fn setSound3CntH(self: *Self, value: u16) void {
+    self.setNr31(@truncate(u8, value));
+    self.vol.raw = (@truncate(u8, value >> 8));
+}
+
+/// NR31
+pub fn setNr31(self: *Self, len: u8) void {
+    self.length = len;
+    self.len_dev.timer = 256 - @as(u9, len);
+}
+
+/// NR33, NR34
+pub fn setSound3CntX(self: *Self, fs: *const FrameSequencer, value: u16) void {
+    self.setNr33(@truncate(u8, value));
+    self.setNr34(fs, @truncate(u8, value >> 8));
+}
+
+/// NR33
+pub fn setNr33(self: *Self, byte: u8) void {
+    self.freq.raw = (self.freq.raw & 0xFF00) | byte;
+}
+
+/// NR34
+pub fn setNr34(self: *Self, fs: *const FrameSequencer, byte: u8) void {
+    var new: io.Frequency = .{ .raw = (@as(u16, byte) << 8) | (self.freq.raw & 0xFF) };
+
+    if (new.trigger.read()) {
+        self.enabled = true;
+
+        if (self.len_dev.timer == 0) {
+            self.len_dev.timer =
+                if (!fs.isLengthNext() and new.length_enable.read()) 255 else 256;
+        }
+
+        // Update The Frequency Timer
+        self.wave_dev.reload(self.freq.frequency.read());
+        self.wave_dev.offset = 0;
+
+        self.enabled = self.select.enabled.read();
+    }
+
+    util.audio.length.update(Self, self, fs, new);
+    self.freq = new;
+}
+
+pub fn onWaveEvent(self: *Self, late: u64) void {
+    self.wave_dev.onWaveTimerExpire(self.freq, self.select, late);
+
+    self.sample = 0;
+    if (!self.select.enabled.read()) return;
+    // Convert unsigned 4-bit wave sample to signed 8-bit sample
+    self.sample = (2 * @as(i8, self.wave_dev.sample(self.select)) - 15) >> self.wave_dev.shift(self.vol);
+}

src/core/apu/device/Envelope.zig

@@ -0,0 +1,28 @@
+const io = @import("../../bus/io.zig");
+
+const Self = @This();
+
+/// Period Timer
+timer: u3,
+/// Current Volume
+vol: u4,
+
+pub fn create() Self {
+    return .{ .timer = 0, .vol = 0 };
+}
+
+pub fn tick(self: *Self, nrx2: io.Envelope) void {
+    if (nrx2.period.read() != 0) {
+        if (self.timer != 0) self.timer -= 1;
+
+        if (self.timer == 0) {
+            self.timer = nrx2.period.read();
+
+            if (nrx2.direction.read()) {
+                if (self.vol < 0xF) self.vol += 1;
+            } else {
+                if (self.vol > 0x0) self.vol -= 1;
+            }
+        }
+    }
+}

src/core/apu/device/Length.zig

@@ -0,0 +1,18 @@
+const Self = @This();
+
+timer: u9,
+
+pub fn create() Self {
+    return .{ .timer = 0 };
+}
+
+pub fn tick(self: *Self, enabled: bool, ch_enable: *bool) void {
+    if (enabled) {
+        if (self.timer == 0) return;
+        self.timer -= 1;
+
+        // By returning early if timer == 0, this is only
+        // true if timer == 0 because of the decrement we just did
+        if (self.timer == 0) ch_enable.* = false;
+    }
+}

src/core/apu/device/Sweep.zig

@@ -0,0 +1,52 @@
+const io = @import("../../bus/io.zig");
+const ToneSweep = @import("../ToneSweep.zig");
+
+const Self = @This();
+
+timer: u8,
+enabled: bool,
+shadow: u11,
+
+calc_performed: bool,
+
+pub fn create() Self {
+    return .{
+        .timer = 0,
+        .enabled = false,
+        .shadow = 0,
+        .calc_performed = false,
+    };
+}
+
+pub fn tick(self: *Self, ch1: *ToneSweep) void {
+    if (self.timer != 0) self.timer -= 1;
+
+    if (self.timer == 0) {
+        const period = ch1.sweep.period.read();
+        self.timer = if (period == 0) 8 else period;
+        if (!self.calc_performed) self.calc_performed = true;
+
+        if (self.enabled and period != 0) {
+            const new_freq = self.calculate(ch1.sweep, &ch1.enabled);
+
+            if (new_freq <= 0x7FF and ch1.sweep.shift.read() != 0) {
+                ch1.freq.frequency.write(@truncate(u11, new_freq));
+                self.shadow = @truncate(u11, new_freq);
+
+                _ = self.calculate(ch1.sweep, &ch1.enabled);
+            }
+        }
+    }
+}
+
+/// Calculates the Sweep Frequency
+pub fn calculate(self: *Self, sweep: io.Sweep, ch_enable: *bool) u12 {
+    const shadow = @as(u12, self.shadow);
+    const shadow_shifted = shadow >> sweep.shift.read();
+    const decrease = sweep.direction.read();
+
+    const freq = if (decrease) shadow - shadow_shifted else shadow + shadow_shifted;
+    if (freq > 0x7FF) ch_enable.* = false;
+
+    return freq;
+}

src/core/apu/signal/Lfsr.zig

@@ -0,0 +1,59 @@
+const io = @import("../../bus/io.zig");
+
+/// Linear Feedback Shift Register
+const Scheduler = @import("../../scheduler.zig").Scheduler;
+const FrameSequencer = @import("../../apu.zig").FrameSequencer;
+const Noise = @import("../Noise.zig");
+
+const Self = @This();
+pub const interval: u64 = (1 << 24) / (1 << 22);
+
+shift: u15,
+timer: u16,
+
+sched: *Scheduler,
+
+pub fn create(sched: *Scheduler) Self {
+    return .{
+        .shift = 0,
+        .timer = 0,
+        .sched = sched,
+    };
+}
+
+pub fn sample(self: *const Self) i8 {
+    return if ((~self.shift & 1) == 1) 1 else -1;
+}
+
+/// Reload LFSR Timer
+pub fn reload(self: *Self, poly: io.PolyCounter) void {
+    self.sched.removeScheduledEvent(.{ .ApuChannel = 3 });
+
+    const div = Self.divisor(poly.div_ratio.read());
+    const timer = div << poly.shift.read();
+    self.sched.push(.{ .ApuChannel = 3 }, @as(u64, timer) * interval);
+}
+
+/// Scheduler Event Handler for LFSR Timer Expire
+/// FIXME: This gets called a lot, clogging up the Scheduler
+pub fn onLfsrTimerExpire(self: *Self, poly: io.PolyCounter, late: u64) void {
+    // Obscure: "Using a noise channel clock shift of 14 or 15
+    // results in the LFSR receiving no clocks."
+    if (poly.shift.read() >= 14) return;
+
+    const div = Self.divisor(poly.div_ratio.read());
+    const timer = div << poly.shift.read();
+
+    const tmp = (self.shift & 1) ^ ((self.shift & 2) >> 1);
+    self.shift = (self.shift >> 1) | (tmp << 14);
+
+    if (poly.width.read())
+        self.shift = (self.shift & ~@as(u15, 0x40)) | tmp << 6;
+
+    self.sched.push(.{ .ApuChannel = 3 }, @as(u64, timer) * interval -| late);
+}
+
+fn divisor(code: u3) u16 {
+    if (code == 0) return 8;
+    return @as(u16, code) << 4;
+}

src/core/apu/signal/Square.zig

@@ -0,0 +1,58 @@
+const std = @import("std");
+const io = @import("../../bus/io.zig");
+
+const Scheduler = @import("../../scheduler.zig").Scheduler;
+const FrameSequencer = @import("../../apu.zig").FrameSequencer;
+const ToneSweep = @import("../ToneSweep.zig");
+const Tone = @import("../Tone.zig");
+
+const Self = @This();
+pub const interval: u64 = (1 << 24) / (1 << 22);
+
+pos: u3,
+sched: *Scheduler,
+timer: u16,
+
+pub fn init(sched: *Scheduler) Self {
+    return .{
+        .timer = 0,
+        .pos = 0,
+        .sched = sched,
+    };
+}
+
+/// Scheduler Event Handler for Square Synth Timer Expire
+pub fn onSquareTimerExpire(self: *Self, comptime T: type, nrx34: io.Frequency, late: u64) void {
+    comptime std.debug.assert(T == ToneSweep or T == Tone);
+    self.pos +%= 1;
+
+    self.timer = (@as(u16, 2048) - nrx34.frequency.read()) * 4;
+    self.sched.push(.{ .ApuChannel = if (T == ToneSweep) 0 else 1 }, @as(u64, self.timer) * interval -| late);
+}
+
+/// Reload Square Wave Timer
+pub fn reload(self: *Self, comptime T: type, value: u11) void {
+    comptime std.debug.assert(T == ToneSweep or T == Tone);
+    const channel = if (T == ToneSweep) 0 else 1;
+
+    self.sched.removeScheduledEvent(.{ .ApuChannel = channel });
+
+    const tmp = (@as(u16, 2048) - value) * 4; // What Freq Timer should be assuming no weird behaviour
+    self.timer = (tmp & ~@as(u16, 0x3)) | self.timer & 0x3; // Keep the last two bits from the old timer;
+
+    self.sched.push(.{ .ApuChannel = channel }, @as(u64, self.timer) * interval);
+}
+
+pub fn sample(self: *const Self, nrx1: io.Duty) i8 {
+    const pattern = nrx1.pattern.read();
+
+    const i = self.pos ^ 7; // index of 0 should get highest bit
+    const result = switch (pattern) {
+        0b00 => @as(u8, 0b00000001) >> i, // 12.5%
+        0b01 => @as(u8, 0b00000011) >> i, // 25%
+        0b10 => @as(u8, 0b00001111) >> i, // 50%
+        0b11 => @as(u8, 0b11111100) >> i, // 75%
+    };
+
+    return if (result & 1 == 1) 1 else -1;
+}

src/core/apu/signal/Wave.zig

@@ -0,0 +1,79 @@
+const std = @import("std");
+const io = @import("../../bus/io.zig");
+
+const Scheduler = @import("../../scheduler.zig").Scheduler;
+const FrameSequencer = @import("../../apu.zig").FrameSequencer;
+const Wave = @import("../Wave.zig");
+
+const buf_len = 0x20;
+pub const interval: u64 = (1 << 24) / (1 << 22);
+const Self = @This();
+
+buf: [buf_len]u8,
+timer: u16,
+offset: u12,
+
+sched: *Scheduler,
+
+pub fn read(self: *const Self, comptime T: type, nr30: io.WaveSelect, addr: u32) T {
+    // TODO: Handle reads when Channel 3 is disabled
+    const base = if (!nr30.bank.read()) @as(u32, 0x10) else 0; // Read from the Opposite Bank in Use
+
+    const i = base + addr - 0x0400_0090;
+    return std.mem.readIntSliceLittle(T, self.buf[i..][0..@sizeOf(T)]);
+}
+
+pub fn write(self: *Self, comptime T: type, nr30: io.WaveSelect, addr: u32, value: T) void {
+    // TODO: Handle writes when Channel 3 is disabled
+    const base = if (!nr30.bank.read()) @as(u32, 0x10) else 0; // Write to the Opposite Bank in Use
+
+    const i = base + addr - 0x0400_0090;
+    std.mem.writeIntSliceLittle(T, self.buf[i..][0..@sizeOf(T)], value);
+}
+
+pub fn init(sched: *Scheduler) Self {
+    return .{
+        .buf = [_]u8{0x00} ** buf_len,
+        .timer = 0,
+        .offset = 0,
+        .sched = sched,
+    };
+}
+
+/// Reload internal Wave Timer
+pub fn reload(self: *Self, value: u11) void {
+    self.sched.removeScheduledEvent(.{ .ApuChannel = 2 });
+
+    self.timer = (@as(u16, 2048) - value) * 2;
+    self.sched.push(.{ .ApuChannel = 2 }, @as(u64, self.timer) * interval);
+}
+
+/// Scheduler Event Handler
+pub fn onWaveTimerExpire(self: *Self, nrx34: io.Frequency, nr30: io.WaveSelect, late: u64) void {
+    if (nr30.dimension.read()) {
+        self.offset = (self.offset + 1) % 0x40; // 0x20 bytes (both banks), which contain 2 samples each
+    } else {
+        self.offset = (self.offset + 1) % 0x20; // 0x10 bytes, which contain 2 samples each
+    }
+
+    self.timer = (@as(u16, 2048) - nrx34.frequency.read()) * 2;
+    self.sched.push(.{ .ApuChannel = 2 }, @as(u64, self.timer) * interval -| late);
+}
+
+/// Generate Sample from Wave Synth
+pub fn sample(self: *const Self, nr30: io.WaveSelect) u4 {
+    const base = if (nr30.bank.read()) @as(u32, 0x10) else 0;
+
+    const value = self.buf[base + self.offset / 2];
+    return if (self.offset & 1 == 0) @truncate(u4, value >> 4) else @truncate(u4, value);
+}
+
+/// TODO: Write comment
+pub fn shift(_: *const Self, nr32: io.WaveVolume) u2 {
+    return switch (nr32.kind.read()) {
+        0b00 => 3, // Mute / Zero
+        0b01 => 0, // 100% Volume
+        0b10 => 1, // 50% Volume
+        0b11 => 2, // 25% Volume
+    };
+}

src/core/bus/Bios.zig

@@ -12,6 +12,36 @@ allocator: Allocator,
 
 addr_latch: u32,
 
+pub fn read(self: *Self, comptime T: type, r15: u32, addr: u32) T {
+    if (r15 < Self.size) {
+        self.addr_latch = addr;
+        return self._read(T, addr);
+    }
+
+    log.debug("Rejected read since r15=0x{X:0>8}", .{r15});
+    return @truncate(T, self._read(T, self.addr_latch + 8));
+}
+
+pub fn dbgRead(self: *const Self, comptime T: type, r15: u32, addr: u32) T {
+    if (r15 < Self.size) return self._read(T, addr);
+    return @truncate(T, self._read(T, self.addr_latch + 8));
+}
+
+/// Read without the GBA safety checks
+fn _read(self: *const Self, comptime T: type, addr: u32) T {
+    const buf = self.buf orelse std.debug.panic("[BIOS] ZBA tried to read {} from 0x{X:0>8} but not BIOS was present", .{ T, addr });
+
+    return switch (T) {
+        u32, u16, u8 => std.mem.readIntSliceLittle(T, buf[addr..][0..@sizeOf(T)]),
+        else => @compileError("BIOS: Unsupported read width"),
+    };
+}
+
+pub fn write(_: *Self, comptime T: type, addr: u32, value: T) void {
+    @setCold(true);
+    log.debug("Tried to write {} 0x{X:} to 0x{X:0>8} ", .{ T, value, addr });
+}
+
 pub fn init(allocator: Allocator, maybe_path: ?[]const u8) !Self {
     const buf: ?[]u8 = if (maybe_path) |path| blk: {
         const file = try std.fs.cwd().openFile(path, .{});
@@ -31,34 +61,3 @@ pub fn deinit(self: *Self) void {
     if (self.buf) |buf| self.allocator.free(buf);
     self.* = undefined;
 }
-
-pub fn read(self: *Self, comptime T: type, r15: u32, addr: u32) T {
-    if (r15 < Self.size) {
-        self.addr_latch = addr;
-        return self.uncheckedRead(T, addr);
-    }
-
-    log.debug("Rejected read since r15=0x{X:0>8}", .{r15});
-    return @truncate(T, self.uncheckedRead(T, self.addr_latch + 8));
-}
-
-pub fn dbgRead(self: *const Self, comptime T: type, r15: u32, addr: u32) T {
-    if (r15 < Self.size) return self.uncheckedRead(T, addr);
-    return @truncate(T, self.uncheckedRead(T, self.addr_latch + 8));
-}
-
-fn uncheckedRead(self: *const Self, comptime T: type, addr: u32) T {
-    if (self.buf) |buf| {
-        return switch (T) {
-            u32, u16, u8 => std.mem.readIntSliceLittle(T, buf[addr..][0..@sizeOf(T)]),
-            else => @compileError("BIOS: Unsupported read width"),
-        };
-    }
-
-    std.debug.panic("[BIOS] ZBA tried to read {} from 0x{X:0>8} but not BIOS was present", .{ T, addr });
-}
-
-pub fn write(_: *Self, comptime T: type, addr: u32, value: T) void {
-    @setCold(true);
-    log.debug("Tried to write {} 0x{X:} to 0x{X:0>8} ", .{ T, value, addr });
-}

src/core/bus/Ewram.zig

@@ -7,21 +7,6 @@ const Self = @This();
 buf: []u8,
 allocator: Allocator,
 
-pub fn init(allocator: Allocator) !Self {
-    const buf = try allocator.alloc(u8, ewram_size);
-    std.mem.set(u8, buf, 0);
-
-    return Self{
-        .buf = buf,
-        .allocator = allocator,
-    };
-}
-
-pub fn deinit(self: *Self) void {
-    self.allocator.free(self.buf);
-    self.* = undefined;
-}
-
 pub fn read(self: *const Self, comptime T: type, address: usize) T {
     const addr = address & 0x3FFFF;
 
@@ -39,3 +24,18 @@ pub fn write(self: *const Self, comptime T: type, address: usize, value: T) void
         else => @compileError("EWRAM: Unsupported write width"),
     };
 }
+
+pub fn init(allocator: Allocator) !Self {
+    const buf = try allocator.alloc(u8, ewram_size);
+    std.mem.set(u8, buf, 0);
+
+    return Self{
+        .buf = buf,
+        .allocator = allocator,
+    };
+}
+
+pub fn deinit(self: *Self) void {
+    self.allocator.free(self.buf);
+    self.* = undefined;
+}

src/core/bus/GamePak.zig

@@ -20,78 +20,11 @@ allocator: Allocator,
 backup: Backup,
 gpio: *Gpio,
 
-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 = file_buf[0xA0..0xAC].*;
-    const kind = Backup.guessKind(file_buf);
-    const device = if (config.config().guest.force_rtc) .Rtc else guessDevice(file_buf);
-
-    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),
-    };
-}
-
-/// Searches the ROM to see if it can determine whether the ROM it's searching uses
-/// any GPIO device, like a RTC for example.
-fn guessDevice(buf: []const u8) Gpio.Device.Kind {
-    // Try to Guess if ROM uses RTC
-    const needle = "RTC_V"; // I was told SIIRTC_V, though Pokemen Firered (USA) is a false negative
-
-    var i: usize = 0;
-    while ((i + needle.len) < buf.len) : (i += 1) {
-        if (std.mem.eql(u8, needle, buf[i..(i + needle.len)])) return .Rtc;
-    }
-
-    // TODO: Detect other GPIO devices
-
-    return .None;
-}
-
-fn logHeader(buf: []const u8, title: *const [12]u8) void {
-    const code = buf[0xAC..0xB0];
-    const maker = buf[0xB0..0xB2];
-    const version = buf[0xBC];
-
-    log.info("Title: {s}", .{title});
-    if (version != 0) log.info("Version: {}", .{version});
-    log.info("Game Code: {s}", .{code});
-    if (lookupMaker(maker)) |c| log.info("Maker: {s}", .{c}) else log.info("Maker Code: {s}", .{maker});
-}
-
-fn lookupMaker(slice: *const [2]u8) ?[]const u8 {
-    const id = @as(u16, slice[1]) << 8 | @as(u16, slice[0]);
-    return switch (id) {
-        0x3130 => "Nintendo",
-        else => null,
-    };
-}
-
-inline fn isLarge(self: *const Self) bool {
-    return self.buf.len > 0x100_0000;
-}
-
-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;
-}
-
 pub fn read(self: *Self, comptime T: type, address: u32) T {
     const addr = address & 0x1FF_FFFF;
 
     if (self.backup.kind == .Eeprom) {
-        if (self.isLarge()) {
+        if (self.buf.len > 0x100_0000) { // Large
             // Addresses 0x1FF_FF00 to 0x1FF_FFFF are reserved from EEPROM accesses if
             // * Backup type is EEPROM
             // * Large ROM (Size is greater than 16MB)
@@ -143,11 +76,19 @@ pub fn read(self: *Self, comptime T: type, address: u32) T {
     };
 }
 
+inline fn get(self: *const Self, i: u32) u8 {
+    @setRuntimeSafety(false);
+    if (i < self.buf.len) return self.buf[i];
+
+    const lhs = i >> 1 & 0xFFFF;
+    return @truncate(u8, lhs >> 8 * @truncate(u5, i & 1));
+}
+
 pub fn dbgRead(self: *const Self, comptime T: type, address: u32) T {
     const addr = address & 0x1FF_FFFF;
 
     if (self.backup.kind == .Eeprom) {
-        if (self.isLarge()) {
+        if (self.buf.len > 0x100_0000) { // Large
             // Addresses 0x1FF_FF00 to 0x1FF_FFFF are reserved from EEPROM accesses if
             // * Backup type is EEPROM
             // * Large ROM (Size is greater than 16MB)
@@ -162,6 +103,35 @@ pub fn dbgRead(self: *const 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)),
@@ -176,7 +146,7 @@ pub fn write(self: *Self, comptime T: type, word_count: u16, address: u32, value
     if (self.backup.kind == .Eeprom) {
         const bit = @truncate(u1, value);
 
-        if (self.isLarge()) {
+        if (self.buf.len > 0x100_0000) { // Large
             // Addresses 0x1FF_FF00 to 0x1FF_FFFF are reserved from EEPROM accesses if
             // * Backup type is EEPROM
             // * Large ROM (Size is greater than 16MB)
@@ -214,12 +184,59 @@ pub fn write(self: *Self, comptime T: type, word_count: u16, address: u32, value
     }
 }
 
-fn get(self: *const Self, i: u32) u8 {
-    @setRuntimeSafety(false);
-    if (i < self.buf.len) return self.buf[i];
+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 lhs = i >> 1 & 0xFFFF;
-    return @truncate(u8, lhs >> 8 * @truncate(u5, i & 1));
+    const file_buf = try file.readToEndAlloc(allocator, try file.getEndPos());
+    const title = file_buf[0xA0..0xAC].*;
+    const kind = Backup.guess(file_buf);
+    const device = if (config.config().guest.force_rtc) .Rtc else guessDevice(file_buf);
+
+    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),
+    };
+}
+
+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;
+}
+
+/// Searches the ROM to see if it can determine whether the ROM it's searching uses
+/// any GPIO device, like a RTC for example.
+fn guessDevice(buf: []const u8) Gpio.Device.Kind {
+    // Try to Guess if ROM uses RTC
+    const needle = "RTC_V"; // I was told SIIRTC_V, though Pokemen Firered (USA) is a false negative
+
+    var i: usize = 0;
+    while ((i + needle.len) < buf.len) : (i += 1) {
+        if (std.mem.eql(u8, needle, buf[i..(i + needle.len)])) return .Rtc;
+    }
+
+    // TODO: Detect other GPIO devices
+
+    return .None;
+}
+
+fn logHeader(buf: []const u8, title: *const [12]u8) void {
+    const code = buf[0xAC..0xB0];
+    const maker = buf[0xB0..0xB2];
+    const version = buf[0xBC];
+
+    log.info("Title: {s}", .{title});
+    if (version != 0) log.info("Version: {}", .{version});
+    log.info("Game Code: {s}", .{code});
+    log.info("Maker Code: {s}", .{maker});
 }
 
 test "OOB Access" {

src/core/bus/Iwram.zig

@@ -7,21 +7,6 @@ const Self = @This();
 buf: []u8,
 allocator: Allocator,
 
-pub fn init(allocator: Allocator) !Self {
-    const buf = try allocator.alloc(u8, iwram_size);
-    std.mem.set(u8, buf, 0);
-
-    return Self{
-        .buf = buf,
-        .allocator = allocator,
-    };
-}
-
-pub fn deinit(self: *Self) void {
-    self.allocator.free(self.buf);
-    self.* = undefined;
-}
-
 pub fn read(self: *const Self, comptime T: type, address: usize) T {
     const addr = address & 0x7FFF;
 
@@ -39,3 +24,18 @@ pub fn write(self: *const Self, comptime T: type, address: usize, value: T) void
         else => @compileError("IWRAM: Unsupported write width"),
     };
 }
+
+pub fn init(allocator: Allocator) !Self {
+    const buf = try allocator.alloc(u8, iwram_size);
+    std.mem.set(u8, buf, 0);
+
+    return Self{
+        .buf = buf,
+        .allocator = allocator,
+    };
+}
+
+pub fn deinit(self: *Self) void {
+    self.allocator.free(self.buf);
+    self.* = undefined;
+}

src/core/bus/backup.zig

@@ -2,9 +2,13 @@ const std = @import("std");
 const Allocator = std.mem.Allocator;
 const log = std.log.scoped(.Backup);
 
+const Eeprom = @import("backup/eeprom.zig").Eeprom;
+const Flash = @import("backup/Flash.zig");
+
 const escape = @import("../../util.zig").escape;
 const span = @import("../../util.zig").span;
 
+const Needle = struct { str: []const u8, kind: Backup.Kind };
 const backup_kinds = [6]Needle{
     .{ .str = "EEPROM_V", .kind = .Eeprom },
     .{ .str = "SRAM_V", .kind = .Sram },
@@ -14,6 +18,8 @@ const backup_kinds = [6]Needle{
     .{ .str = "FLASH1M_V", .kind = .Flash1M },
 };
 
+const SaveError = error{Unsupported};
+
 pub const Backup = struct {
     const Self = @This();
 
@@ -35,122 +41,6 @@ pub const Backup = struct {
         None,
     };
 
-    pub fn init(allocator: Allocator, kind: Kind, title: [12]u8, path: ?[]const u8) !Self {
-        log.info("Kind: {}", .{kind});
-
-        const buf_size: usize = switch (kind) {
-            .Sram => 0x8000, // 32K
-            .Flash => 0x10000, // 64K
-            .Flash1M => 0x20000, // 128K
-            .None, .Eeprom => 0, // EEPROM is handled upon first Read Request to it
-        };
-
-        const buf = try allocator.alloc(u8, buf_size);
-        std.mem.set(u8, buf, 0xFF);
-
-        var backup = Self{
-            .buf = buf,
-            .allocator = allocator,
-            .kind = kind,
-            .title = title,
-            .save_path = path,
-            .flash = Flash.init(),
-            .eeprom = Eeprom.init(allocator),
-        };
-
-        if (backup.save_path) |p| backup.loadSaveFromDisk(allocator, p) catch |e| log.err("Failed to load save: {}", .{e});
-        return backup;
-    }
-
-    pub fn guessKind(rom: []const u8) Kind {
-        for (backup_kinds) |needle| {
-            const needle_len = needle.str.len;
-
-            var i: usize = 0;
-            while ((i + needle_len) < rom.len) : (i += 1) {
-                if (std.mem.eql(u8, needle.str, rom[i..][0..needle_len])) return needle.kind;
-            }
-        }
-
-        return .None;
-    }
-
-    pub fn deinit(self: *Self) void {
-        if (self.save_path) |path| self.writeSaveToDisk(self.allocator, path) catch |e| log.err("Failed to write save: {}", .{e});
-        self.allocator.free(self.buf);
-        self.* = undefined;
-    }
-
-    fn loadSaveFromDisk(self: *Self, allocator: Allocator, path: []const u8) !void {
-        const file_path = try self.getSaveFilePath(allocator, path);
-        defer allocator.free(file_path);
-
-        // FIXME: Don't rely on this lol
-        if (std.mem.eql(u8, file_path[file_path.len - 12 .. file_path.len], "untitled.sav")) {
-            return log.err("ROM header lacks title, no save loaded", .{});
-        }
-
-        const file: std.fs.File = try std.fs.openFileAbsolute(file_path, .{});
-        const file_buf = try file.readToEndAlloc(allocator, try file.getEndPos());
-        defer allocator.free(file_buf);
-
-        switch (self.kind) {
-            .Sram, .Flash, .Flash1M => {
-                if (self.buf.len == file_buf.len) {
-                    std.mem.copy(u8, self.buf, file_buf);
-                    return log.info("Loaded Save from {s}", .{file_path});
-                }
-
-                log.err("{s} is {} bytes, but we expected {} bytes", .{ file_path, file_buf.len, self.buf.len });
-            },
-            .Eeprom => {
-                if (file_buf.len == 0x200 or file_buf.len == 0x2000) {
-                    self.eeprom.kind = if (file_buf.len == 0x200) .Small else .Large;
-
-                    self.buf = try allocator.alloc(u8, file_buf.len);
-                    std.mem.copy(u8, self.buf, file_buf);
-                    return log.info("Loaded Save from {s}", .{file_path});
-                }
-
-                log.err("EEPROM can either be 0x200 bytes or 0x2000 byes, but {s} was {X:} bytes", .{
-                    file_path,
-                    file_buf.len,
-                });
-            },
-            .None => return SaveError.UnsupportedBackupKind,
-        }
-    }
-
-    fn getSaveFilePath(self: *const Self, allocator: Allocator, path: []const u8) ![]const u8 {
-        const filename = try self.getSaveFilename(allocator);
-        defer allocator.free(filename);
-
-        return try std.fs.path.join(allocator, &[_][]const u8{ path, filename });
-    }
-
-    fn getSaveFilename(self: *const Self, allocator: Allocator) ![]const u8 {
-        const title_str = span(&escape(self.title));
-        const name = if (title_str.len != 0) title_str else "untitled";
-
-        return try std.mem.concat(allocator, u8, &[_][]const u8{ name, ".sav" });
-    }
-
-    fn writeSaveToDisk(self: Self, allocator: Allocator, path: []const u8) !void {
-        const file_path = try self.getSaveFilePath(allocator, path);
-        defer allocator.free(file_path);
-
-        switch (self.kind) {
-            .Sram, .Flash, .Flash1M, .Eeprom => {
-                const file = try std.fs.createFileAbsolute(file_path, .{});
-                defer file.close();
-
-                try file.writeAll(self.buf);
-                log.info("Wrote Save to {s}", .{file_path});
-            },
-            else => return SaveError.UnsupportedBackupKind,
-        }
-    }
-
     pub fn read(self: *const Self, address: usize) u8 {
         const addr = address & 0xFFFF;
 
@@ -184,7 +74,7 @@ pub const Backup = struct {
         switch (self.kind) {
             .Flash, .Flash1M => {
                 if (self.flash.prep_write) return self.flash.write(self.buf, addr, byte);
-                if (self.flash.shouldEraseSector(addr, byte)) return self.flash.eraseSector(self.buf, addr);
+                if (self.flash.shouldEraseSector(addr, byte)) return self.flash.erase(self.buf, addr);
 
                 switch (addr) {
                     0x0000 => if (self.kind == .Flash1M and self.flash.set_bank) {
@@ -209,358 +99,121 @@ pub const Backup = struct {
             .None, .Eeprom => {},
         }
     }
+
+    pub fn init(allocator: Allocator, kind: Kind, title: [12]u8, path: ?[]const u8) !Self {
+        log.info("Kind: {}", .{kind});
+
+        const buf_size: usize = switch (kind) {
+            .Sram => 0x8000, // 32K
+            .Flash => 0x10000, // 64K
+            .Flash1M => 0x20000, // 128K
+            .None, .Eeprom => 0, // EEPROM is handled upon first Read Request to it
         };
 
-const Needle = struct {
-    const Self = @This();
-
-    str: []const u8,
-    kind: Backup.Kind,
-
-    fn init(str: []const u8, kind: Backup.Kind) Self {
-        return .{
-            .str = str,
-            .kind = kind,
-        };
-    }
-};
-
-const SaveError = error{
-    UnsupportedBackupKind,
-};
-
-const Flash = struct {
-    const Self = @This();
-
-    state: State,
-
-    id_mode: bool,
-    set_bank: bool,
-    prep_erase: bool,
-    prep_write: bool,
-
-    bank: u1,
-
-    const State = enum {
-        Ready,
-        Set,
-        Command,
-    };
-
-    fn init() Self {
-        return .{
-            .state = .Ready,
-            .id_mode = false,
-            .set_bank = false,
-            .prep_erase = false,
-            .prep_write = false,
-            .bank = 0,
-        };
-    }
-
-    fn handleCommand(self: *Self, buf: []u8, byte: u8) void {
-        switch (byte) {
-            0x90 => self.id_mode = true,
-            0xF0 => self.id_mode = false,
-            0xB0 => self.set_bank = true,
-            0x80 => self.prep_erase = true,
-            0x10 => {
+        const buf = try allocator.alloc(u8, buf_size);
         std.mem.set(u8, buf, 0xFF);
-                self.prep_erase = false;
-            },
-            0xA0 => self.prep_write = true,
-            else => std.debug.panic("Unhandled Flash Command: 0x{X:0>2}", .{byte}),
-        }
 
-        self.state = .Ready;
-    }
-
-    fn shouldEraseSector(self: *const Self, addr: usize, byte: u8) bool {
-        return self.state == .Command and self.prep_erase and byte == 0x30 and addr & 0xFFF == 0x000;
-    }
-
-    fn write(self: *Self, buf: []u8, idx: usize, byte: u8) void {
-        buf[self.baseAddress() + idx] = byte;
-        self.prep_write = false;
-    }
-
-    fn read(self: *const Self, buf: []u8, idx: usize) u8 {
-        return buf[self.baseAddress() + idx];
-    }
-
-    fn eraseSector(self: *Self, buf: []u8, idx: usize) void {
-        const start = self.baseAddress() + (idx & 0xF000);
-
-        std.mem.set(u8, buf[start..][0..0x1000], 0xFF);
-        self.prep_erase = false;
-        self.state = .Ready;
-    }
-
-    inline fn baseAddress(self: *const Self) usize {
-        return if (self.bank == 1) 0x10000 else @as(usize, 0);
-    }
-};
-
-const Eeprom = struct {
-    const Self = @This();
-
-    addr: u14,
-
-    kind: Kind,
-    state: State,
-    writer: Writer,
-    reader: Reader,
-
-    allocator: Allocator,
-
-    const Kind = enum {
-        Unknown,
-        Small, // 512B
-        Large, // 8KB
-    };
-
-    const State = enum {
-        Ready,
-        Read,
-        Write,
-        WriteTransfer,
-        RequestEnd,
-    };
-
-    fn init(allocator: Allocator) Self {
-        return .{
-            .kind = .Unknown,
-            .state = .Ready,
-            .writer = Writer.init(),
-            .reader = Reader.init(),
-            .addr = 0,
+        var backup = Self{
+            .buf = buf,
             .allocator = allocator,
-        };
-    }
-
-    pub fn read(self: *Self) u1 {
-        return self.reader.read();
-    }
-
-    pub fn dbgRead(self: *const Self) u1 {
-        return self.reader.dbgRead();
-    }
-
-    pub fn write(self: *Self, word_count: u16, buf: *[]u8, bit: u1) void {
-        if (self.guessKind(word_count)) |found| {
-            log.info("EEPROM Kind: {}", .{found});
-            self.kind = found;
-
-            // buf.len will not equal zero when a save file was found and loaded.
-            // Right now, we assume that the save file is of the correct size which
-            // isn't necessarily true, since we can't trust anything a user can influence
-            // TODO: use ?[]u8 instead of a 0-sized slice?
-            if (buf.len == 0) {
-                const len: usize = switch (found) {
-                    .Small => 0x200,
-                    .Large => 0x2000,
-                    else => unreachable,
+            .kind = kind,
+            .title = title,
+            .save_path = path,
+            .flash = Flash.create(),
+            .eeprom = Eeprom.create(allocator),
         };
 
-                buf.* = self.allocator.alloc(u8, len) catch |e| {
-                    log.err("Failed to resize EEPROM buf to {} bytes", .{len});
-                    std.debug.panic("EEPROM entered irrecoverable state {}", .{e});
-                };
-                std.mem.set(u8, buf.*, 0xFF);
+        if (backup.save_path) |p| backup.readSave(allocator, p) catch |e| log.err("Failed to load save: {}", .{e});
+        return backup;
+    }
+
+    pub fn deinit(self: *Self) void {
+        if (self.save_path) |path| self.writeSave(self.allocator, path) catch |e| log.err("Failed to write save: {}", .{e});
+        self.allocator.free(self.buf);
+        self.* = undefined;
+    }
+
+    /// Guesses the Backup Kind of a GBA ROM
+    pub fn guess(rom: []const u8) Kind {
+        for (backup_kinds) |needle| {
+            const needle_len = needle.str.len;
+
+            var i: usize = 0;
+            while ((i + needle_len) < rom.len) : (i += 1) {
+                if (std.mem.eql(u8, needle.str, rom[i..][0..needle_len])) return needle.kind;
             }
         }
 
-        if (self.state == .RequestEnd) {
-            if (bit != 0) log.debug("EEPROM Request did not end in 0u1. TODO: is this ok?", .{});
-            self.state = .Ready;
-            return;
+        return .None;
     }
 
-        switch (self.state) {
-            .Ready => self.writer.requestWrite(bit),
-            .Read, .Write => self.writer.addressWrite(self.kind, bit),
-            .WriteTransfer => self.writer.dataWrite(bit),
-            .RequestEnd => unreachable, // We return early just above this block
+    fn readSave(self: *Self, allocator: Allocator, path: []const u8) !void {
+        const file_path = try self.savePath(allocator, path);
+        defer allocator.free(file_path);
+
+        // FIXME: Don't rely on this lol
+        if (std.mem.eql(u8, file_path[file_path.len - 12 .. file_path.len], "untitled.sav")) {
+            return log.err("ROM header lacks title, no save loaded", .{});
         }
 
-        self.tick(buf.*);
-    }
+        const file: std.fs.File = try std.fs.openFileAbsolute(file_path, .{});
+        const file_buf = try file.readToEndAlloc(allocator, try file.getEndPos());
+        defer allocator.free(file_buf);
 
-    fn guessKind(self: *const Self, word_count: u16) ?Kind {
-        if (self.kind != .Unknown or self.state != .Read) return null;
-
-        return switch (word_count) {
-            17 => .Large,
-            9 => .Small,
-            else => blk: {
-                log.err("Unexpected length of DMA3 Transfer upon initial EEPROM read: {}", .{word_count});
-                break :blk null;
-            },
-        };
-    }
-
-    fn tick(self: *Self, buf: []u8) void {
-        switch (self.state) {
-            .Ready => {
-                if (self.writer.len() == 2) {
-                    const req = @intCast(u2, self.writer.finish());
-                    switch (req) {
-                        0b11 => self.state = .Read,
-                        0b10 => self.state = .Write,
-                        else => log.err("Unknown EEPROM Request 0b{b:0>2}", .{req}),
-                    }
-                }
-            },
-            .Read => {
         switch (self.kind) {
-                    .Large => {
-                        if (self.writer.len() == 14) {
-                            const addr = @intCast(u10, self.writer.finish());
-                            const value = std.mem.readIntSliceLittle(u64, buf[@as(u13, addr) * 8 ..][0..8]);
+            .Sram, .Flash, .Flash1M => {
+                if (self.buf.len == file_buf.len) {
+                    std.mem.copy(u8, self.buf, file_buf);
+                    return log.info("Loaded Save from {s}", .{file_path});
+                }
 
-                            self.reader.configure(value);
-                            self.state = .RequestEnd;
-                        }
+                log.err("{s} is {} bytes, but we expected {} bytes", .{ file_path, file_buf.len, self.buf.len });
             },
-                    .Small => {
-                        if (self.writer.len() == 6) {
-                            // FIXME: Duplicated code from above
-                            const addr = @intCast(u6, self.writer.finish());
-                            const value = std.mem.readIntSliceLittle(u64, buf[@as(u13, addr) * 8 ..][0..8]);
+            .Eeprom => {
+                if (file_buf.len == 0x200 or file_buf.len == 0x2000) {
+                    self.eeprom.kind = if (file_buf.len == 0x200) .Small else .Large;
 
-                            self.reader.configure(value);
-                            self.state = .RequestEnd;
+                    self.buf = try allocator.alloc(u8, file_buf.len);
+                    std.mem.copy(u8, self.buf, file_buf);
+                    return log.info("Loaded Save from {s}", .{file_path});
                 }
+
+                log.err("EEPROM can either be 0x200 bytes or 0x2000 byes, but {s} was {X:} bytes", .{
+                    file_path,
+                    file_buf.len,
+                });
             },
-                    else => log.err("Unable to calculate EEPROM read address. EEPROM size UNKNOWN", .{}),
+            .None => return SaveError.Unsupported,
         }
-            },
-            .Write => {
+    }
+
+    fn savePath(self: *const Self, allocator: Allocator, path: []const u8) ![]const u8 {
+        const filename = try self.saveName(allocator);
+        defer allocator.free(filename);
+
+        return try std.fs.path.join(allocator, &[_][]const u8{ path, filename });
+    }
+
+    fn saveName(self: *const Self, allocator: Allocator) ![]const u8 {
+        const title_str = span(&escape(self.title));
+        const name = if (title_str.len != 0) title_str else "untitled";
+
+        return try std.mem.concat(allocator, u8, &[_][]const u8{ name, ".sav" });
+    }
+
+    fn writeSave(self: Self, allocator: Allocator, path: []const u8) !void {
+        const file_path = try self.savePath(allocator, path);
+        defer allocator.free(file_path);
+
         switch (self.kind) {
-                    .Large => {
-                        if (self.writer.len() == 14) {
-                            self.addr = @intCast(u10, self.writer.finish());
-                            self.state = .WriteTransfer;
-                        }
+            .Sram, .Flash, .Flash1M, .Eeprom => {
+                const file = try std.fs.createFileAbsolute(file_path, .{});
+                defer file.close();
+
+                try file.writeAll(self.buf);
+                log.info("Wrote Save to {s}", .{file_path});
             },
-                    .Small => {
-                        if (self.writer.len() == 6) {
-                            self.addr = @intCast(u6, self.writer.finish());
-                            self.state = .WriteTransfer;
+            else => return SaveError.Unsupported,
         }
-                    },
-                    else => log.err("Unable to calculate EEPROM write address. EEPROM size UNKNOWN", .{}),
-                }
-            },
-            .WriteTransfer => {
-                if (self.writer.len() == 64) {
-                    std.mem.writeIntSliceLittle(u64, buf[self.addr * 8 ..][0..8], self.writer.finish());
-                    self.state = .RequestEnd;
-                }
-            },
-            .RequestEnd => unreachable, // We return early in write() if state is .RequestEnd
-        }
-    }
-
-    const Reader = struct {
-        const This = @This();
-
-        data: u64,
-        i: u8,
-        enabled: bool,
-
-        fn init() This {
-            return .{
-                .data = 0,
-                .i = 0,
-                .enabled = false,
-            };
-        }
-
-        fn configure(self: *This, value: u64) void {
-            self.data = value;
-            self.i = 0;
-            self.enabled = true;
-        }
-
-        fn read(self: *This) u1 {
-            if (!self.enabled) return 1;
-
-            const bit = if (self.i < 4) blk: {
-                break :blk 0;
-            } else blk: {
-                const idx = @intCast(u6, 63 - (self.i - 4));
-                break :blk @truncate(u1, self.data >> idx);
-            };
-
-            self.i = (self.i + 1) % (64 + 4);
-            if (self.i == 0) self.enabled = false;
-
-            return bit;
-        }
-
-        fn dbgRead(self: *const This) u1 {
-            if (!self.enabled) return 1;
-
-            const bit = if (self.i < 4) blk: {
-                break :blk 0;
-            } else blk: {
-                const idx = @intCast(u6, 63 - (self.i - 4));
-                break :blk @truncate(u1, self.data >> idx);
-            };
-
-            return bit;
     }
 };
-
-    const Writer = struct {
-        const This = @This();
-
-        data: u64,
-        i: u8,
-
-        fn init() This {
-            return .{ .data = 0, .i = 0 };
-        }
-
-        fn requestWrite(self: *This, bit: u1) void {
-            const idx = @intCast(u1, 1 - self.i);
-            self.data = (self.data & ~(@as(u64, 1) << idx)) | (@as(u64, bit) << idx);
-            self.i += 1;
-        }
-
-        fn addressWrite(self: *This, kind: Eeprom.Kind, bit: u1) void {
-            if (kind == .Unknown) return;
-
-            const size: u4 = switch (kind) {
-                .Large => 13,
-                .Small => 5,
-                .Unknown => unreachable,
-            };
-
-            const idx = @intCast(u4, size - self.i);
-            self.data = (self.data & ~(@as(u64, 1) << idx)) | (@as(u64, bit) << idx);
-            self.i += 1;
-        }
-
-        fn dataWrite(self: *This, bit: u1) void {
-            const idx = @intCast(u6, 63 - self.i);
-            self.data = (self.data & ~(@as(u64, 1) << idx)) | (@as(u64, bit) << idx);
-            self.i += 1;
-        }
-
-        fn len(self: *const This) u8 {
-            return self.i;
-        }
-
-        fn finish(self: *This) u64 {
-            defer self.reset();
-            return self.data;
-        }
-
-        fn reset(self: *This) void {
-            self.i = 0;
-            self.data = 0;
-        }
-    };
-};

src/core/bus/backup/Flash.zig

@@ -0,0 +1,72 @@
+const std = @import("std");
+
+const Self = @This();
+
+state: State,
+
+id_mode: bool,
+set_bank: bool,
+prep_erase: bool,
+prep_write: bool,
+
+bank: u1,
+
+const State = enum {
+    Ready,
+    Set,
+    Command,
+};
+
+pub fn read(self: *const Self, buf: []u8, idx: usize) u8 {
+    return buf[self.address() + idx];
+}
+
+pub fn write(self: *Self, buf: []u8, idx: usize, byte: u8) void {
+    buf[self.address() + idx] = byte;
+    self.prep_write = false;
+}
+
+pub fn create() Self {
+    return .{
+        .state = .Ready,
+        .id_mode = false,
+        .set_bank = false,
+        .prep_erase = false,
+        .prep_write = false,
+        .bank = 0,
+    };
+}
+
+pub fn handleCommand(self: *Self, buf: []u8, byte: u8) void {
+    switch (byte) {
+        0x90 => self.id_mode = true,
+        0xF0 => self.id_mode = false,
+        0xB0 => self.set_bank = true,
+        0x80 => self.prep_erase = true,
+        0x10 => {
+            std.mem.set(u8, buf, 0xFF);
+            self.prep_erase = false;
+        },
+        0xA0 => self.prep_write = true,
+        else => std.debug.panic("Unhandled Flash Command: 0x{X:0>2}", .{byte}),
+    }
+
+    self.state = .Ready;
+}
+
+pub fn shouldEraseSector(self: *const Self, addr: usize, byte: u8) bool {
+    return self.state == .Command and self.prep_erase and byte == 0x30 and addr & 0xFFF == 0x000;
+}
+
+pub fn erase(self: *Self, buf: []u8, sector: usize) void {
+    const start = self.address() + (sector & 0xF000);
+
+    std.mem.set(u8, buf[start..][0..0x1000], 0xFF);
+    self.prep_erase = false;
+    self.state = .Ready;
+}
+
+/// Base Address
+inline fn address(self: *const Self) usize {
+    return if (self.bank == 1) 0x10000 else @as(usize, 0);
+}

src/core/bus/backup/eeprom.zig

@@ -0,0 +1,269 @@
+const std = @import("std");
+
+const Allocator = std.mem.Allocator;
+
+const log = std.log.scoped(.Eeprom);
+
+pub const Eeprom = struct {
+    const Self = @This();
+
+    addr: u14,
+
+    kind: Kind,
+    state: State,
+    writer: Writer,
+    reader: Reader,
+
+    allocator: Allocator,
+
+    const Kind = enum {
+        Unknown,
+        Small, // 512B
+        Large, // 8KB
+    };
+
+    const State = enum {
+        Ready,
+        Read,
+        Write,
+        WriteTransfer,
+        RequestEnd,
+    };
+
+    pub fn read(self: *Self) u1 {
+        return self.reader.read();
+    }
+
+    pub fn dbgRead(self: *const Self) u1 {
+        return self.reader.dbgRead();
+    }
+
+    pub fn write(self: *Self, word_count: u16, buf: *[]u8, bit: u1) void {
+        if (self.guessKind(word_count)) |found| {
+            log.info("EEPROM Kind: {}", .{found});
+            self.kind = found;
+
+            // buf.len will not equal zero when a save file was found and loaded.
+            // Right now, we assume that the save file is of the correct size which
+            // isn't necessarily true, since we can't trust anything a user can influence
+            // TODO: use ?[]u8 instead of a 0-sized slice?
+            if (buf.len == 0) {
+                const len: usize = switch (found) {
+                    .Small => 0x200,
+                    .Large => 0x2000,
+                    else => unreachable,
+                };
+
+                buf.* = self.allocator.alloc(u8, len) catch |e| {
+                    log.err("Failed to resize EEPROM buf to {} bytes", .{len});
+                    std.debug.panic("EEPROM entered irrecoverable state {}", .{e});
+                };
+                std.mem.set(u8, buf.*, 0xFF);
+            }
+        }
+
+        if (self.state == .RequestEnd) {
+            if (bit != 0) log.debug("EEPROM Request did not end in 0u1. TODO: is this ok?", .{});
+            self.state = .Ready;
+            return;
+        }
+
+        switch (self.state) {
+            .Ready => self.writer.requestWrite(bit),
+            .Read, .Write => self.writer.addressWrite(self.kind, bit),
+            .WriteTransfer => self.writer.dataWrite(bit),
+            .RequestEnd => unreachable, // We return early just above this block
+        }
+
+        self.tick(buf.*);
+    }
+
+    pub fn create(allocator: Allocator) Self {
+        return .{
+            .kind = .Unknown,
+            .state = .Ready,
+            .writer = Writer.create(),
+            .reader = Reader.create(),
+            .addr = 0,
+            .allocator = allocator,
+        };
+    }
+
+    fn guessKind(self: *const Self, word_count: u16) ?Kind {
+        if (self.kind != .Unknown or self.state != .Read) return null;
+
+        return switch (word_count) {
+            17 => .Large,
+            9 => .Small,
+            else => blk: {
+                log.err("Unexpected length of DMA3 Transfer upon initial EEPROM read: {}", .{word_count});
+                break :blk null;
+            },
+        };
+    }
+
+    fn tick(self: *Self, buf: []u8) void {
+        switch (self.state) {
+            .Ready => {
+                if (self.writer.len() == 2) {
+                    const req = @intCast(u2, self.writer.finish());
+                    switch (req) {
+                        0b11 => self.state = .Read,
+                        0b10 => self.state = .Write,
+                        else => log.err("Unknown EEPROM Request 0b{b:0>2}", .{req}),
+                    }
+                }
+            },
+            .Read => {
+                switch (self.kind) {
+                    .Large => {
+                        if (self.writer.len() == 14) {
+                            const addr = @intCast(u10, self.writer.finish());
+                            const value = std.mem.readIntSliceLittle(u64, buf[@as(u13, addr) * 8 ..][0..8]);
+
+                            self.reader.configure(value);
+                            self.state = .RequestEnd;
+                        }
+                    },
+                    .Small => {
+                        if (self.writer.len() == 6) {
+                            // FIXME: Duplicated code from above
+                            const addr = @intCast(u6, self.writer.finish());
+                            const value = std.mem.readIntSliceLittle(u64, buf[@as(u13, addr) * 8 ..][0..8]);
+
+                            self.reader.configure(value);
+                            self.state = .RequestEnd;
+                        }
+                    },
+                    else => log.err("Unable to calculate EEPROM read address. EEPROM size UNKNOWN", .{}),
+                }
+            },
+            .Write => {
+                switch (self.kind) {
+                    .Large => {
+                        if (self.writer.len() == 14) {
+                            self.addr = @intCast(u10, self.writer.finish());
+                            self.state = .WriteTransfer;
+                        }
+                    },
+                    .Small => {
+                        if (self.writer.len() == 6) {
+                            self.addr = @intCast(u6, self.writer.finish());
+                            self.state = .WriteTransfer;
+                        }
+                    },
+                    else => log.err("Unable to calculate EEPROM write address. EEPROM size UNKNOWN", .{}),
+                }
+            },
+            .WriteTransfer => {
+                if (self.writer.len() == 64) {
+                    std.mem.writeIntSliceLittle(u64, buf[self.addr * 8 ..][0..8], self.writer.finish());
+                    self.state = .RequestEnd;
+                }
+            },
+            .RequestEnd => unreachable, // We return early in write() if state is .RequestEnd
+        }
+    }
+};
+
+const Reader = struct {
+    const Self = @This();
+
+    data: u64,
+    i: u8,
+    enabled: bool,
+
+    fn create() Self {
+        return .{
+            .data = 0,
+            .i = 0,
+            .enabled = false,
+        };
+    }
+
+    fn read(self: *Self) u1 {
+        if (!self.enabled) return 1;
+
+        const bit = if (self.i < 4) blk: {
+            break :blk 0;
+        } else blk: {
+            const idx = @intCast(u6, 63 - (self.i - 4));
+            break :blk @truncate(u1, self.data >> idx);
+        };
+
+        self.i = (self.i + 1) % (64 + 4);
+        if (self.i == 0) self.enabled = false;
+
+        return bit;
+    }
+
+    fn dbgRead(self: *const Self) u1 {
+        if (!self.enabled) return 1;
+
+        const bit = if (self.i < 4) blk: {
+            break :blk 0;
+        } else blk: {
+            const idx = @intCast(u6, 63 - (self.i - 4));
+            break :blk @truncate(u1, self.data >> idx);
+        };
+
+        return bit;
+    }
+
+    fn configure(self: *Self, value: u64) void {
+        self.data = value;
+        self.i = 0;
+        self.enabled = true;
+    }
+};
+
+const Writer = struct {
+    const Self = @This();
+
+    data: u64,
+    i: u8,
+
+    fn create() Self {
+        return .{ .data = 0, .i = 0 };
+    }
+
+    fn requestWrite(self: *Self, bit: u1) void {
+        const idx = @intCast(u1, 1 - self.i);
+        self.data = (self.data & ~(@as(u64, 1) << idx)) | (@as(u64, bit) << idx);
+        self.i += 1;
+    }
+
+    fn addressWrite(self: *Self, kind: Eeprom.Kind, bit: u1) void {
+        if (kind == .Unknown) return;
+
+        const size: u4 = switch (kind) {
+            .Large => 13,
+            .Small => 5,
+            .Unknown => unreachable,
+        };
+
+        const idx = @intCast(u4, size - self.i);
+        self.data = (self.data & ~(@as(u64, 1) << idx)) | (@as(u64, bit) << idx);
+        self.i += 1;
+    }
+
+    fn dataWrite(self: *Self, bit: u1) void {
+        const idx = @intCast(u6, 63 - self.i);
+        self.data = (self.data & ~(@as(u64, 1) << idx)) | (@as(u64, bit) << idx);
+        self.i += 1;
+    }
+
+    fn len(self: *const Self) u8 {
+        return self.i;
+    }
+
+    fn finish(self: *Self) u64 {
+        defer self.reset();
+        return self.data;
+    }
+
+    fn reset(self: *Self) void {
+        self.i = 0;
+        self.data = 0;
+    }
+};

src/core/bus/dma.zig

@@ -8,6 +8,9 @@ const Arm7tdmi = @import("../cpu.zig").Arm7tdmi;
 pub const DmaTuple = std.meta.Tuple(&[_]type{ DmaController(0), DmaController(1), DmaController(2), DmaController(3) });
 const log = std.log.scoped(.DmaTransfer);
 
+const setHi = util.setHi;
+const setLo = util.setLo;
+
 pub fn create() DmaTuple {
     return .{ DmaController(0).init(), DmaController(1).init(), DmaController(2).init(), DmaController(3).init() };
 }
@@ -40,48 +43,48 @@ pub fn write(comptime T: type, dma: *DmaTuple, addr: u32, value: T) void {
 
     switch (T) {
         u32 => switch (byte) {
-            0xB0 => dma.*[0].setSad(value),
-            0xB4 => dma.*[0].setDad(value),
-            0xB8 => dma.*[0].setCnt(value),
-            0xBC => dma.*[1].setSad(value),
-            0xC0 => dma.*[1].setDad(value),
-            0xC4 => dma.*[1].setCnt(value),
-            0xC8 => dma.*[2].setSad(value),
-            0xCC => dma.*[2].setDad(value),
-            0xD0 => dma.*[2].setCnt(value),
-            0xD4 => dma.*[3].setSad(value),
-            0xD8 => dma.*[3].setDad(value),
-            0xDC => dma.*[3].setCnt(value),
+            0xB0 => dma.*[0].setDmasad(value),
+            0xB4 => dma.*[0].setDmadad(value),
+            0xB8 => dma.*[0].setDmacnt(value),
+            0xBC => dma.*[1].setDmasad(value),
+            0xC0 => dma.*[1].setDmadad(value),
+            0xC4 => dma.*[1].setDmacnt(value),
+            0xC8 => dma.*[2].setDmasad(value),
+            0xCC => dma.*[2].setDmadad(value),
+            0xD0 => dma.*[2].setDmacnt(value),
+            0xD4 => dma.*[3].setDmasad(value),
+            0xD8 => dma.*[3].setDmadad(value),
+            0xDC => dma.*[3].setDmacnt(value),
             else => util.io.write.undef(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
         u16 => switch (byte) {
-            0xB0 => dma.*[0].setSad(setU32L(dma.*[0].sad, value)),
-            0xB2 => dma.*[0].setSad(setU32H(dma.*[0].sad, value)),
-            0xB4 => dma.*[0].setDad(setU32L(dma.*[0].dad, value)),
-            0xB6 => dma.*[0].setDad(setU32H(dma.*[0].dad, value)),
-            0xB8 => dma.*[0].setCntL(value),
-            0xBA => dma.*[0].setCntH(value),
+            0xB0 => dma.*[0].setDmasad(setLo(u32, dma.*[0].sad, value)),
+            0xB2 => dma.*[0].setDmasad(setHi(u32, dma.*[0].sad, value)),
+            0xB4 => dma.*[0].setDmadad(setLo(u32, dma.*[0].dad, value)),
+            0xB6 => dma.*[0].setDmadad(setHi(u32, dma.*[0].dad, value)),
+            0xB8 => dma.*[0].setDmacntL(value),
+            0xBA => dma.*[0].setDmacntH(value),
 
-            0xBC => dma.*[1].setSad(setU32L(dma.*[1].sad, value)),
-            0xBE => dma.*[1].setSad(setU32H(dma.*[1].sad, value)),
-            0xC0 => dma.*[1].setDad(setU32L(dma.*[1].dad, value)),
-            0xC2 => dma.*[1].setDad(setU32H(dma.*[1].dad, value)),
-            0xC4 => dma.*[1].setCntL(value),
-            0xC6 => dma.*[1].setCntH(value),
+            0xBC => dma.*[1].setDmasad(setLo(u32, dma.*[1].sad, value)),
+            0xBE => dma.*[1].setDmasad(setHi(u32, dma.*[1].sad, value)),
+            0xC0 => dma.*[1].setDmadad(setLo(u32, dma.*[1].dad, value)),
+            0xC2 => dma.*[1].setDmadad(setHi(u32, dma.*[1].dad, value)),
+            0xC4 => dma.*[1].setDmacntL(value),
+            0xC6 => dma.*[1].setDmacntH(value),
 
-            0xC8 => dma.*[2].setSad(setU32L(dma.*[2].sad, value)),
-            0xCA => dma.*[2].setSad(setU32H(dma.*[2].sad, value)),
-            0xCC => dma.*[2].setDad(setU32L(dma.*[2].dad, value)),
-            0xCE => dma.*[2].setDad(setU32H(dma.*[2].dad, value)),
-            0xD0 => dma.*[2].setCntL(value),
-            0xD2 => dma.*[2].setCntH(value),
+            0xC8 => dma.*[2].setDmasad(setLo(u32, dma.*[2].sad, value)),
+            0xCA => dma.*[2].setDmasad(setHi(u32, dma.*[2].sad, value)),
+            0xCC => dma.*[2].setDmadad(setLo(u32, dma.*[2].dad, value)),
+            0xCE => dma.*[2].setDmadad(setHi(u32, dma.*[2].dad, value)),
+            0xD0 => dma.*[2].setDmacntL(value),
+            0xD2 => dma.*[2].setDmacntH(value),
 
-            0xD4 => dma.*[3].setSad(setU32L(dma.*[3].sad, value)),
-            0xD6 => dma.*[3].setSad(setU32H(dma.*[3].sad, value)),
-            0xD8 => dma.*[3].setDad(setU32L(dma.*[3].dad, value)),
-            0xDA => dma.*[3].setDad(setU32H(dma.*[3].dad, value)),
-            0xDC => dma.*[3].setCntL(value),
-            0xDE => dma.*[3].setCntH(value),
+            0xD4 => dma.*[3].setDmasad(setLo(u32, dma.*[3].sad, value)),
+            0xD6 => dma.*[3].setDmasad(setHi(u32, dma.*[3].sad, value)),
+            0xD8 => dma.*[3].setDmadad(setLo(u32, dma.*[3].dad, value)),
+            0xDA => dma.*[3].setDmadad(setHi(u32, dma.*[3].dad, value)),
+            0xDC => dma.*[3].setDmacntL(value),
+            0xDE => dma.*[3].setDmacntH(value),
             else => util.io.write.undef(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
         u8 => util.io.write.undef(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, addr }),
@@ -110,15 +113,12 @@ fn DmaController(comptime id: u2) type {
         cnt: DmaControl,
 
         /// Internal. Currrent Source Address
-        _sad: u32,
+        sad_latch: u32,
         /// Internal. Current Destination Address
-        _dad: u32,
+        dad_latch: u32,
         /// Internal. Word Count
         _word_count: if (id == 3) u16 else u14,
 
-        // Internal. FIFO Word Count
-        _fifo_word_count: u8,
-
         /// Some DMA Transfers are enabled during Hblank / VBlank and / or
         /// have delays. Thefore bit 15 of DMACNT isn't actually something
         /// we can use to control when we do or do not execute a step in a DMA Transfer
@@ -132,33 +132,32 @@ fn DmaController(comptime id: u2) type {
                 .cnt = .{ .raw = 0x000 },
 
                 // Internals
-                ._sad = 0,
-                ._dad = 0,
+                .sad_latch = 0,
+                .dad_latch = 0,
                 ._word_count = 0,
-                ._fifo_word_count = 4,
                 .in_progress = false,
             };
         }
 
-        pub fn setSad(self: *Self, addr: u32) void {
+        pub fn setDmasad(self: *Self, addr: u32) void {
             self.sad = addr & sad_mask;
         }
 
-        pub fn setDad(self: *Self, addr: u32) void {
+        pub fn setDmadad(self: *Self, addr: u32) void {
             self.dad = addr & dad_mask;
         }
 
-        pub fn setCntL(self: *Self, halfword: u16) void {
+        pub fn setDmacntL(self: *Self, halfword: u16) void {
             self.word_count = @truncate(@TypeOf(self.word_count), halfword);
         }
 
-        pub fn setCntH(self: *Self, halfword: u16) void {
+        pub fn setDmacntH(self: *Self, halfword: u16) void {
             const new = DmaControl{ .raw = halfword };
 
             if (!self.cnt.enabled.read() and new.enabled.read()) {
                 // Reload Internals on Rising Edge.
-                self._sad = self.sad;
-                self._dad = self.dad;
+                self.sad_latch = self.sad;
+                self.dad_latch = self.dad;
                 self._word_count = if (self.word_count == 0) std.math.maxInt(@TypeOf(self._word_count)) else self.word_count;
 
                 // Only a Start Timing of 00 has a DMA Transfer immediately begin
@@ -168,15 +167,15 @@ fn DmaController(comptime id: u2) type {
             self.cnt.raw = halfword;
         }
 
-        pub fn setCnt(self: *Self, word: u32) void {
-            self.setCntL(@truncate(u16, word));
-            self.setCntH(@truncate(u16, word >> 16));
+        pub fn setDmacnt(self: *Self, word: u32) void {
+            self.setDmacntL(@truncate(u16, word));
+            self.setDmacntH(@truncate(u16, word >> 16));
         }
 
         pub fn step(self: *Self, cpu: *Arm7tdmi) void {
             const is_fifo = (id == 1 or id == 2) and self.cnt.start_timing.read() == 0b11;
-            const sad_adj = Self.adjustment(self.cnt.sad_adj.read());
-            const dad_adj = if (is_fifo) .Fixed else Self.adjustment(self.cnt.dad_adj.read());
+            const sad_adj = @intToEnum(Adjustment, self.cnt.sad_adj.read());
+            const dad_adj = if (is_fifo) .Fixed else @intToEnum(Adjustment, self.cnt.dad_adj.read());
 
             const transfer_type = is_fifo or self.cnt.transfer_type.read();
             const offset: u32 = if (transfer_type) @sizeOf(u32) else @sizeOf(u16);
@@ -184,22 +183,22 @@ fn DmaController(comptime id: u2) type {
             const mask = if (transfer_type) ~@as(u32, 3) else ~@as(u32, 1);
 
             if (transfer_type) {
-                cpu.bus.write(u32, self._dad & mask, cpu.bus.read(u32, self._sad & mask));
+                cpu.bus.write(u32, self.dad_latch & mask, cpu.bus.read(u32, self.sad_latch & mask));
             } else {
-                cpu.bus.write(u16, self._dad & mask, cpu.bus.read(u16, self._sad & mask));
+                cpu.bus.write(u16, self.dad_latch & mask, cpu.bus.read(u16, self.sad_latch & mask));
             }
 
             switch (sad_adj) {
-                .Increment => self._sad +%= offset,
-                .Decrement => self._sad -%= offset,
-                // TODO: Is just ignoring this ok?
+                .Increment => self.sad_latch +%= offset,
+                .Decrement => self.sad_latch -%= offset,
+                // FIXME: Is just ignoring this ok?
                 .IncrementReload => log.err("{} is a prohibited adjustment on SAD", .{sad_adj}),
                 .Fixed => {},
             }
 
             switch (dad_adj) {
-                .Increment, .IncrementReload => self._dad +%= offset,
-                .Decrement => self._dad -%= offset,
+                .Increment, .IncrementReload => self.dad_latch +%= offset,
+                .Decrement => self.dad_latch -%= offset,
                 .Fixed => {},
             }
 
@@ -227,7 +226,7 @@ fn DmaController(comptime id: u2) type {
             }
         }
 
-        pub fn pollBlankingDma(self: *Self, comptime kind: DmaKind) void {
+        fn poll(self: *Self, comptime kind: DmaKind) void {
             if (self.in_progress) return; // If there's an ongoing DMA Transfer, exit early
 
             // No ongoing DMA Transfer, We want to check if we should repeat an existing one
@@ -243,11 +242,11 @@ fn DmaController(comptime id: u2) type {
             // Reload internal DAD latch if we are in IncrementRelaod
             if (self.in_progress) {
                 self._word_count = if (self.word_count == 0) std.math.maxInt(@TypeOf(self._word_count)) else self.word_count;
-                if (Self.adjustment(self.cnt.dad_adj.read()) == .IncrementReload) self._dad = self.dad;
+                if (@intToEnum(Adjustment, self.cnt.dad_adj.read()) == .IncrementReload) self.dad_latch = self.dad;
             }
         }
 
-        pub fn requestSoundDma(self: *Self, _: u32) void {
+        pub fn requestAudio(self: *Self, _: u32) void {
             comptime std.debug.assert(id == 1 or id == 2);
             if (self.in_progress) return; // APU must wait their turn
 
@@ -259,23 +258,19 @@ fn DmaController(comptime id: u2) type {
             // We Assume DMACNT_L is set to 4
 
             // FIXME: Safe to just assume whatever DAD is set to is the FIFO Address?
-            // self._dad = fifo_addr;
+            // self.dad_latch = fifo_addr;
             self.cnt.repeat.set();
             self._word_count = 4;
             self.in_progress = true;
         }
-
-        fn adjustment(idx: u2) Adjustment {
-            return std.meta.intToEnum(Adjustment, idx) catch unreachable;
-        }
     };
 }
 
-pub fn pollBlankingDma(bus: *Bus, comptime kind: DmaKind) void {
-    bus.dma[0].pollBlankingDma(kind);
-    bus.dma[1].pollBlankingDma(kind);
-    bus.dma[2].pollBlankingDma(kind);
-    bus.dma[3].pollBlankingDma(kind);
+pub fn pollDmaOnBlank(bus: *Bus, comptime kind: DmaKind) void {
+    bus.dma[0].poll(kind);
+    bus.dma[1].poll(kind);
+    bus.dma[2].poll(kind);
+    bus.dma[3].poll(kind);
 }
 
 const Adjustment = enum(u2) {
@@ -291,11 +286,3 @@ const DmaKind = enum(u2) {
     VBlank,
     Special,
 };
-
-fn setU32L(left: u32, right: u16) u32 {
-    return (left & 0xFFFF_0000) | right;
-}
-
-fn setU32H(left: u32, right: u16) u32 {
-    return (left & 0x0000_FFFF) | (@as(u32, right) << 16);
-}

src/core/bus/gpio.zig

@@ -288,7 +288,7 @@ pub const Clock = struct {
         cpu.sched.push(.RealTimeClock, 1 << 24); // Every Second
     }
 
-    pub fn updateTime(self: *Self, late: u64) void {
+    pub fn onClockUpdate(self: *Self, late: u64) void {
         self.cpu.sched.push(.RealTimeClock, (1 << 24) -| late); // Reschedule
 
         const now = DateTime.now();

src/core/bus/io.zig

@@ -11,6 +11,9 @@ const Bus = @import("../Bus.zig");
 const DmaController = @import("dma.zig").DmaController;
 const Scheduler = @import("../scheduler.zig").Scheduler;
 
+const setHi = util.setLo;
+const setLo = util.setHi;
+
 const log = std.log.scoped(.@"I/O");
 
 pub const Io = struct {
@@ -233,18 +236,18 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
             0x0400_0022 => bus.ppu.aff_bg[0].pb = @bitCast(i16, value),
             0x0400_0024 => bus.ppu.aff_bg[0].pc = @bitCast(i16, value),
             0x0400_0026 => bus.ppu.aff_bg[0].pd = @bitCast(i16, value),
-            0x0400_0028 => bus.ppu.aff_bg[0].x = @bitCast(i32, @bitCast(u32, bus.ppu.aff_bg[0].x) & 0xFFFF_0000 | value),
-            0x0400_002A => bus.ppu.aff_bg[0].x = @bitCast(i32, @bitCast(u32, bus.ppu.aff_bg[0].x) & 0x0000_FFFF | (@as(u32, value) << 16)),
-            0x0400_002C => bus.ppu.aff_bg[0].y = @bitCast(i32, @bitCast(u32, bus.ppu.aff_bg[0].y) & 0xFFFF_0000 | value),
-            0x0400_002E => bus.ppu.aff_bg[0].y = @bitCast(i32, @bitCast(u32, bus.ppu.aff_bg[0].y) & 0x0000_FFFF | (@as(u32, value) << 16)),
+            0x0400_0028 => bus.ppu.aff_bg[0].x = @bitCast(i32, setLo(u32, @bitCast(u32, bus.ppu.aff_bg[0].x), value)),
+            0x0400_002A => bus.ppu.aff_bg[0].x = @bitCast(i32, setHi(u32, @bitCast(u32, bus.ppu.aff_bg[0].x), value)),
+            0x0400_002C => bus.ppu.aff_bg[0].y = @bitCast(i32, setLo(u32, @bitCast(u32, bus.ppu.aff_bg[0].y), value)),
+            0x0400_002E => bus.ppu.aff_bg[0].y = @bitCast(i32, setHi(u32, @bitCast(u32, bus.ppu.aff_bg[0].y), value)),
             0x0400_0030 => bus.ppu.aff_bg[1].pa = @bitCast(i16, value),
             0x0400_0032 => bus.ppu.aff_bg[1].pb = @bitCast(i16, value),
             0x0400_0034 => bus.ppu.aff_bg[1].pc = @bitCast(i16, value),
             0x0400_0036 => bus.ppu.aff_bg[1].pd = @bitCast(i16, value),
-            0x0400_0038 => bus.ppu.aff_bg[1].x = @bitCast(i32, @bitCast(u32, bus.ppu.aff_bg[1].x) & 0xFFFF_0000 | value),
-            0x0400_003A => bus.ppu.aff_bg[1].x = @bitCast(i32, @bitCast(u32, bus.ppu.aff_bg[1].x) & 0x0000_FFFF | (@as(u32, value) << 16)),
-            0x0400_003C => bus.ppu.aff_bg[1].y = @bitCast(i32, @bitCast(u32, bus.ppu.aff_bg[1].y) & 0xFFFF_0000 | value),
-            0x0400_003E => bus.ppu.aff_bg[1].y = @bitCast(i32, @bitCast(u32, bus.ppu.aff_bg[1].y) & 0x0000_FFFF | (@as(u32, value) << 16)),
+            0x0400_0038 => bus.ppu.aff_bg[1].x = @bitCast(i32, setLo(u32, @bitCast(u32, bus.ppu.aff_bg[1].x), value)),
+            0x0400_003A => bus.ppu.aff_bg[1].x = @bitCast(i32, setHi(u32, @bitCast(u32, bus.ppu.aff_bg[1].x), value)),
+            0x0400_003C => bus.ppu.aff_bg[1].y = @bitCast(i32, setLo(u32, @bitCast(u32, bus.ppu.aff_bg[1].y), value)),
+            0x0400_003E => bus.ppu.aff_bg[1].y = @bitCast(i32, setHi(u32, @bitCast(u32, bus.ppu.aff_bg[1].y), value)),
             0x0400_0040 => bus.ppu.win.h[0].raw = value,
             0x0400_0042 => bus.ppu.win.h[1].raw = value,
             0x0400_0044 => bus.ppu.win.v[0].raw = value,
@@ -296,16 +299,16 @@ pub fn write(bus: *Bus, comptime T: type, address: u32, value: T) void {
         },
         u8 => switch (address) {
             // Display
-            0x0400_0004 => bus.ppu.dispstat.raw = (bus.ppu.dispstat.raw & 0xFF00) | value,
-            0x0400_0005 => bus.ppu.dispstat.raw = (@as(u16, value) << 8) | (bus.ppu.dispstat.raw & 0xFF),
-            0x0400_0008 => bus.ppu.bg[0].cnt.raw = (bus.ppu.bg[0].cnt.raw & 0xFF00) | value,
-            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_0048 => bus.ppu.win.setInL(value),
-            0x0400_0049 => bus.ppu.win.setInH(value),
-            0x0400_004A => bus.ppu.win.setOutL(value),
-            0x0400_0054 => bus.ppu.bldy.raw = (bus.ppu.bldy.raw & 0xFF00) | value,
+            0x0400_0004 => bus.ppu.dispstat.raw = setLo(u16, bus.ppu.dispstat.raw, value),
+            0x0400_0005 => bus.ppu.dispstat.raw = setHi(u16, bus.ppu.dispstat.raw, value),
+            0x0400_0008 => bus.ppu.bg[0].cnt.raw = setLo(u16, bus.ppu.bg[0].cnt.raw, value),
+            0x0400_0009 => bus.ppu.bg[0].cnt.raw = setHi(u16, bus.ppu.bg[0].cnt.raw, value),
+            0x0400_000A => bus.ppu.bg[1].cnt.raw = setLo(u16, bus.ppu.bg[1].cnt.raw, value),
+            0x0400_000B => bus.ppu.bg[1].cnt.raw = setHi(u16, bus.ppu.bg[1].cnt.raw, value),
+            0x0400_0048 => bus.ppu.win.in.raw = setLo(u16, bus.ppu.win.in.raw, value),
+            0x0400_0049 => bus.ppu.win.in.raw = setHi(u16, bus.ppu.win.in.raw, value),
+            0x0400_004A => bus.ppu.win.out.raw = setLo(u16, bus.ppu.win.out.raw, value),
+            0x0400_0054 => bus.ppu.bldy.raw = setLo(u16, bus.ppu.bldy.raw, value),
 
             // Sound
             0x0400_0060...0x0400_00A7 => apu.write(T, &bus.apu, address, value),

src/core/bus/timer.zig

@@ -19,20 +19,20 @@ pub fn read(comptime T: type, tim: *const TimerTuple, addr: u32) ?T {
 
     return switch (T) {
         u32 => switch (nybble) {
-            0x0 => @as(T, tim.*[0].cnt.raw) << 16 | tim.*[0].getCntL(),
-            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(),
+            0x0 => @as(T, tim.*[0].cnt.raw) << 16 | tim.*[0].timcntL(),
+            0x4 => @as(T, tim.*[1].cnt.raw) << 16 | tim.*[1].timcntL(),
+            0x8 => @as(T, tim.*[2].cnt.raw) << 16 | tim.*[2].timcntL(),
+            0xC => @as(T, tim.*[3].cnt.raw) << 16 | tim.*[3].timcntL(),
             else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         },
         u16 => switch (nybble) {
-            0x0 => tim.*[0].getCntL(),
+            0x0 => tim.*[0].timcntL(),
             0x2 => tim.*[0].cnt.raw,
-            0x4 => tim.*[1].getCntL(),
+            0x4 => tim.*[1].timcntL(),
             0x6 => tim.*[1].cnt.raw,
-            0x8 => tim.*[2].getCntL(),
+            0x8 => tim.*[2].timcntL(),
             0xA => tim.*[2].cnt.raw,
-            0xC => tim.*[3].getCntL(),
+            0xC => tim.*[3].timcntL(),
             0xE => tim.*[3].cnt.raw,
             else => util.io.read.undef(T, log, "Tried to perform a {} read to 0x{X:0>8}", .{ T, addr }),
         },
@@ -46,21 +46,21 @@ pub fn write(comptime T: type, tim: *TimerTuple, addr: u32, value: T) void {
 
     return switch (T) {
         u32 => switch (nybble) {
-            0x0 => tim.*[0].setCnt(value),
-            0x4 => tim.*[1].setCnt(value),
-            0x8 => tim.*[2].setCnt(value),
-            0xC => tim.*[3].setCnt(value),
+            0x0 => tim.*[0].setTimcnt(value),
+            0x4 => tim.*[1].setTimcnt(value),
+            0x8 => tim.*[2].setTimcnt(value),
+            0xC => tim.*[3].setTimcnt(value),
             else => util.io.write.undef(log, "Tried to write 0x{X:0>8}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
         u16 => switch (nybble) {
-            0x0 => tim.*[0].setCntL(value),
-            0x2 => tim.*[0].setCntH(value),
-            0x4 => tim.*[1].setCntL(value),
-            0x6 => tim.*[1].setCntH(value),
-            0x8 => tim.*[2].setCntL(value),
-            0xA => tim.*[2].setCntH(value),
-            0xC => tim.*[3].setCntL(value),
-            0xE => tim.*[3].setCntH(value),
+            0x0 => tim.*[0].setTimcntL(value),
+            0x2 => tim.*[0].setTimcntH(value),
+            0x4 => tim.*[1].setTimcntL(value),
+            0x6 => tim.*[1].setTimcntH(value),
+            0x8 => tim.*[2].setTimcntL(value),
+            0xA => tim.*[2].setTimcntH(value),
+            0xC => tim.*[3].setTimcntL(value),
+            0xE => tim.*[3].setTimcntH(value),
             else => util.io.write.undef(log, "Tried to write 0x{X:0>4}{} to 0x{X:0>8}", .{ value, T, addr }),
         },
         u8 => util.io.write.undef(log, "Tried to write 0x{X:0>2}{} to 0x{X:0>8}", .{ value, T, addr }),
@@ -72,13 +72,13 @@ fn Timer(comptime id: u2) type {
     return struct {
         const Self = @This();
 
-        /// Read Only, Internal. Please use self.getCntL()
+        /// Read Only, Internal. Please use self.timcntL()
         _counter: u16,
 
-        /// Write Only, Internal. Please use self.setCntL()
+        /// Write Only, Internal. Please use self.setTimcntL()
         _reload: u16,
 
-        /// Write Only, Internal. Please use self.setCntH()
+        /// Write Only, Internal. Please use self.setTimcntH()
         cnt: TimerControl,
 
         /// Internal.
@@ -97,26 +97,26 @@ fn Timer(comptime id: u2) type {
             };
         }
 
-        /// TIMCNT_L
-        pub fn getCntL(self: *const Self) u16 {
+        /// TIMCNT_L Getter
+        pub fn timcntL(self: *const Self) u16 {
             if (self.cnt.cascade.read() or !self.cnt.enabled.read()) return self._counter;
 
             return self._counter +% @truncate(u16, (self.sched.now() - self._start_timestamp) / self.frequency());
         }
 
-        /// TIMCNT_L
-        pub fn setCntL(self: *Self, halfword: u16) void {
+        /// TIMCNT_L Setter
+        pub fn setTimcntL(self: *Self, halfword: u16) void {
             self._reload = halfword;
         }
 
         /// TIMCNT_L & TIMCNT_H
-        pub fn setCnt(self: *Self, word: u32) void {
-            self.setCntL(@truncate(u16, word));
-            self.setCntH(@truncate(u16, word >> 16));
+        pub fn setTimcnt(self: *Self, word: u32) void {
+            self.setTimcntL(@truncate(u16, word));
+            self.setTimcntH(@truncate(u16, word >> 16));
         }
 
         /// TIMCNT_H
-        pub fn setCntH(self: *Self, halfword: u16) void {
+        pub fn setTimcntH(self: *Self, halfword: u16) void {
             const new = TimerControl{ .raw = halfword };
 
             // If Timer happens to be enabled, It will either be resheduled or disabled
@@ -132,12 +132,12 @@ fn Timer(comptime id: u2) type {
             if (!self.cnt.enabled.read() and new.enabled.read()) self._counter = self._reload;
 
             // If Timer is enabled and we're not cascading, we need to schedule an overflow event
-            if (new.enabled.read() and !new.cascade.read()) self.scheduleOverflow(0);
+            if (new.enabled.read() and !new.cascade.read()) self.rescheduleTimerExpire(0);
 
             self.cnt.raw = halfword;
         }
 
-        pub fn handleOverflow(self: *Self, cpu: *Arm7tdmi, late: u64) void {
+        pub fn onTimerExpire(self: *Self, cpu: *Arm7tdmi, late: u64) void {
             // Fire IRQ if enabled
             const io = &cpu.bus.io;
 
@@ -154,22 +154,22 @@ fn Timer(comptime id: u2) type {
 
             // DMA Sound Things
             if (id == 0 or id == 1) {
-                cpu.bus.apu.handleTimerOverflow(cpu, id);
+                cpu.bus.apu.onDmaAudioSampleRequest(cpu, id);
             }
 
             // Perform Cascade Behaviour
             switch (id) {
                 0 => if (cpu.bus.tim[1].cnt.cascade.read()) {
                     cpu.bus.tim[1]._counter +%= 1;
-                    if (cpu.bus.tim[1]._counter == 0) cpu.bus.tim[1].handleOverflow(cpu, late);
+                    if (cpu.bus.tim[1]._counter == 0) cpu.bus.tim[1].onTimerExpire(cpu, late);
                 },
                 1 => if (cpu.bus.tim[2].cnt.cascade.read()) {
                     cpu.bus.tim[2]._counter +%= 1;
-                    if (cpu.bus.tim[2]._counter == 0) cpu.bus.tim[2].handleOverflow(cpu, late);
+                    if (cpu.bus.tim[2]._counter == 0) cpu.bus.tim[2].onTimerExpire(cpu, late);
                 },
                 2 => if (cpu.bus.tim[3].cnt.cascade.read()) {
                     cpu.bus.tim[3]._counter +%= 1;
-                    if (cpu.bus.tim[3]._counter == 0) cpu.bus.tim[3].handleOverflow(cpu, late);
+                    if (cpu.bus.tim[3]._counter == 0) cpu.bus.tim[3].onTimerExpire(cpu, late);
                 },
                 3 => {}, // There is no Timer for TIM3 to "cascade" to,
             }
@@ -177,11 +177,11 @@ fn Timer(comptime id: u2) type {
             // Reschedule Timer if we're not cascading
             if (!self.cnt.cascade.read()) {
                 self._counter = self._reload;
-                self.scheduleOverflow(late);
+                self.rescheduleTimerExpire(late);
             }
         }
 
-        fn scheduleOverflow(self: *Self, late: u64) void {
+        fn rescheduleTimerExpire(self: *Self, late: u64) void {
             const when = (@as(u64, 0x10000) - self._counter) * self.frequency();
 
             self._start_timestamp = self.sched.now();

src/core/ppu.zig

@@ -10,7 +10,7 @@ const Bitfield = @import("bitfield").Bitfield;
 
 const Allocator = std.mem.Allocator;
 const log = std.log.scoped(.PPU);
-const pollBlankingDma = @import("bus/dma.zig").pollBlankingDma;
+const pollDmaOnBlank = @import("bus/dma.zig").pollDmaOnBlank;
 
 /// This is used to generate byuu / Talurabi's Color Correction algorithm
 const COLOUR_LUT = genColourLut();
@@ -562,7 +562,7 @@ pub const Ppu = struct {
         };
     }
 
-    pub fn handleHDrawEnd(self: *Self, cpu: *Arm7tdmi, late: u64) void {
+    pub fn onHdrawEnd(self: *Self, cpu: *Arm7tdmi, late: u64) void {
         // Transitioning to a Hblank
         if (self.dispstat.hblank_irq.read()) {
             cpu.bus.io.irq.hblank.set();
@@ -572,13 +572,13 @@ pub const Ppu = struct {
         // See if HBlank DMA is present and not enabled
 
         if (!self.dispstat.vblank.read())
-            pollBlankingDma(cpu.bus, .HBlank);
+            pollDmaOnBlank(cpu.bus, .HBlank);
 
         self.dispstat.hblank.set();
         self.sched.push(.HBlank, 68 * 4 -| late);
     }
 
-    pub fn handleHBlankEnd(self: *Self, cpu: *Arm7tdmi, late: u64) void {
+    pub fn onHblankEnd(self: *Self, cpu: *Arm7tdmi, late: u64) void {
         // The End of a Hblank (During Draw or Vblank)
         const old_scanline = self.vcount.scanline.read();
         const scanline = (old_scanline + 1) % 228;
@@ -614,7 +614,7 @@ pub const Ppu = struct {
                 self.aff_bg[1].latchRefPoints();
 
                 // See if Vblank DMA is present and not enabled
-                pollBlankingDma(cpu.bus, .VBlank);
+                pollDmaOnBlank(cpu.bus, .VBlank);
             }
 
             if (scanline == 227) self.dispstat.vblank.unset();
@@ -808,18 +808,6 @@ const Window = struct {
         self.in.raw = @truncate(u16, value);
         self.out.raw = @truncate(u16, value >> 16);
     }
-
-    pub fn setInL(self: *Self, value: u8) void {
-        self.in.raw = (self.in.raw & 0xFF00) | value;
-    }
-
-    pub fn setInH(self: *Self, value: u8) void {
-        self.in.raw = (self.in.raw & 0x00FF) | (@as(u16, value) << 8);
-    }
-
-    pub fn setOutL(self: *Self, value: u8) void {
-        self.out.raw = (self.out.raw & 0xFF00) | value;
-    }
 };
 
 const Background = struct {

src/core/scheduler.zig

@@ -43,22 +43,22 @@ pub const Scheduler = struct {
                 .Draw => {
                     // The end of a VDraw
                     cpu.bus.ppu.drawScanline();
-                    cpu.bus.ppu.handleHDrawEnd(cpu, late);
+                    cpu.bus.ppu.onHdrawEnd(cpu, late);
                 },
                 .TimerOverflow => |id| {
                     switch (id) {
-                        0 => cpu.bus.tim[0].handleOverflow(cpu, late),
-                        1 => cpu.bus.tim[1].handleOverflow(cpu, late),
-                        2 => cpu.bus.tim[2].handleOverflow(cpu, late),
-                        3 => cpu.bus.tim[3].handleOverflow(cpu, late),
+                        0 => cpu.bus.tim[0].onTimerExpire(cpu, late),
+                        1 => cpu.bus.tim[1].onTimerExpire(cpu, late),
+                        2 => cpu.bus.tim[2].onTimerExpire(cpu, late),
+                        3 => cpu.bus.tim[3].onTimerExpire(cpu, late),
                     }
                 },
                 .ApuChannel => |id| {
                     switch (id) {
-                        0 => cpu.bus.apu.ch1.channelTimerOverflow(late),
-                        1 => cpu.bus.apu.ch2.channelTimerOverflow(late),
-                        2 => cpu.bus.apu.ch3.channelTimerOverflow(late),
-                        3 => cpu.bus.apu.ch4.channelTimerOverflow(late),
+                        0 => cpu.bus.apu.ch1.onToneSweepEvent(late),
+                        1 => cpu.bus.apu.ch2.onToneEvent(late),
+                        2 => cpu.bus.apu.ch3.onWaveEvent(late),
+                        3 => cpu.bus.apu.ch4.onNoiseEvent(late),
                     }
                 },
                 .RealTimeClock => {
@@ -66,12 +66,12 @@ pub const Scheduler = struct {
                     if (device.kind != .Rtc or device.ptr == null) return;
 
                     const clock = @ptrCast(*Clock, @alignCast(@alignOf(*Clock), device.ptr.?));
-                    clock.updateTime(late);
+                    clock.onClockUpdate(late);
                 },
-                .FrameSequencer => cpu.bus.apu.tickFrameSequencer(late),
+                .FrameSequencer => cpu.bus.apu.onSequencerTick(late),
                 .SampleAudio => cpu.bus.apu.sampleAudio(late),
-                .HBlank => cpu.bus.ppu.handleHBlankEnd(cpu, late), // The end of a HBlank
-                .VBlank => cpu.bus.ppu.handleHDrawEnd(cpu, late), // The end of a VBlank
+                .HBlank => cpu.bus.ppu.onHblankEnd(cpu, late), // The end of a HBlank
+                .VBlank => cpu.bus.ppu.onHdrawEnd(cpu, late), // The end of a VBlank
             }
         }
     }

src/util.zig

@@ -162,17 +162,6 @@ pub const io = struct {
         }
     };
 };
-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", .{});
-
-    return 0;
-}
-
-pub fn writeUndefined(log: anytype, comptime format: []const u8, args: anytype) void {
-    log.warn(format, args);
-    if (builtin.mode == .Debug) std.debug.panic("TODO: Implement I/O Register", .{});
-}
 
 pub const Logger = struct {
     const Self = @This();
@@ -234,3 +223,81 @@ pub const Logger = struct {
 };
 
 const FmtArgTuple = std.meta.Tuple(&.{ u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32, u32 });
+
+pub const audio = struct {
+    const _io = @import("core/bus/io.zig");
+
+    const ToneSweep = @import("core/apu/ToneSweep.zig");
+    const Tone = @import("core/apu/Tone.zig");
+    const Wave = @import("core/apu/Wave.zig");
+    const Noise = @import("core/apu/Noise.zig");
+
+    pub const length = struct {
+        const FrameSequencer = @import("core/apu.zig").FrameSequencer;
+
+        /// Update State of Ch1, Ch2 and Ch3 length timer
+        pub fn update(comptime T: type, self: *T, fs: *const FrameSequencer, nrx34: _io.Frequency) void {
+            comptime std.debug.assert(T == ToneSweep or T == Tone or T == Wave);
+
+            // Write to NRx4 when FS's next step is not one that clocks the length counter
+            if (!fs.isLengthNext()) {
+                // If length_enable was disabled but is now enabled and length timer is not 0 already,
+                // decrement the length timer
+
+                if (!self.freq.length_enable.read() and nrx34.length_enable.read() and self.len_dev.timer != 0) {
+                    self.len_dev.timer -= 1;
+
+                    // If Length Timer is now 0 and trigger is clear, disable the channel
+                    if (self.len_dev.timer == 0 and !nrx34.trigger.read()) self.enabled = false;
+                }
+            }
+        }
+
+        pub const ch4 = struct {
+            /// update state of ch4 length timer
+            pub fn update(self: *Noise, fs: *const FrameSequencer, nr44: _io.NoiseControl) void {
+                // Write to NRx4 when FS's next step is not one that clocks the length counter
+                if (!fs.isLengthNext()) {
+                    // If length_enable was disabled but is now enabled and length timer is not 0 already,
+                    // decrement the length timer
+
+                    if (!self.cnt.length_enable.read() and nr44.length_enable.read() and self.len_dev.timer != 0) {
+                        self.len_dev.timer -= 1;
+
+                        // If Length Timer is now 0 and trigger is clear, disable the channel
+                        if (self.len_dev.timer == 0 and !nr44.trigger.read()) self.enabled = false;
+                    }
+                }
+            }
+        };
+    };
+};
+
+/// Sets the high bits of an integer to a value
+pub inline fn setHi(comptime T: type, left: T, right: HalfInt(T)) T {
+    return switch (T) {
+        u32 => (left & 0xFFFF_0000) | right,
+        u16 => (left & 0xFF00) | right,
+        u8 => (left & 0xF0) | right,
+        else => @compileError("unsupported type"),
+    };
+}
+
+/// sets the low bits of an integer to a value
+pub inline fn setLo(comptime T: type, left: T, right: HalfInt(T)) T {
+    return switch (T) {
+        u32 => (left & 0x0000_FFFF) | @as(u32, right) << 16,
+        u16 => (left & 0x00FF) | @as(u16, right) << 8,
+        u8 => (left & 0x0F) | @as(u8, right) << 4,
+        else => @compileError("unsupported type"),
+    };
+}
+
+/// The Integer type which corresponds to T with exactly half the amount of bits
+fn HalfInt(comptime T: type) type {
+    const type_info = @typeInfo(T);
+    comptime std.debug.assert(type_info == .Int); // Type must be an integer
+    comptime std.debug.assert(type_info.Int.bits % 2 == 0); // Type must have an even amount of bits
+
+    return std.meta.Int(type_info.Int.signedness, type_info.Int.bits >> 1);
+}