feat: improve DMA Transfer support

src/bus/dma.zig

@@ -35,6 +35,11 @@ pub fn DmaController(comptime id: u2) type {
         /// Internal. Word Count
         _word_count: if (id == 3) u16 else u14,
 
+        /// 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
+        enabled: bool,
+
         pub fn init() Self {
             return .{
                 .id = id,
@@ -47,6 +52,7 @@ pub fn DmaController(comptime id: u2) type {
                 ._sad = 0,
                 ._dad = 0,
                 ._word_count = 0,
+                .enabled = false,
             };
         }
 
@@ -70,6 +76,9 @@ pub fn DmaController(comptime id: u2) type {
                 self._sad = self.sad;
                 self._dad = 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
+                self.enabled = new.start_timing.read() == 0b00;
             }
 
             self.cnt.raw = halfword;
@@ -80,7 +89,9 @@ pub fn DmaController(comptime id: u2) type {
             self.writeCntHigh(@truncate(u16, word >> 16));
         }
 
-        pub fn step(self: *Self, bus: *Bus) void {
+        pub fn step(self: *Self, bus: *Bus) bool {
+            if (!self.enabled or !self.cnt.enabled.read()) return false;
+
             const sad_adj = std.meta.intToEnum(Adjustment, self.cnt.sad_adj.read()) catch unreachable;
             const dad_adj = std.meta.intToEnum(Adjustment, self.cnt.dad_adj.read()) catch unreachable;
 
@@ -123,14 +134,46 @@ pub fn DmaController(comptime id: u2) type {
                 }
 
                 self.cnt.enabled.unset();
+                self.enabled = false;
+            }
+
+            return true;
+        }
+
+        pub fn isBlocking(self: *const Self) bool {
+            // A DMA Transfer is Blocking if it is Immediate
+            return self.cnt.start_timing.read() == 0b00;
+        }
+
+        pub fn pollBlankingDma(self: *Self, comptime kind: DmaKind) void {
+            if (self.enabled) return;
+
+            switch (kind) {
+                .HBlank => self.enabled = self.cnt.enabled.read() and self.cnt.start_timing.read() == 0b10,
+                .VBlank => self.enabled = self.cnt.enabled.read() and self.cnt.start_timing.read() == 0b01,
+                .Immediate, .Special => {},
             }
         }
     };
 }
 
+pub fn pollBlankingDma(bus: *Bus, comptime kind: DmaKind) void {
+    bus.io.dma0.pollBlankingDma(kind);
+    bus.io.dma1.pollBlankingDma(kind);
+    bus.io.dma2.pollBlankingDma(kind);
+    bus.io.dma3.pollBlankingDma(kind);
+}
+
 const Adjustment = enum(u2) {
     Increment = 0,
     Decrement = 1,
     Fixed = 2,
     IncrementReload = 3,
 };
+
+const DmaKind = enum(u2) {
+    Immediate = 0,
+    HBlank,
+    VBlank,
+    Special,
+};

src/cpu.zig

@@ -296,30 +296,10 @@ pub const Arm7tdmi = struct {
     }
 
     fn handleDMATransfers(self: *Self) bool {
-        const dma0 = &self.bus.io.dma0;
+        if (self.bus.io.dma0.step(self.bus)) return self.bus.io.dma0.isBlocking();
-        const dma1 = &self.bus.io.dma1;
+        if (self.bus.io.dma1.step(self.bus)) return self.bus.io.dma1.isBlocking();
-        const dma2 = &self.bus.io.dma2;
+        if (self.bus.io.dma2.step(self.bus)) return self.bus.io.dma2.isBlocking();
-        const dma3 = &self.bus.io.dma3;
+        if (self.bus.io.dma3.step(self.bus)) return self.bus.io.dma3.isBlocking();
-
-        if (dma0.cnt.enabled.read() and dma0.cnt.start_timing.read() == 0) {
-            dma0.step(self.bus);
-            return true;
-        }
-
-        if (dma1.cnt.enabled.read() and dma1.cnt.start_timing.read() == 0) {
-            dma1.step(self.bus);
-            return true;
-        }
-
-        if (dma2.cnt.enabled.read() and dma2.cnt.start_timing.read() == 0) {
-            dma2.step(self.bus);
-            return true;
-        }
-
-        if (dma3.cnt.enabled.read() and dma3.cnt.start_timing.read() == 0) {
-            dma3.step(self.bus);
-            return true;
-        }
 
         return false;
     }

src/scheduler.zig

@@ -1,5 +1,7 @@
 const std = @import("std");
 
+const pollBlankingDma = @import("bus/dma.zig").pollBlankingDma;
+
 const Bus = @import("Bus.zig");
 const Arm7tdmi = @import("cpu.zig").Arm7tdmi;
 
@@ -68,6 +70,9 @@ pub const Scheduler = struct {
                                 irq.vblank.set();
                                 cpu.handleInterrupt();
                             }
+
+                            // See if Vblank DMA is present and not enabled
+                            pollBlankingDma(bus, .VBlank);
                         }
 
                         if (scanline == 227) stat.vblank.unset();
@@ -84,6 +89,9 @@ pub const Scheduler = struct {
                         cpu.handleInterrupt();
                     }
 
+                    // See if Hblank DMA is present and not enabled
+                    pollBlankingDma(bus, .HBlank);
+
                     bus.ppu.dispstat.hblank.set();
                     self.push(.HBlank, self.tick + (68 * 4));
                 },
@@ -96,6 +104,9 @@ pub const Scheduler = struct {
                         cpu.handleInterrupt();
                     }
 
+                    // See if Hblank DMA is present and not enabled
+                    pollBlankingDma(bus, .HBlank);
+
                     bus.ppu.dispstat.hblank.set();
                     self.push(.HBlank, self.tick + (68 * 4));
                 },