gba: latch timer register writes for 1 cycle (#1489)

Latches writes to timer registers for 1 cycle. This fixes common
off-by-one errors with precise timer use, which for example positively
impacts the test cases in #154.

ares/gba/cpu/cpu.cpp

@@ -70,6 +70,13 @@ auto CPU::step(u32 clocks) -> void {
     timer[1].run();
     timer[2].run();
     timer[3].run();
+    if(context.timerLatched) {
+      timer[0].stepLatch();
+      timer[1].stepLatch();
+      timer[2].stepLatch();
+      timer[3].stepLatch();
+      context.timerLatched = 0;
+    }
     context.clock++;
   }
 

ares/gba/cpu/cpu.hpp

@@ -128,6 +128,7 @@ struct CPU : ARM7TDMI, Thread, IO {
 
   struct Timer {
     //timer.cpp
+    auto stepLatch() -> void;
     auto run() -> void;
     auto step() -> void;
 
@@ -142,6 +143,13 @@ struct CPU : ARM7TDMI, Thread, IO {
     n1  cascade;
     n1  irq;
     n1  enable;
+    
+    struct Latch {
+      n16 reload;
+      n8  control;
+      n2  reloadFlags;
+      n1  controlFlag;
+    } latch;
   } timer[4];
 
   struct Serial {
@@ -229,6 +237,7 @@ struct CPU : ARM7TDMI, Thread, IO {
     n1  stopped;
     n1  booted;  //set to true by the GBA BIOS
     n1  dmaActive;
+    n1  timerLatched;
   } context;
 };
 

ares/gba/cpu/io.cpp

@@ -301,24 +301,23 @@ auto CPU::writeIO(n32 address, n8 data) -> void {
   }
 
   //TM0CNT_L, TM1CNT_L, TM2CNT_L, TM3CNT_L
-  case 0x0400'0100: case 0x0400'0104: case 0x0400'0108: case 0x0400'010c: timer().reload.byte(0) = data; return;
-  case 0x0400'0101: case 0x0400'0105: case 0x0400'0109: case 0x0400'010d: timer().reload.byte(1) = data; return;
+  case 0x0400'0100: case 0x0400'0104: case 0x0400'0108: case 0x0400'010c:
+    timer().latch.reload.byte(0) = data;
+    timer().latch.reloadFlags.bit(0) = 1;
+    context.timerLatched = 1;
+    return;
+  case 0x0400'0101: case 0x0400'0105: case 0x0400'0109: case 0x0400'010d:
+    timer().latch.reload.byte(1) = data;
+    timer().latch.reloadFlags.bit(1) = 1;
+    context.timerLatched = 1;
+    return;
 
   //TM0CNT_H, TM1CNT_H, TM2CNT_H, TM3CNT_H
-  case 0x0400'0102: case 0x0400'0106: case 0x0400'010a: case 0x0400'010e: {
-    bool enable = timer().enable;
-
-    timer().frequency = data.bit(0,1);
-    timer().irq       = data.bit(6);
-    timer().enable    = data.bit(7);
-
-    if(address != 0x0400'0102) timer().cascade = data.bit(2);
-
-    if(!enable && timer().enable) {  //0->1 transition
-      timer().pending = true;
-    }
+  case 0x0400'0102: case 0x0400'0106: case 0x0400'010a: case 0x0400'010e:
+    timer().latch.control = data;
+    timer().latch.controlFlag = 1;
+    context.timerLatched = 1;
     return;
-  }
   case 0x0400'0103: case 0x0400'0107: case 0x0400'010b: case 0x0400'010f:
     return;
 

ares/gba/cpu/serialization.cpp

@@ -42,6 +42,10 @@ auto CPU::serialize(serializer& s) -> void {
     s(timer.cascade);
     s(timer.irq);
     s(timer.enable);
+    s(timer.latch.reload);
+    s(timer.latch.control);
+    s(timer.latch.reloadFlags);
+    s(timer.latch.controlFlag);
   }
 
   s(serial.shiftClockSelect);
@@ -104,4 +108,5 @@ auto CPU::serialize(serializer& s) -> void {
   s(context.stopped);
   s(context.booted);
   s(context.dmaActive);
+  s(context.timerLatched);
 }

ares/gba/cpu/timer.cpp

@@ -1,3 +1,30 @@
+auto CPU::Timer::stepLatch() -> void {
+  if(latch.reloadFlags.bit(0)) {
+    reload.byte(0) = latch.reload.byte(0);
+    latch.reloadFlags.bit(0) = 0;
+  }
+  
+  if(latch.reloadFlags.bit(1)) {
+    reload.byte(1) = latch.reload.byte(1);
+    latch.reloadFlags.bit(1) = 0;
+  }
+  
+  if(latch.controlFlag) {
+    n1 wasEnabled = enable;
+
+    frequency = latch.control.bit(0,1);
+    irq       = latch.control.bit(6);
+    enable    = latch.control.bit(7);
+
+    if(id != 0) cascade = latch.control.bit(2);
+
+    if(!wasEnabled && enable) {  //0->1 transition
+      pending = true;
+    }
+    latch.controlFlag = 0;
+  }
+}
+
 inline auto CPU::Timer::run() -> void {
   if(pending) {
     pending = false;

ares/gba/system/serialization.cpp

@@ -1,4 +1,4 @@
-static const string SerializerVersion = "v131";
+static const string SerializerVersion = "v138";
 
 auto System::serialize(bool synchronize) -> serializer {
   if(synchronize) scheduler.enter(Scheduler::Mode::Synchronize);