fix(timer): Reading TCNT in 2-cycle instructions

close #40

3 files changed, 35 insertions, 16 deletions

diff --git a/src/cpu/instruction.ts b/src/cpu/instruction.ts
index 223a320..51bc3ab 100644
--- a/src/cpu/instruction.ts
+++ b/src/cpu/instruction.ts
@@ -347,78 +347,78 @@ export function avrInstruction(cpu: ICPU) {
     cpu.data[((opcode & 0xf0) >> 4) + 16] = (opcode & 0xf) | ((opcode & 0xf00) >> 4);
   } else if ((opcode & 0xfe0f) === 0x9000) {
     /* LDS, 1001 000d dddd 0000 kkkk kkkk kkkk kkkk */
+    cpu.cycles++;
     const value = cpu.readData(cpu.progMem[cpu.pc + 1]);
     cpu.data[(opcode & 0x1f0) >> 4] = value;
     cpu.pc++;
-    cpu.cycles++;
   } else if ((opcode & 0xfe0f) === 0x900c) {
     /* LDX, 1001 000d dddd 1100 */
-    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(cpu.dataView.getUint16(26, true));
     cpu.cycles++;
+    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(cpu.dataView.getUint16(26, true));
   } else if ((opcode & 0xfe0f) === 0x900d) {
     /* LDX(INC), 1001 000d dddd 1101 */
     const x = cpu.dataView.getUint16(26, true);
+    cpu.cycles++;
     cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(x);
     cpu.dataView.setUint16(26, x + 1, true);
-    cpu.cycles++;
   } else if ((opcode & 0xfe0f) === 0x900e) {
     /* LDX(DEC), 1001 000d dddd 1110 */
     const x = cpu.dataView.getUint16(26, true) - 1;
     cpu.dataView.setUint16(26, x, true);
-    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(x);
     cpu.cycles++;
+    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(x);
   } else if ((opcode & 0xfe0f) === 0x8008) {
     /* LDY, 1000 000d dddd 1000 */
-    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(cpu.dataView.getUint16(28, true));
     cpu.cycles++;
+    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(cpu.dataView.getUint16(28, true));
   } else if ((opcode & 0xfe0f) === 0x9009) {
     /* LDY(INC), 1001 000d dddd 1001 */
     const y = cpu.dataView.getUint16(28, true);
+    cpu.cycles++;
     cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(y);
     cpu.dataView.setUint16(28, y + 1, true);
-    cpu.cycles++;
   } else if ((opcode & 0xfe0f) === 0x900a) {
     /* LDY(DEC), 1001 000d dddd 1010 */
     const y = cpu.dataView.getUint16(28, true) - 1;
     cpu.dataView.setUint16(28, y, true);
-    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(y);
     cpu.cycles++;
+    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(y);
   } else if (
     (opcode & 0xd208) === 0x8008 &&
     (opcode & 7) | ((opcode & 0xc00) >> 7) | ((opcode & 0x2000) >> 8)
   ) {
     /* LDDY, 10q0 qq0d dddd 1qqq */
+    cpu.cycles++;
     cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(
       cpu.dataView.getUint16(28, true) +
         ((opcode & 7) | ((opcode & 0xc00) >> 7) | ((opcode & 0x2000) >> 8))
     );
-    cpu.cycles++;
   } else if ((opcode & 0xfe0f) === 0x8000) {
     /* LDZ, 1000 000d dddd 0000 */
-    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(cpu.dataView.getUint16(30, true));
     cpu.cycles++;
+    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(cpu.dataView.getUint16(30, true));
   } else if ((opcode & 0xfe0f) === 0x9001) {
     /* LDZ(INC), 1001 000d dddd 0001 */
     const z = cpu.dataView.getUint16(30, true);
+    cpu.cycles++;
     cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(z);
     cpu.dataView.setUint16(30, z + 1, true);
-    cpu.cycles++;
   } else if ((opcode & 0xfe0f) === 0x9002) {
     /* LDZ(DEC), 1001 000d dddd 0010 */
     const z = cpu.dataView.getUint16(30, true) - 1;
     cpu.dataView.setUint16(30, z, true);
-    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(z);
     cpu.cycles++;
+    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(z);
   } else if (
     (opcode & 0xd208) === 0x8000 &&
     (opcode & 7) | ((opcode & 0xc00) >> 7) | ((opcode & 0x2000) >> 8)
   ) {
     /* LDDZ, 10q0 qq0d dddd 0qqq */
+    cpu.cycles++;
     cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(
       cpu.dataView.getUint16(30, true) +
         ((opcode & 7) | ((opcode & 0xc00) >> 7) | ((opcode & 0x2000) >> 8))
     );
-    cpu.cycles++;
   } else if (opcode === 0x95c8) {
     /* LPM, 1001 0101 1100 1000 */
     cpu.data[0] = cpu.progBytes[cpu.dataView.getUint16(30, true)];
diff --git a/src/peripherals/timer.spec.ts b/src/peripherals/timer.spec.ts
index 7d280ed..8733f73 100644
--- a/src/peripherals/timer.spec.ts
+++ b/src/peripherals/timer.spec.ts
@@ -95,7 +95,7 @@ describe('timer', () => {
     cpu.cycles = 1;
     timer.tick();
     const tcnt = cpu.readData(0x46);
-    expect(tcnt).toEqual(0); // TCNT should be 0
+    expect(tcnt).toEqual(2); // TCNT should be 2 (one tick above + 2 cycles for interrupt)
     expect(cpu.data[0x35]).toEqual(0); // TOV bit in TIFR should be clear
     expect(cpu.pc).toEqual(0x20);
     expect(cpu.cycles).toEqual(3);
@@ -183,7 +183,7 @@ describe('timer', () => {
     cpu.cycles = 1;
     timer.tick();
     const tcnt = cpu.readData(0x46);
-    expect(tcnt).toEqual(0x21); // TCNT should be 0x21
+    expect(tcnt).toEqual(0x23); // TCNT should be 0x23 (one tick above + 2 cycles for interrupt)
     expect(cpu.data[0x35]).toEqual(0); // OCFA bit in TIFR should be clear
     expect(cpu.pc).toEqual(0x1c);
     expect(cpu.cycles).toEqual(3);
@@ -216,7 +216,7 @@ describe('timer', () => {
     cpu.cycles = 1;
     timer.tick();
     const tcnt = cpu.readData(0x46);
-    expect(tcnt).toEqual(0x21); // TCNT should be 0x21
+    expect(tcnt).toEqual(0x23); // TCNT should be 0x23 (one tick above + 2 cycles for interrupt)
     expect(cpu.data[0x35]).toEqual(0); // OCFB bit in TIFR should be clear
     expect(cpu.pc).toEqual(0x1e);
     expect(cpu.cycles).toEqual(3);
@@ -255,6 +255,24 @@ describe('timer', () => {
     expect(cpu.readData(0xb2)).toEqual(2); // TCNT2 should be 2
   });
 
+  it('should update TCNT as it is being read by a 2-cycle instruction (issue #40)', () => {
+    const program = [
+      'LDI r16, 0x1', // TCCR0B = 1 << CS00;
+      'OUT 0x25, r16',
+      'LDI r16, 0x0', // TCNT0 <- 0x30
+      'OUT 0x26, r16',
+      'NOP',
+      'LDS r1, 0x46', // r17 <- TCNT0 (2 cycles)
+    ];
+    loadProgram(...program);
+    const timer = new AVRTimer(cpu, timer0Config);
+    for (let i = 0; i < program.length; i++) {
+      avrInstruction(cpu);
+      timer.tick();
+    }
+    expect(cpu.data[1]).toEqual(2); // r1 should equal 2
+  });
+
   describe('16 bit timers', () => {
     it('should increment 16-bit TCNT by 1', () => {
       const timer = new AVRTimer(cpu, timer1Config);
@@ -301,7 +319,7 @@ describe('timer', () => {
       cpu.cycles = 1;
       timer.tick();
       cpu.readData(0x84); // Refresh TCNT1
-      expect(cpu.dataView.getUint16(0x84, true)).toEqual(0); // TCNT1 should be 0
+      expect(cpu.dataView.getUint16(0x84, true)).toEqual(2); // TCNT1 should be 0 (one tick above + 2 cycles for interrupt)
       expect(cpu.data[0x36]).toEqual(0); // TOV bit in TIFR should be clear
       expect(cpu.pc).toEqual(0x1a);
       expect(cpu.cycles).toEqual(3);
diff --git a/src/peripherals/timer.ts b/src/peripherals/timer.ts
index 04c09aa..7cae2ac 100644
--- a/src/peripherals/timer.ts
+++ b/src/peripherals/timer.ts
@@ -197,6 +197,7 @@ export class AVRTimer {
   constructor(private cpu: CPU, private config: AVRTimerConfig) {
     this.updateWGMConfig();
     this.cpu.readHooks[config.TCNT] = (addr: u8) => {
+      this.tick();
       if (this.config.bits === 16) {
         this.cpu.data[addr + 1] = this.tcnt >> 8;
       }