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Cpu.cs
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using System;
using System.Diagnostics;
// References
// - GitHub Copilot -for a very fast start (500+ instructions in 3-4h), which would prove rather error prone later
// - https://gbdev.io/gb-opcodes/optables/ for visual reference of the instructions while coding them, or for timing
// - http://marc.rawer.de/Gameboy/Docs/GBCPUman.pdf probably the best GameBoy CPU manual -though not exempt of inaccuracies
// - https://github.com/retrio/gb-test-roms as source for Blargg test roms for CPU instuctions
// - https://github.com/TheSorm/Boy (for the toughest instructions to pass the Blargg tests, 2-3 would have been impossible to guess)
namespace ZarthGB
{
class Cpu
{
private Memory memory;
public bool Stopped { get; private set; }
public bool Halted { get; private set; }
private ushort[] pcTrace = new ushort[6553600];
private int pcTracePtr = 0;
#region Flags
private byte flags;
public byte Flags
{
get => flags;
private set => flags = (byte) (value & 0xF0); // Do not attempt to write the unused flag space
}
public bool FlagZ
{
get => (Flags & 0x80) != 0;
private set => Flags = (byte)((Flags & 0x7F) | (value ? 0x80 : 0));
}
public bool FlagN
{
get => (Flags & 0x40) != 0;
private set => Flags = (byte)((Flags & 0xBF) | (value ? 0x40 : 0));
}
public bool FlagH
{
get => (Flags & 0x20) != 0;
private set => Flags = (byte)((Flags & 0xDF) | (value ? 0x20 : 0));
}
public bool FlagC
{
get => (Flags & 0x10) != 0;
private set => Flags = (byte)((Flags & 0xEF) | (value ? 0x10 : 0));
}
#endregion
#region Registers
public byte A { get; private set; }
public byte B { get; private set; }
public byte C { get; private set; }
public byte D { get; private set; }
public byte E { get; private set; }
public byte H { get; private set; }
public byte L { get; private set; }
public ushort SP { get; private set; }
public ushort PC { get; private set; }
public ushort AF
{
get => (ushort) ((A << 8) | Flags);
private set
{
A = (byte)(value >> 8);
Flags = (byte) (value & 0xFF);
}
}
public ushort BC
{
get => (ushort)((B << 8) | C);
private set
{
B = (byte)(value >> 8);
C = (byte)(value & 0xFF);
}
}
public ushort DE
{
get => (ushort)((D << 8) | E);
private set
{
D = (byte)(value >> 8);
E = (byte)(value & 0xFF);
}
}
public ushort HL
{
get => (ushort)((H << 8) | L);
private set
{
H = (byte)(value >> 8);
L = (byte)(value & 0xFF);
}
}
#endregion
#region Cycles
public int Steps { get; private set; }
//private int TransferInProgressTicks = 0;
public int Ticks
{
get => memory.Ticks;
private set
{
int increment = value - memory.Ticks;
for (int i = 0; i < increment; i++)
{
if ((memory.Ticks + i) % 268 == 0) // DIV goes 16384 times per second, so 1/268th of 4.19 MHz
memory.IncrementDiv();
if (memory.TimerEnabled)
{
if ((memory.Ticks + i) % memory.TimerTick == 0)
{
int newCounterValue = memory[0xff05] + 1;
memory[0xff05] = (byte) (newCounterValue & 0xFF);
if (newCounterValue > 0xFF)
{
memory[0xff05] = memory[0xff06];
if ((InterruptEnable & InterruptsTimer) > 0)
{
InterruptFlags |= InterruptsTimer;
}
}
}
}
}
// Process serial port if needed
/*if (TransferInProgressTicks > 0) // Transfer in progress
{
TransferInProgressTicks--;
if (TransferInProgressTicks == 0)
{
memory[0xff01] = 0xFF; // No other GameBoy present
if ((memory[0xffff] & Cpu.InterruptsSerial) > 0)
memory[0xff0f] |= Cpu.InterruptsSerial;
}
}
if (memory[0xff02] >> 7 > 0) // New transfer
{
// Mark transfer finished -we don't have serial transfers here
memory[0xff02] = (byte)(memory[0xff02] & 0x7F);
TransferInProgressTicks = 8;
}*/
memory.Ticks = value;
}
}
private int[] normalInstructionHalfTicks = {
2, 6, 4, 4, 2, 2, 4, 4, 10, 4, 4, 4, 2, 2, 4, 2, // 0x0_
2, 6, 4, 4, 2, 2, 4, 4, 6, 4, 4, 4, 2, 2, 4, 2, // 0x1_
4, 6, 4, 4, 2, 2, 4, 2, 4, 4, 4, 4, 2, 2, 4, 2, // 0x2_
4, 6, 4, 4, 6, 6, 6, 2, 4, 4, 4, 4, 2, 2, 4, 2, // 0x3_
2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 4, 2, // 0x4_
2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 4, 2, // 0x5_
2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 4, 2, // 0x6_
4, 4, 4, 4, 4, 4, 2, 4, 2, 2, 2, 2, 2, 2, 4, 2, // 0x7_
2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 4, 2, // 0x8_
2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 4, 2, // 0x9_
2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 4, 2, // 0xa_
2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 4, 2, // 0xb_
4, 6, 6, 8, 6, 8, 4, 8, 4, 8, 6, 2, 6,12, 4, 8, // 0xc_
4, 6, 6, 0, 6, 8, 4, 8, 4, 8, 6, 0, 6, 0, 4, 8, // 0xd_
6, 6, 4, 0, 0, 8, 4, 8, 8, 2, 8, 0, 0, 0, 4, 8, // 0xe_
6, 6, 4, 2, 0, 8, 4, 8, 6, 4, 8, 2, 0, 0, 4, 8 // 0xf_
};
private int[] prefixInstructionHalfTicks = {
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0x0_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0x1_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0x2_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0x3_
4, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 4, 4, 4, 6, 4, // 0x4_
4, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 4, 4, 4, 6, 4, // 0x5_
4, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 4, 4, 4, 6, 4, // 0x6_
4, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 4, 4, 4, 6, 4, // 0x7_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0x8_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0x9_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0xa_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0xb_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0xc_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0xd_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4, // 0xe_
4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 4 // 0xf_
};
#endregion
#region Interrupts
public const byte InterruptsVblank = (1 << 0);
public const byte InterruptsLcdstat = (1 << 1);
public const byte InterruptsTimer = (1 << 2);
public const byte InterruptsSerial = (1 << 3);
public const byte InterruptsJoypad = (1 << 4);
private bool InterruptMasterEnable { get; set; }
private byte InterruptEnable
{
get => memory[0xffff];
set => memory[0xffff] = value;
}
private byte InterruptFlags
{
get => memory[0xff0f];
set => memory[0xff0f] = value;
}
private void InterruptStep()
{
if (InterruptMasterEnable || Halted)
if (InterruptEnable != 0 && InterruptFlags != 0)
{
byte fire = (byte) (InterruptEnable & InterruptFlags);
if ((fire & InterruptsVblank) != 0)
{
Halted = false;
if (InterruptMasterEnable)
{
InterruptFlags = (byte) (InterruptFlags & ~InterruptsVblank);
Vblank();
}
}
if ((fire & InterruptsLcdstat) != 0)
{
Halted = false;
if (InterruptMasterEnable)
{
InterruptFlags = (byte)(InterruptFlags & ~InterruptsLcdstat);
LcdStat();
}
}
if ((fire & InterruptsTimer) != 0)
{
Halted = false;
if (InterruptMasterEnable)
{
InterruptFlags = (byte)(InterruptFlags & ~InterruptsTimer);
Timer();
}
}
if ((fire & InterruptsSerial) != 0)
{
Halted = false;
if (InterruptMasterEnable)
{
InterruptFlags = (byte) (InterruptFlags & ~InterruptsSerial);
Serial();
}
}
if ((fire & InterruptsJoypad) != 0)
{
Halted = false;
if (InterruptMasterEnable)
{
InterruptFlags = (byte)(InterruptFlags & ~InterruptsJoypad);
Joypad();
}
}
}
}
private void Vblank()
{
//drawFramebuffer();
InterruptMasterEnable = false;
PushWord(PC);
PC = 0x40;
Ticks += 12;
}
private void LcdStat()
{
InterruptMasterEnable = false;
PushWord(PC);
PC = 0x48;
Ticks += 12;
}
private void Timer()
{
InterruptMasterEnable = false;
PushWord(PC);
PC = 0x50;
Ticks += 12;
}
private void Serial()
{
InterruptMasterEnable = false;
PushWord(PC);
PC = 0x58;
Ticks += 12;
}
private void Joypad()
{
InterruptMasterEnable = false;
PushWord(PC);
PC = 0x60;
Ticks += 12;
}
#endregion
public Cpu(Memory memory)
{
this.memory = memory;
Reset();
}
public void Reset()
{
// Default values for DMG (from TCAGBD.pdf)
A = 0x01;
Flags = 0xB0;
B = 0x00;
C = 0x13;
D = 0x00;
E = 0xD8;
H = 0x01;
L = 0x4D;
SP = 0xFFFE;
PC = 0x0000; // But force to go through Boot ROM -because I can
//PC = 0x0100; // Skip Boot ROM
//memory[0xFF50] = 0x01; // Disable Boot ROM
Stopped = false;
Halted = false;
Steps = 0;
}
public void KeyPressed()
{
Stopped = false;
if((InterruptEnable & InterruptsJoypad) > 0)
InterruptFlags |= InterruptsJoypad;
}
public void Step()
{
InterruptStep();
if (Stopped || Halted)
{
Ticks += 4;
return;
}
#region Tracing
pcTrace[pcTracePtr++] = PC;
if (pcTracePtr == pcTrace.Length) pcTracePtr = 0;
if (pcTracePtr > 20 &&
pcTrace[pcTracePtr - 2] == PC - 1 &&
pcTrace[pcTracePtr - 3] == PC - 2 &&
pcTrace[pcTracePtr - 4] == PC - 3 &&
pcTrace[pcTracePtr - 5] == PC - 4 &&
pcTrace[pcTracePtr - 6] == PC - 5 &&
pcTrace[pcTracePtr - 7] == PC - 6 &&
pcTrace[pcTracePtr - 8] == PC - 7 &&
pcTrace[pcTracePtr - 9] == PC - 8 &&
pcTrace[pcTracePtr - 10] == PC - 9 &&
pcTrace[pcTracePtr - 11] == PC - 10 &&
pcTrace[pcTracePtr - 12] == PC - 11 &&
pcTrace[pcTracePtr - 13] == PC - 12 &&
pcTrace[pcTracePtr - 14] == PC - 13 &&
pcTrace[pcTracePtr - 15] == PC - 14 &&
pcTrace[pcTracePtr - 16] == PC - 15 &&
pcTrace[pcTracePtr - 17] == PC - 16 &&
pcTrace[pcTracePtr - 18] == PC - 17 &&
pcTrace[pcTracePtr - 19] == PC - 18 &&
pcTrace[pcTracePtr - 20] == PC - 19
)
PC = PC;
if (PC == 0x100)
PC = PC;
if (PC == 0xC32F)
PC = PC;
#endregion
Steps++;
byte opcode = ReadByte();
Ticks += normalInstructionHalfTicks[opcode] << 1;
//Debug.Print($"Step={Steps} PC={PC:X4} OP={opcode:X2} A={A:X2} F={Flags:X2} B={B:X2} C={C:X2} D={D:X2} E={E:X2} H={H:X2} L={L:X2} SP={SP:X4}");
switch(opcode)
{
case 0x00: // NOP
break;
case 0x01: // LD BC, nn
BC = ReadWord();
break;
case 0x02: // LD (BC), A
WriteByte(BC, A);
break;
case 0x03: // INC BC
BC++;
break;
case 0x04: // INC B
B = Increment(B);
break;
case 0x05: // DEC B
B = Decrement(B);
break;
case 0x06: // LD B, n
B = ReadByte();
break;
case 0x07: // RLCA
A = Rlc(A);
FlagZ = false;
break;
case 0x08: // LD (nn), SP
WriteWord(ReadWord(), SP);
break;
case 0x09: // ADD HL, BC
HL = Add16(HL, BC);
break;
case 0x0A: // LD A, (BC)
A = ReadByte(BC);
break;
case 0x0B: // DEC BC
BC--;
break;
case 0x0C: // INC C
C = Increment(C);
break;
case 0x0D: // DEC C
C = Decrement(C);
break;
case 0x0E: // LD C, n
C = ReadByte();
break;
case 0x0F: // RRCA
A = Rrc(A);
FlagZ = false;
break;
case 0x10: // STOP
// Wait until button pressed
Stopped = true;
break;
case 0x11: // LD DE, nn
DE = ReadWord();
break;
case 0x12: // LD (DE), A
WriteByte(DE, A);
break;
case 0x13: // INC DE
DE++;
break;
case 0x14: // INC D
D = Increment(D);
break;
case 0x15: // DEC D
D = Decrement(D);
break;
case 0x16: // LD D, n
D = ReadByte();
break;
case 0x17: // RLA
A = Rl(A);
FlagZ = false;
break;
case 0x18: // JR n // Verified OK - 01
PC += (ushort)(sbyte)ReadByte();
PC++;
break;
case 0x19: // ADD HL, DE
HL = Add16(HL, DE);
break;
case 0x1A: // LD A, (DE)
A = ReadByte(DE);
break;
case 0x1B: // DEC DE
DE--;
break;
case 0x1C: // INC E
E = Increment(E);
break;
case 0x1D: // DEC E
E = Decrement(E);
break;
case 0x1E: // LD E, n
E = ReadByte();
break;
case 0x1F: // RRA
A = Rr(A);
FlagZ = false;
break;
case 0x20: // JR NZ, n
if (!FlagZ)
{
PC += (ushort)(sbyte)ReadByte();
Ticks += 4;
}
PC++;
break;
case 0x21: // LD HL, nn
HL = ReadWord();
break;
case 0x22: // LD (HL+), A
WriteByte(HL++, A);
break;
case 0x23: // INC HL
HL++;
break;
case 0x24: // INC H
H = Increment(H);
break;
case 0x25: // DEC H
H = Decrement(H);
break;
case 0x26: // LD H, n
H = ReadByte();
break;
case 0x27: // DAA // Verified OK - 01, 11
int resultDAA = A;
if (!FlagN)
{
if (FlagH || (resultDAA & 0xF) > 9)
resultDAA += 0x06;
if (FlagC || resultDAA > 0x9F)
resultDAA += 0x60;
}
else
{
if (FlagH)
resultDAA = (resultDAA - 6) & 0xFF;
if (FlagC)
resultDAA -= 0x60;
}
FlagZ = false;
FlagH = false;
if ((resultDAA & 0x100) == 0x100)
FlagC = true;
resultDAA &= 0xFF;
if (resultDAA == 0)
FlagZ = true;
A = (byte) (resultDAA & 0xFF);
break;
case 0x28: // JR Z, n
if (FlagZ)
{
PC += (ushort)(sbyte)ReadByte();
Ticks += 4;
}
PC++;
break;
case 0x29: // ADD HL, HL
HL = Add16(HL, HL);
break;
case 0x2A: // LD A, (HL+)
A = ReadByte(HL++);
break;
case 0x2B: // DEC HL
HL--;
break;
case 0x2C: // INC L
L = Increment(L);
break;
case 0x2D: // DEC L
L = Decrement(L);
break;
case 0x2E: // LD L, n
L = ReadByte();
break;
case 0x2F: // CPL
A = (byte)~A;
FlagN = true;
FlagH = true;
break;
case 0x30: // JR NC, n
if (!FlagC)
{
PC += (ushort)(sbyte)ReadByte();
Ticks += 4;
}
PC++;
break;
case 0x31: // LD SP, nn
SP = ReadWord();
break;
case 0x32: // LD (HL-), A
WriteByte(HL--, A);
break;
case 0x33: // INC SP
SP++;
break;
case 0x34: // INC (HL)
WriteByte(HL, Increment(ReadByte(HL)));
break;
case 0x35: // DEC (HL)
WriteByte(HL, Decrement(ReadByte(HL)));
break;
case 0x36: // LD (HL), n
WriteByte(HL, ReadByte());
break;
case 0x37: // SCF
FlagN = false;
FlagH = false;
FlagC = true;
break;
case 0x38: // JR C, n
if (FlagC)
{
PC += (ushort)(sbyte)ReadByte();
Ticks += 4;
}
PC++;
break;
case 0x39: // ADD HL, SP
HL = Add16(HL, SP);
break;
case 0x3A: // LD A, (HL-)
A = ReadByte(HL--);
break;
case 0x3B: // DEC SP
SP--;
break;
case 0x3C: // INC A
A = Increment(A);
break;
case 0x3D: // DEC A
A = Decrement(A);
break;
case 0x3E: // LD A, n
A = ReadByte();
break;
case 0x3F: // CCF
FlagN = false;
FlagH = false;
FlagC = !FlagC;
break;
case 0x40: // LD B, B
B = B;
break;
case 0x41: // LD B, C
B = C;
break;
case 0x42: // LD B, D
B = D;
break;
case 0x43: // LD B, E
B = E;
break;
case 0x44: // LD B, H
B = H;
break;
case 0x45: // LD B, L
B = L;
break;
case 0x46: // LD B, (HL)
B = ReadByte(HL);
break;
case 0x47: // LD B, A
B = A;
break;
case 0x48: // LD C, B
C = B;
break;
case 0x49: // LD C, C
C = C;
break;
case 0x4A: // LD C, D
C = D;
break;
case 0x4B: // LD C, E
C = E;
break;
case 0x4C: // LD C, H
C = H;
break;
case 0x4D: // LD C, L
C = L;
break;
case 0x4E: // LD C, (HL)
C = ReadByte(HL);
break;
case 0x4F: // LD C, A
C = A;
break;
case 0x50: // LD D, B
D = B;
break;
case 0x51: // LD D, C
D = C;
break;
case 0x52: // LD D, D
D = D;
break;
case 0x53: // LD D, E
D = E;
break;
case 0x54: // LD D, H
D = H;
break;
case 0x55: // LD D, L
D = L;
break;
case 0x56: // LD D, (HL)
D = ReadByte(HL);
break;
case 0x57: // LD D, A
D = A;
break;
case 0x58: // LD E, B
E = B;
break;
case 0x59: // LD E, C
E = C;
break;
case 0x5A: // LD E, D
E = D;
break;
case 0x5B: // LD E, E
E = E;
break;
case 0x5C: // LD E, H
E = H;
break;
case 0x5D: // LD E, L
E = L;
break;
case 0x5E: // LD E, (HL)
E = ReadByte(HL);
break;
case 0x5F: // LD E, A
E = A;
break;
case 0x60: // LD H, B
H = B;
break;
case 0x61: // LD H, C
H = C;
break;
case 0x62: // LD H, D
H = D;
break;
case 0x63: // LD H, E
H = E;
break;
case 0x64: // LD H, H
H = H;
break;
case 0x65: // LD H, L
H = L;
break;
case 0x66: // LD H, (HL)
H = ReadByte(HL);
break;
case 0x67: // LD H, A
H = A;
break;
case 0x68: // LD L, B
L = B;
break;
case 0x69: // LD L, C
L = C;
break;
case 0x6A: // LD L, D
L = D;
break;
case 0x6B: // LD L, E
L = E;
break;
case 0x6C: // LD L, H
L = H;
break;
case 0x6D: // LD L, L
L = L;
break;
case 0x6E: // LD L, (HL)
L = ReadByte(HL);
break;
case 0x6F: // LD L, A
L = A;
break;
case 0x70: // LD (HL), B
WriteByte(HL, B);
break;
case 0x71: // LD (HL), C
WriteByte(HL, C);
break;
case 0x72: // LD (HL), D
WriteByte(HL, D);
break;
case 0x73: // LD (HL), E
WriteByte(HL, E);
break;
case 0x74: // LD (HL), H
WriteByte(HL, H);
break;
case 0x75: // LD (HL), L
WriteByte(HL, L);
break;
case 0x76: // HALT
/*if (InterruptMasterEnable)
Halted = true; // Halt execution until interrupt occurs
else
PC++; // DMG issue skips one instruction when in DI
*/ // In theory this is the behavior, but if fails the tests
Halted = true;
break;
case 0x77: // LD (HL), A
WriteByte(HL, A);
break;
case 0x78: // LD A, B
A = B;
break;
case 0x79: // LD A, C
A = C;
break;
case 0x7A: // LD A, D
A = D;
break;
case 0x7B: // LD A, E
A = E;
break;
case 0x7C: // LD A, H
A = H;
break;
case 0x7D: // LD A, L
A = L;
break;
case 0x7E: // LD A, (HL)
A = ReadByte(HL);
break;
case 0x7F: // LD A, A
A = A;
break;
case 0x80: // ADD A, B
Add(B);
break;
case 0x81: // ADD A, C
Add(C);
break;
case 0x82: // ADD A, D
Add(D);
break;
case 0x83: // ADD A, E
Add(E);
break;
case 0x84: // ADD A, H
Add(H);
break;
case 0x85: // ADD A, L
Add(L);