private static Mos6502State Asl(int address, Mos6502State currentState, MemoryUnit memory)
{
var operand = address == 0 ? currentState.A : memory.ReadByte(address);
var r = operand << 1;
var flags = currentState.P;
flags = flags.SetIf(r > 0xFF, Mos6502Flags.Carry);
r = (byte)r;
// Set Zero flag **only** if the accumulator changes to zero.
if (address == 0)
{
flags = flags.SetIf(r == 0, Mos6502Flags.Zero);
}
flags = flags.SetIf((r & 0b1000_0000) != 0, Mos6502Flags.Negative);
if (address == 0)
{
return(currentState.With(a: (byte)r, p: flags));
}
else
{
memory.WriteByte(address, (byte)r);
return(currentState.With(p: flags));
}
}
public Mos6502Cpu(Mos6502State initialState, MemoryUnit memory, ILoggerFactory loggerFactory)
{
CurrentState = initialState;
Memory = memory;
_logger = loggerFactory.CreateLogger <Mos6502Cpu>();
_logger.LogTrace(new EventId(0, "Initializing"), "Initializing with state: {InitialMos6502State}", CurrentState);
}
private static Mos6502State And(int address, Mos6502State currentState, MemoryUnit memory)
{
var value = memory.ReadByte(address);
var r = (byte)(value & currentState.A);
var flags = currentState.P;
flags = flags.SetIf(r == 0, Mos6502Flags.Zero);
flags = flags.SetIf((r & 0b1000_0000) != 0, Mos6502Flags.Negative);
return(currentState.With(a: r, p: flags));
}
private static Mos6502State Ahx(int address, Mos6502State currentState, MemoryUnit memory)
{
// AHX is an unofficial opcode with odd behavior
// Based this test on https://github.com/anurse/remy/blob/4f01c0e16ac9b8db1553d97588c9d8927884cef1/src/hw/mos6502/exec/store.rs#L158
// I don't remember where I figured that behavior out then :(.
var highByte = (address & 0xFF00) >> 8;
var val = currentState.A & currentState.X & highByte;
memory.WriteByte(address, (byte)val);
return(currentState);
}
private static Mos6502State Bit(int address, Mos6502State currentState, MemoryUnit memory)
{
var m = memory.ReadByte(address);
var result = currentState.A & m;
var flags = currentState.P;
flags = flags.SetIf(result == 0, Mos6502Flags.Zero);
flags = flags.SetIf((result & 0b0100_0000) != 0, Mos6502Flags.Overflow);
flags = flags.SetIf((result & 0b1000_0000) != 0, Mos6502Flags.Negative);
return(currentState.With(p: flags));
}
private static Mos6502State Adc(int address, Mos6502State currentState, MemoryUnit memory)
{
// ADC: A = A + M + P[Carry]
var m = memory.ReadByte(address);
var r = currentState.A + m + (int)(currentState.P & Mos6502Flags.Carry);
var flags = currentState.P;
// Carry out if r overflows
flags = flags.SetIf(r > 0xFF, Mos6502Flags.Carry);
r = (byte)r;
flags = flags.SetIf(r == 0, Mos6502Flags.Zero);
flags = flags.SetIf((r & 0b1000_0000) != 0, Mos6502Flags.Negative);
// Check if the overflow bit should be set.
// The bit should be set when both A and M have the same sign bit, and R has a different sign bit.
// This indicates that signed addition overflowed and the sign bit is inaccurate.
// We set based on this truth table
// | # | A | M | R | A ^ M | A ^ R | Value |
// | 1 | 0b0xxx_xxxx | 0b0xxx_xxxx | 0b0xxx_xxxx | 0b0xxx_xxxx | 0b0xxx_xxxx | F |
// | 2 | 0b0xxx_xxxx | 0b0xxx_xxxx | 0b1xxx_xxxx | 0b0xxx_xxxx | 0b1xxx_xxxx | T |
// | 3 | 0b0xxx_xxxx | 0b1xxx_xxxx | 0b0xxx_xxxx | 0b1xxx_xxxx | 0b0xxx_xxxx | F |
// | 4 | 0b0xxx_xxxx | 0b1xxx_xxxx | 0b1xxx_xxxx | 0b1xxx_xxxx | 0b1xxx_xxxx | F |
// | 5 | 0b1xxx_xxxx | 0b0xxx_xxxx | 0b0xxx_xxxx | 0b1xxx_xxxx | 0b1xxx_xxxx | F |
// | 6 | 0b1xxx_xxxx | 0b0xxx_xxxx | 0b1xxx_xxxx | 0b1xxx_xxxx | 0b0xxx_xxxx | F |
// | 7 | 0b1xxx_xxxx | 0b1xxx_xxxx | 0b0xxx_xxxx | 0b0xxx_xxxx | 0b1xxx_xxxx | T |
// | 8 | 0b1xxx_xxxx | 0b1xxx_xxxx | 0b1xxx_xxxx | 0b0xxx_xxxx | 0b0xxx_xxxx | F |
// From the above, we can see that overflow is set in rows 2 and 7, when:
// * A^M has the sign bit clear
// * AND, A^R has the sign bit set.
flags = ((currentState.A ^ m) & 0x80) == 0 && ((currentState.A ^ r) & 0x80) == 0x80 ?
flags | Mos6502Flags.Overflow :
flags & ~Mos6502Flags.Overflow;
return(currentState.With(a: r, p: flags));
}
private void ChangeMos6502State(Mos6502State newState)
{
_logger.LogTrace(new EventId(0, "ChangingMos6502State"), "Changing state from {OldMos6502State} to {NewMos6502State}", CurrentState, newState);
CurrentState = newState;
}
public Mos6502Cpu(Mos6502State initialState, MemoryUnit memory) : this(initialState, memory, NullLoggerFactory.Instance)
{
}
private static Mos6502State Arr(int address, Mos6502State currentState, MemoryUnit memory)
{
throw new NotImplementedException();
}