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 static Mos6502Instruction Decode(MemoryUnit memory, int offset)
{
var opcode = memory.ReadByte(offset);
return(new Mos6502Instruction(
(int)opcode,
_operationTable[opcode],
_addressingModeTable[opcode],
_cycleCountTable[opcode],
_instructionSizeTable[opcode]));
}
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 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));
}