opcode match

[narakai]

Hi,

In CPUOpcodes.h, I found there are opcodes and respective hex number in enum OperationImplied, but other enums like Operation1, Operation2..., there are only opcodes defined, no hex number at all. So I am wondering why we can still use opcodes(should be a hex number) in 'Switch-Case' to match the codes in these enum?

Say enum Operation1
{
ORA, /* there's no hex like ORA = E9 defined ?*/
AND,
EOR,
ADC,
STA,
LDA,
CMP,
SBC,
};

In CPU.cc:

switch (op){
case ORA:
// do some stuff
break;
}

[amhndu]

In the opcode, operation is represented by the highest 3 bits. Thus if you mask the opcode by 11100000b (0xE0) and shift by 5, you get the operation which is a number from 0 to 7. The enum value is just this number.

See constants OperationMask and OperationShift and example decode logic.

Reference: https://llx.com/Neil/a2/opcodes.html

[narakai]

Thanks for the reply, it helps a lot!