Lesson 14 · Compare - Build a Game Boy Emulator

Compare

Today you implement CP, a subtraction that sets flags without touching the accumulator, so a program can compare two values without losing data. This is the instruction every conditional branch in the next chapter relies on.

The goal

Implement CP A, B as a subtraction that sets the flags but discards the numeric result.

Start here - the target

TO DO

Scenario: Comparing equal values

GivenA is 0x42 and B is 0x42

WhenCP A, B (opcode 0xB8) runs

Thenthe Z flag is set and A is still 0x42

Andwith B of 0x40 instead, Z is clear and the C flag is clear (A is not below B)

Background

Compare (CP A, B, opcode 0xB8) does the arithmetic of SUB - it computes A - B and sets all four flags exactly as subtraction would - but throws the numeric result away, leaving A untouched. It exists so a program can ask “how do these two bytes relate?” without clobbering the accumulator.

Read the flags afterward and you know everything: Z set means they were equal, C set means A was less than B, and both clear means A was greater. This is the instruction every if and every loop condition compiles down to, so once you pair it with conditional jumps in the next chapter, real control flow becomes possible.

Make it work

case 0xB8: // CP A, B  - like SUB but A is left unchanged
    r := c.A - c.B
    c.SetFlag(FlagZ, r == 0)
    c.SetFlag(FlagN, true)
    c.SetFlag(FlagH, c.B&0x0F > c.A&0x0F)
    c.SetFlag(FlagC, c.B > c.A)
    return 4