Lesson 24 · Conditional branches - Build a Game Boy Emulator

Conditional branches

Today you implement JR NZ, e, the first conditional branch, which only jumps when the Z flag is clear and costs a different number of cycles depending on whether the branch is taken - the mechanism behind every if and loop test.

The goal

Implement JR NZ, e so it jumps only when the Z flag is clear, and make sure it reports the correct cycle count whether the branch is taken or not.

Start here - the target

TO DO

Scenario: A branch taken and not taken

Givenmemory at 0x0100 holds 0x20, 0x05 and the Z flag is clear

Whenthe CPU executes one step

ThenPC is 0x0107 and the step reports 12 cycles (branch taken)

Andwith Z set instead, PC is 0x0102 and the step reports 8 cycles (not taken)

Background

Real decisions come from conditional branches. JR NZ, e (opcode 0x20) jumps only when the Z flag is clear (“not zero”); otherwise it reads past the offset and falls through. The four conditions - NZ, Z, NC, C - pair with the flags a CP or DEC just set, which is how if and loop tests execute.

There is a timing wrinkle unique to conditional instructions: they cost more cycles when the branch is taken (12) than when it is not (8), because taking the branch does extra work. You must fetch the operand in both cases so PC advances correctly on a fall-through. Return the right cycle count for each path

the graphics timing you build later depends on these being exact.

Make it work

case 0x20: // JR NZ, e
    e := int8(c.fetch())
    if c.F&FlagZ == 0 {
        c.PC = uint16(int(c.PC) + int(e))
        return 12
    }
    return 8