Lesson 29 · The CB prefix - Build a Game Boy Emulator

The CB prefix

Today you decode the 0xCB prefix byte, which opens a whole second 256-entry opcode table for bit operations, and implement RLC B as the first instruction in it - unlocking rotates, shifts, swaps, and bit tests for the rest of the chapter.

The goal

Decode the 0xCB prefix and implement RLC B, using the normal zero-flag rule that CB-prefixed rotates follow.

Start here - the target

TO DO

Scenario: A prefixed rotate that sets Z

Givenmemory at 0x0100 holds 0xCB, 0x00 and B is 0x85

Whenthe CPU executes one step

ThenB is 0x0B and the C flag is set

Andthe step reports 8 cycles and PC is 0x0102

Background

The main opcode table only has room for 256 instructions, so the Game Boy hides a second table behind the prefix byte 0xCB. When the CPU fetches 0xCB, it fetches another byte and decodes that in the CB table - 256 more instructions, all bit manipulation. Model it as a second switch reached from the 0xCB case.

RLC B (CB opcode 0x00) rotates like yesterday’s RLCA, but on any register and with one difference: the CB rotates set Z normally when the result is zero. Everything prefixed costs at least 8 cycles because of the extra fetch. Getting the prefix dispatch right unlocks rotates, shifts, swaps, and the bit-test instructions - the whole toolkit for working on individual bits.

Make it work

case 0xCB: // prefix - the real opcode is the next byte
    return c.stepCB(c.fetch())
// in stepCB:
case 0x00: // RLC B
    carry := c.B >> 7
    c.B = c.B<<1 | carry
    c.SetFlag(FlagZ, c.B == 0) // CB rotates set Z normally
    // ...N=0, H=0, C=carry