Lesson 23 · Relative jumps - Build a Game Boy Emulator

Relative jumps

Today you implement JR e, a relative jump using a signed 8-bit offset from the following instruction - the compact, backward-capable jump that makes loops possible.

The goal

Implement JR e using a signed 8-bit offset measured from the address after the operand, so it can jump both forward and backward.

Start here - the target

TO DO

Scenario: Jumping forward by a signed offset

Givenmemory at 0x0100 holds 0x18, 0x05

Whenthe CPU executes one step

ThenPC is 0x0107

Andwith the operand 0xFE instead, PC returns to 0x0100 (an offset of -2)

Background

JR e (opcode 0x18) jumps relative to the current position rather than to an absolute address. The operand is a signed 8-bit offset - read it as a value from -128 to +127 - added to the address of the next instruction. Since you fetch the opcode and the offset first, PC already points at that next instruction when you add, so the math lands correctly.

The signed part is the crux: an offset of 0x05 jumps forward to 0x0107, but 0xFE is -2, which jumps backward two bytes - right onto the JR itself, an infinite loop. Backward relative jumps are how every loop on the machine repeats, and they are compact (two bytes) which is why compilers favor them over absolute jumps.

Make it work

case 0x18: // JR e
    e := int8(c.fetch())          // signed offset
    c.PC = uint16(int(c.PC) + int(e))
    return 12