Lesson 20 · Post-increment addressing - Build a Game Boy Emulator

Post-increment addressing

Today you implement LD (HL+), A and its decrementing sibling LD (HL-), A, which store a byte through HL and then automatically advance the pointer - the single-instruction building block behind every memory-filling loop you will write later.

The goal

Implement LD (HL+), A and LD (HL-), A, which store through HL and then increment or decrement HL in the same instruction.

Start here - the target

TO DO

Scenario: Storing and advancing in one instruction

GivenHL is 0xC000 and A is 0x42

WhenLD (HL+), A (opcode 0x22) runs

Thenmemory at 0xC000 holds 0x42

AndHL is now 0xC001

Andthe decrement form LD (HL-), A (opcode 0x32) instead leaves HL at 0xBFFF after the store

Background

Copying a block of memory means “write a byte, move the pointer, repeat,” and the Game Boy folds the first two steps into one instruction. LD (HL+), A (opcode 0x22) stores A at the address in HL and then increments HL so it points at the next slot. LD (HL-), A (0x32) does the same but decrements.

These auto-updating forms are everywhere in real code because the most common loop on the machine is “fill a region of memory.” Clearing video RAM at startup or copying a sprite table both lean on (HL+). It is a small convenience, but it turns a two-instruction inner loop into a one-instruction one - and once you add conditional jumps, you will write exactly that loop.

Make it work

case 0x22: // LD (HL+), A
    c.mem.Write(c.HL(), c.A)
    c.SetHL(c.HL() + 1)
    return 8
case 0x32: // LD (HL-), A - same, but decrement