Lesson 08 · Register-to-register loads - Build a Game Boy Emulator

Today you implement register-to-register loads and run your first real program, a short sequence of opcodes that carries a value from one register into another. It is a small proof that your fetch-decode-execute loop, registers, and memory all work together as one machine.

The goal

Implement LD A, B and run a short program of loads end to end.

Start here - the target

TO DO

Scenario: Running three loads in a row

Givenmemory at 0x0100 holds 0x06, 0x99, 0x78 (LD B,0x99 then LD A,B)

Whenthe CPU runs two steps

ThenB is 0x99 and A is 0x99

AndPC is 0x0103

Background

The largest single block of the opcode table, 0x40 through 0x7F, is nothing but register-to-register copies: LD A, B, LD C, H, LD E, A, and so on. Each just moves one register’s byte into another and costs 4 cycles. LD A, B (opcode 0x78) is a representative example.

This is also your first real program: load a constant into B, then copy B into A, and watch two steps of your loop carry the value through. That end -to-end run - a sequence of opcodes producing a predictable final state - is exactly what an emulator does, just at a smaller scale than a whole game. You now have registers, memory, flags, and a working fetch–decode–execute core.

Make it work

case 0x78: // LD A, B
    c.A = c.B
    return 4
// every opcode in 0x40..0x7F is one LD r, r' - same shape, different pair.
// Implement just LD A, B today; the rest of the block follows identically.