Learn Emulators by Writing One CHIP-8 Instruction

The Central Loop

Tiny Emulators is popular today because small systems make invisible computer behavior visible. You do not need to build a complete emulator to learn the central loop.

CHIP-8 instructions are two bytes. The instruction 6xkk stores byte kk in register Vx.

memory = bytearray(4096)
V = [0] * 16
pc = 0x200

def step():
    global pc
    opcode = (memory[pc] << 8) | memory[pc + 1]
    pc += 2
    if opcode & 0xF000 == 0x6000:
        x = (opcode >> 8) & 0xF
        V[x] = opcode & 0xFF
    else:
        raise ValueError(f"unsupported opcode {opcode:04x}")

memory[0x200:0x204] = bytes([0x61, 0x2A, 0x6F, 0xFF])
step(); step()
assert V[1] == 42 and V[15] == 255 and pc == 0x204

The important details are already here: big-endian fetch, program-counter movement, bit masks, register selection, and a deterministic failure for unknown instructions.

Next Steps

Next, add 7xkk (addition), then write a table-driven test for overflow. Only after the CPU state is trustworthy should you add timers, keyboard input, or graphics.

Compare behavior against a public CHIP-8 test ROM and document which interpreter quirks you support.

An emulator is a specification made executable. One instruction with a precise test teaches more than copying a finished 1,000-line implementation.