016. Opcode table refactor

File to create

emulator/cpu/opcodes.py

File to update

emulator/cpu/cpu.py

Locations

opcodes.lda_immediate
opcodes.lda_zero_page
opcodes.lda_absolute
opcodes.OPCODE_TABLE
CPU.step

Why this step exists

Direct branches work for three opcodes but scale poorly across the 6502 instruction set. A table makes byte-to-handler dispatch data-driven while retaining the existing addressing and instruction boundaries.

Complete example implementation

# emulator/cpu/opcodes.py
from emulator.cpu.addressing_modes import absolute, immediate, zero_page
from emulator.cpu.instructions import lda


def lda_immediate(cpu) -> None:
    lda(cpu, immediate(cpu))


def lda_zero_page(cpu) -> None:
    address = zero_page(cpu)
    lda(cpu, cpu.bus.read(address))


def lda_absolute(cpu) -> None:
    address = absolute(cpu)
    lda(cpu, cpu.bus.read(address))


OPCODE_TABLE = {
    0xA9: lda_immediate,
    0xA5: lda_zero_page,
    0xAD: lda_absolute,
}


# emulator/cpu/cpu.py
from emulator.cpu.opcodes import OPCODE_TABLE


class CPU:
    def step(self) -> None:
        opcode = self.fetch_byte()
        handler = OPCODE_TABLE.get(opcode)

        if handler is None:
            raise NotImplementedError(
                f"Opcode {opcode:02X} not implemented"
            )

        return handler(self)

Important boundary

CPU.step dispatches; an opcode handler coordinates addressing, memory access, and the instruction; lda performs the register-and-flags state transition.

Common misconception

An opcode handler is not the same as an instruction. Three LDA opcodes share one lda instruction but use different operand acquisition mechanisms.

Out of scope

• indexed LDA handlers
• cycle-count return values
• a generic instruction decoder object