209. Nop 隐含寻址

接通官方隐含寻址 NOP 操作码 $EA。

第 209 / 356 · tests/chapter_01_cpu/test_209_NOP_implied.py

本步骤存在的原因

在本步骤中，更新 `emulator/cpu/opcodes.py 的指令导入以及 OPCODE_TABLE；emulator/cpu/instructions.py::nop` 已在步骤 208 中存在。该映射使 CPU.step 能够在真实程序中执行填充指令。

建议实现

from emulator.cpu.instructions import nop  # add to the existing import

OPCODE_TABLE = {
    # existing entries
    0xEA: nop,
}

不变式：$EA 为单字节指令，因此从 $8000 开始的一步会结束于 $8001，这完全是因为 CPU.step 取指了该操作码；寄存器、S、P 和内存均保持不变。常见的误解是在 `nop` 中递增 PC，或者取一个虚构的操作数。

范围之外：非官方的多字节 NOP 操作码以及时序变化属于后续步骤。多指令 CPU 验证属于步骤 210。

运行本课

uv run pytest tests/chapter_01_cpu/test_209_NOP_implied.py -v

示例实现

建议实现

from emulator.cpu.instructions import nop  # add to the existing import

OPCODE_TABLE = {
    # existing entries
    0xEA: nop,
}

查看测试代码test_209_NOP_implied.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
from emulator.cpu.instructions import nop
from emulator.memory.fake_rom import FakeROM


def make_cpu_with_rom():
    rom = FakeROM()
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)
    bus = CpuBus(program_rom=rom)
    return CPU(bus), bus, rom


def test_nop_implied_is_in_opcode_table():
    """Objective: opcode 0xEA is the official NOP opcode."""
    assert opcodes.OPCODE_TABLE[0xEA] is nop


def test_nop_instruction_signature_takes_only_cpu():
    """Objective: NOP is implied, so nop(cpu) does not need an operand argument."""
    assert list(inspect.signature(nop).parameters) == ["cpu"]


def test_opcode_EA_nop_advances_pc_only_by_opcode_fetch():
    """
    Objective:
    Executing opcode 0xEA advances PC from $8000 to $8001.

    That one-byte advance comes from CPU.step() fetching the opcode, not from
    nop(cpu) itself.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xEA)

    cpu.reset()
    cpu.step()

    assert cpu.pc == 0x8001


def test_opcode_EA_nop_does_not_fetch_operand_bytes():
    """
    Objective:
    NOP is one byte long. The byte after NOP is the next instruction, not an
    operand consumed by NOP.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xEA)
    rom.write(0x0001, 0x99)

    cpu.reset()
    cpu.step()

    assert cpu.pc == 0x8001


def test_opcode_EA_nop_preserves_registers_and_flags():
    """Objective: executing NOP preserves registers and flags."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xEA)

    cpu.reset()
    cpu.a = 0x11
    cpu.x = 0x22
    cpu.y = 0x33
    cpu.s = 0xFD
    cpu.p = 0b1100_1111
    cpu.step()

    assert cpu.a == 0x11
    assert cpu.x == 0x22
    assert cpu.y == 0x33
    assert cpu.s == 0xFD
    assert cpu.p == 0b1100_1111

测试函数 (5)

test_nop_implied_is_in_opcode_table
test_nop_instruction_signature_takes_only_cpu
test_opcode_EA_nop_advances_pc_only_by_opcode_fetch
test_opcode_EA_nop_does_not_fetch_operand_bytes
test_opcode_EA_nop_preserves_registers_and_flags