209. Nop implícito

conectar el opcode implícito oficial NOP $EA.

Lección 209 de 356 · tests/chapter_01_cpu/test_209_NOP_implied.py

Por qué existe este paso

En este paso, actualiza la importación de instrucciones y `OPCODE_TABLE de emulator/cpu/opcodes.py; emulator/cpu/instructions.py::nop` ya existe desde el paso 208. Este mapeo permite que CPU.step ejecute relleno en programas reales.

Implementación sugerida

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

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

Invariante: $EA ocupa un byte, así que un paso desde $8000 termina en $8001 únicamente porque CPU.step obtuvo el opcode; los registros, S, P y la memoria permanecen sin cambios. El error conceptual habitual es incrementar PC en `nop` u obtener un operando fantasma.

Fuera de alcance: los opcodes NOP no oficiales multibyte y los cambios de temporización corresponden a pasos posteriores. La verificación de la CPU con múltiples instrucciones corresponde al paso 210.

Ejecutar esta lección

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

Implementación de ejemplo

Implementación sugerida

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

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

Ver código del testtest_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

Funciones de test (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