200. Plp implícito

registrar PLP implícito.

Lección 200 de 356 · tests/chapter_01_cpu/test_200_PLP_implied.py

Requisito previo: el paso 199 añadió `plp. En este paso, modifica únicamente emulator/cpu/opcodes.py importando plp y añadiendo su entrada en OPCODE_TABLE`.

Por qué existe este paso

PLP obtiene su valor de la pila seleccionada por S. El opcode $28 no tiene operando y debe despachar directamente a `plp(cpu)`.

Implementación sugerida

from emulator.cpu.instructions import plp

OPCODE_TABLE = {
    # existing entries
    0x28: plp,
}

Invariantes: conserva todas las asignaciones existentes; asigna exactamente $28 a `plp`; no uses envoltorio de modo de direccionamiento; consume solo el byte de opcode; conserva la semántica de sustitución y enmascarado del paso 199.

Concepto erróneo: el byte tras $28 es la siguiente instrucción, no el byte de estado a restaurar; el estado procede exclusivamente de la posición de pila ya incrementada.

Fuera de alcance: el comportamiento de PLP es el paso 199. TXS/TSX corresponden a los pasos 201-204. No debe anticiparse el comportamiento posterior de Break/ONE.

Ejecutar esta lección

uv run pytest tests/chapter_01_cpu/test_200_PLP_implied.py -v

Implementación de ejemplo

Implementación sugerida

from emulator.cpu.instructions import plp

OPCODE_TABLE = {
    # existing entries
    0x28: plp,
}

Ver código del testtest_200_PLP_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 plp
from emulator.memory.fake_rom import FakeROM


CARRY_FLAG = 1 << 0
BREAK_FLAG = 1 << 4
ONE_FLAG = 1 << 5
NEGATIVE_FLAG = 1 << 7
STACK_BASE = 0x0100


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_plp_implied_is_in_opcode_table():
    """Objective: opcode 0x28 is the official PLP opcode."""
    assert opcodes.OPCODE_TABLE[0x28] is plp


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


def test_opcode_28_plp_restores_processor_status_from_stack():
    """Objective: executing opcode 0x28 pulls saved status into cpu.p."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x28)

    cpu.reset()
    cpu.s = 0xFC
    bus.write(STACK_BASE | 0xFD, CARRY_FLAG | NEGATIVE_FLAG)
    cpu.step()

    assert cpu.p == (CARRY_FLAG | NEGATIVE_FLAG)
    assert cpu.s == 0xFD


def test_opcode_28_plp_masks_break_from_restored_status():
    """
    Objective:
    If the saved status byte has Break/ONE set, PLP does not keep those bits as
    persistent CPU state in this emulator model.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x28)

    cpu.reset()
    cpu.s = 0xFC
    bus.write(STACK_BASE | 0xFD, BREAK_FLAG | ONE_FLAG | CARRY_FLAG)
    cpu.step()

    assert cpu.p == CARRY_FLAG
    assert cpu.flags.get_break_flag() is False
    assert cpu.flags.get_one_flag() is False


def test_opcode_28_plp_does_not_fetch_operand_bytes():
    """
    Objective:
    PLP is one byte long. The byte after PLP must not be consumed as an operand.

    The restored status comes from the stack, not from program memory after 0x28.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x28)
    rom.write(0x0001, 0x99)

    cpu.reset()
    cpu.s = 0xFC
    bus.write(STACK_BASE | 0xFD, NEGATIVE_FLAG)
    cpu.step()

    assert cpu.p == NEGATIVE_FLAG
    assert cpu.pc == 0x8001


def test_opcode_28_plp_replaces_previous_status_not_merges_it():
    """Objective: PLP replaces cpu.p with the pulled status bits."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x28)

    cpu.reset()
    cpu.s = 0xFC
    cpu.p = NEGATIVE_FLAG
    bus.write(STACK_BASE | 0xFD, CARRY_FLAG)
    cpu.step()

    assert cpu.p == CARRY_FLAG

Funciones de test (6)

test_plp_implied_is_in_opcode_table
test_plp_instruction_signature_takes_only_cpu
test_opcode_28_plp_restores_processor_status_from_stack
test_opcode_28_plp_masks_break_from_restored_status
test_opcode_28_plp_does_not_fetch_operand_bytes
test_opcode_28_plp_replaces_previous_status_not_merges_it