198. Php implícito

registrar PHP implícito.

Lección 198 de 356 · tests/chapter_01_cpu/test_198_PHP_implied.py

Requisito previo: el paso 197 añadió `php. En este paso, modifica únicamente emulator/cpu/opcodes.py importando php y registrándolo en OPCODE_TABLE`.

Por qué existe este paso

PHP obtiene P y S directamente del estado de la CPU. El opcode $08, por tanto, no tiene operando y despacha directamente a `php(cpu)`.

Implementación sugerida

from emulator.cpu.instructions import php

OPCODE_TABLE = {
    # existing entries
    0x08: php,
}

Invariantes: conserva las asignaciones existentes; asigna exactamente $08 al objeto de función `php`; no uses envoltorio de direccionamiento ni lectura de operando; consume un byte de opcode y conserva la semántica de pila/estado del paso 197.

Concepto erróneo: el byte tras $08 no es un dato de estado; PHP apila el P real.

Fuera de alcance: el comportamiento de PHP corresponde al paso 197, y PLP empieza en el paso 199. Los cambios al byte apilado por PHP no corresponden a este paso de registro de opcode.

Ejecutar esta lección

uv run pytest tests/chapter_01_cpu/test_198_PHP_implied.py -v

Implementación de ejemplo

Implementación sugerida

from emulator.cpu.instructions import php

OPCODE_TABLE = {
    # existing entries
    0x08: php,
}

Ver código del testtest_198_PHP_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 php
from emulator.memory.fake_rom import FakeROM


CARRY_FLAG = 1 << 0
BREAK_FLAG = 1 << 4
ONE_FLAG = 1 << 5
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_php_implied_is_in_opcode_table():
    """Objective: opcode 0x08 is the official PHP opcode."""
    assert opcodes.OPCODE_TABLE[0x08] is php


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


def test_opcode_08_php_pushes_status_to_stack():
    """Objective: executing opcode 0x08 pushes P with Break and ONE set."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x08)

    cpu.reset()
    cpu.p = CARRY_FLAG
    cpu.step()

    pushed_status = bus.read(STACK_BASE | 0xFD)
    assert (pushed_status & CARRY_FLAG) != 0
    assert (pushed_status & BREAK_FLAG) != 0
    assert (pushed_status & ONE_FLAG) != 0
    assert cpu.s == 0xFC


def test_opcode_08_php_does_not_fetch_operand_bytes():
    """
    Objective:
    PHP is one byte long. The byte after PHP must not be consumed as an operand.

    The pushed value comes from cpu.p, not from program memory after 0x08.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x08)
    rom.write(0x0001, 0x99)

    cpu.reset()
    cpu.p = CARRY_FLAG
    cpu.step()

    pushed_status = bus.read(STACK_BASE | 0xFD)
    assert (pushed_status & CARRY_FLAG) != 0
    assert cpu.pc == 0x8001


def test_opcode_08_php_does_not_leave_break_set_in_cpu_state():
    """
    Objective:
    PHP pushes Break in the saved status byte, but does not keep Break set in
    cpu.p afterward in this emulator model.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x08)

    cpu.reset()
    cpu.p = 0x00
    cpu.step()

    pushed_status = bus.read(STACK_BASE | 0xFD)
    assert (pushed_status & BREAK_FLAG) != 0
    assert (pushed_status & ONE_FLAG) != 0
    assert cpu.flags.get_break_flag() is False
    assert cpu.flags.get_one_flag() is False


def test_opcode_08_php_preserves_existing_cpu_flags_except_temporary_break():
    """Objective: PHP should not damage existing CPU status flags."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x08)

    cpu.reset()
    cpu.p = CARRY_FLAG
    cpu.step()

    assert cpu.p == CARRY_FLAG

Funciones de test (6)

test_php_implied_is_in_opcode_table
test_php_instruction_signature_takes_only_cpu
test_opcode_08_php_pushes_status_to_stack
test_opcode_08_php_does_not_fetch_operand_bytes
test_opcode_08_php_does_not_leave_break_set_in_cpu_state
test_opcode_08_php_preserves_existing_cpu_flags_except_temporary_break