198. 隐含寻址的 Php

注册隐含寻址的 PHP。

第 198 / 356 · tests/chapter_01_cpu/test_198_PHP_implied.py

前置条件：第 197 步已添加 `php。在本步骤中，只修改 emulator/cpu/opcodes.py，导入 php 并在 OPCODE_TABLE` 中注册它。

本步骤存在的原因

PHP 直接从 CPU 状态中取得 P 和 S。因此操作码 $08 没有操作数，可以直接分发给 `php(cpu)`。

建议实现

from emulator.cpu.instructions import php

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

不变量：保留已有的映射；把 $08 精确映射到 `php` 函数对象；不使用寻址包装函数或操作数读取；只消耗一个操作码字节，并保留第 197 步的栈/状态语义。

常见误解：$08 之后的字节并不是状态数据；PHP 压入的是当前实时的 P。

范围之外：PHP 的行为属于第 197 步，PLP 从第 199 步开始。对 PHP 压栈字节的改动不属于本次操作码注册步骤。

运行本课

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

示例实现

建议实现

from emulator.cpu.instructions import php

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

查看测试代码test_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

测试函数 (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