200. 隐含寻址的 Plp

注册隐含寻址的 PLP。

第 200 / 356 · tests/chapter_01_cpu/test_200_PLP_implied.py

前置条件：第 199 步已添加 `plp。在本步骤中，只修改 emulator/cpu/opcodes.py，导入 plp 并添加它的 OPCODE_TABLE` 条目。

本步骤存在的原因

PLP 从由 S 选定的栈中取得数值。操作码 $28 没有操作数，必须直接分发给 `plp(cpu)`。

建议实现

from emulator.cpu.instructions import plp

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

不变量：保留所有已有的映射；把 $28 精确映射到 `plp`；不使用寻址方式包装函数；只消耗操作码字节；保留第 199 步中先替换后屏蔽的语义。

常见误解：$28 之后的字节是下一条指令，而不是待恢复的状态字节；状态只能来自递增后的栈槽位。

范围之外：PLP 的行为属于第 199 步。TXS/TSX 属于第 201-204 步。这里不得提前引入之后的 Break/ONE 相关行为。

运行本课

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

示例实现

建议实现

from emulator.cpu.instructions import plp

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

查看测试代码test_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

测试函数 (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