196. 隐含寻址的 Pla

注册隐含寻址的 PLA。

第 196 / 356 · tests/chapter_01_cpu/test_196_PLA_implied.py

前置条件：第 195 步已添加 `pla。在本步骤中，只修改 emulator/cpu/opcodes.py，导入 pla 并添加它的 OPCODE_TABLE` 条目。

本步骤存在的原因

PLA 的输入是由 S 选定的硬件栈。操作码 $68 没有操作数，因此 `CPU.step() 必须直接分发给 pla(cpu)`。

建议实现

from emulator.cpu.instructions import pla

OPCODE_TABLE = {
    # existing entries
    0x68: pla,
}

不变量：保留已有的映射；把 $68 精确映射到 `pla`；不使用寻址方式包装函数；只消耗操作码字节；保留第 195 步实现的 Z/N 影响。

常见误解：$68 之后的程序内存并不能提供出栈的值；提供该值的是递增后的栈地址。

范围之外：`pla` 的行为属于第 195 步。PHP 从第 197 步开始，这次改动不应引入它的函数或操作码映射。

运行本课

uv run pytest tests/chapter_01_cpu/test_196_PLA_implied.py -v

示例实现

建议实现

from emulator.cpu.instructions import pla

OPCODE_TABLE = {
    # existing entries
    0x68: pla,
}

查看测试代码test_196_PLA_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 pla
from emulator.memory.fake_rom import FakeROM


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_pla_implied_is_in_opcode_table():
    """Objective: opcode 0x68 is the official PLA opcode."""
    assert opcodes.OPCODE_TABLE[0x68] is pla


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


def test_opcode_68_pla_pulls_stack_value_into_accumulator():
    """Objective: executing opcode 0x68 pulls the next stack byte into A."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x68)

    cpu.reset()
    cpu.s = 0xFC
    bus.write(STACK_BASE | 0xFD, 0x42)
    cpu.step()

    assert cpu.a == 0x42
    assert cpu.s == 0xFD


def test_opcode_68_pla_updates_zero_and_negative_flags():
    """Objective: PLA opcode behavior includes Zero/Negative flag updates."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x68)

    cpu.reset()
    cpu.s = 0xFC
    bus.write(STACK_BASE | 0xFD, 0x80)
    cpu.step()

    assert cpu.a == 0x80
    assert cpu.flags.get_zero_flag() is False
    assert cpu.flags.get_negative_flag() is True


def test_opcode_68_pla_does_not_fetch_operand_bytes():
    """
    Objective:
    PLA is one byte long. The byte after PLA must not be consumed as an operand.

    The pulled value comes from the stack, not from program memory after 0x68.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x68)
    rom.write(0x0001, 0x99)

    cpu.reset()
    cpu.s = 0xFC
    bus.write(STACK_BASE | 0xFD, 0x55)
    cpu.step()

    assert cpu.a == 0x55
    assert cpu.pc == 0x8001


def test_opcode_68_pla_preserves_other_flags():
    """Objective: PLA updates Z/N but does not damage unrelated status flags."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x68)

    cpu.reset()
    cpu.s = 0xFC
    cpu.p = 0b0100_0001
    bus.write(STACK_BASE | 0xFD, 0x01)
    cpu.step()

    assert cpu.a == 0x01
    assert cpu.p & 0b0100_0001 == 0b0100_0001
    assert cpu.flags.get_zero_flag() is False
    assert cpu.flags.get_negative_flag() is False

测试函数 (6)

test_pla_implied_is_in_opcode_table
test_pla_instruction_signature_takes_only_cpu
test_opcode_68_pla_pulls_stack_value_into_accumulator
test_opcode_68_pla_updates_zero_and_negative_flags
test_opcode_68_pla_does_not_fetch_operand_bytes
test_opcode_68_pla_preserves_other_flags