036. stx 指令

添加核心 STX 指令。

第 36 / 356 · tests/chapter_01_cpu/test_036_instruction_stx.py

要更新的文件

emulator/cpu/instructions.py

位置

instructions.stx, beside ldx

为什么需要这一步

STX 引入与 LDX 相对应的存储指令。它遵循 STA 确立的边界：核心指令接收一个已解析好的目标地址，并通过 CPU 总线写入寄存器的值，且不改变标志。

完整示例实现

# emulator/cpu/instructions.py
def stx(cpu: CPU, address: int):
    value = cpu.x
    cpu.bus.write(address, value)

重要不变式

stx 接收的是地址，不是值
写入的值是 X，且写入通过 cpu.bus.write 进行
X 和处理器标志保持不变

常见误区

不要根据 X 更新 Zero 或 Negative。存储指令只写入内存，即使被存储的字节是 $00 或第 7 位为 1。

超出范围

所有 STX 操作码处理函数和操作码表项
对寻址模式辅助函数的改动
周期时序和写入侧硬件效应

运行本课

uv run pytest tests/chapter_01_cpu/test_036_instruction_stx.py -v

示例实现

完整示例实现

# emulator/cpu/instructions.py
def stx(cpu: CPU, address: int):
    value = cpu.x
    cpu.bus.write(address, value)

查看测试代码test_036_instruction_stx.py

import inspect

from emulator.cpu import instructions
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG, make_cpu


def test_stx_instruction_exists():
    """
    Objective:
    Create in instructions.py:
        def stx(cpu, address):
            ...

    Example implementation:
        value = cpu.x
        cpu.bus.write(address, value)

    Important:
    STX receives an address, not a value.
    STX does not update flags.
    """
    assert hasattr(instructions, "stx")
    assert callable(instructions.stx)
    assert list(inspect.signature(instructions.stx).parameters) == ["cpu", "address"]


def test_stx_writes_register_x_to_address():
    """
    Objective:
    stx(cpu, address) must store register X into memory.
    """
    cpu = make_cpu()
    cpu.x = 0x42

    instructions.stx(cpu, 0x0010)

    assert cpu.bus.read(0x0010) == 0x42


def test_stx_does_not_change_zero_or_negative_flags():
    """
    Objective:
    STX must not update Zero or Negative flags.
    """
    cpu = make_cpu()
    cpu.x = 0x00
    cpu.p = NEGATIVE_FLAG

    instructions.stx(cpu, 0x0010)

    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) != 0

测试函数 (3)

test_stx_instruction_exists
test_stx_writes_register_x_to_address
test_stx_does_not_change_zero_or_negative_flags