037. Stx 零页寻址

添加 STX 零页寻址（$86）。

第 37 / 356 · tests/chapter_01_cpu/test_037_STX_zero_page.py

要更新的文件

emulator/cpu/opcodes.py

位置

opcodes import of stx
opcodes.stx_zero_page
opcodes.OPCODE_TABLE[$86]

为什么需要这一步

测试 036 添加了地址层面的 stx 操作。本课通过解析一个零页目标地址，并把该地址传给 stx，为它接入第一个编码。

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import lda, sta, ldx, stx


def stx_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    stx(cpu, addr)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x86: stx_zero_page,
}

重要不变式

$86 映射到 stx_zero_page
zero_page 消耗一个操作数字节并返回一个位于 $00 页的地址
传给 stx 的是地址，而不是该地址当前的内容
整条指令使 PC 前进 2 字节，且标志保持不变

常见误区

不要在调用 stx 之前从已解析的地址读取内容；STX 是把 X 写入该目标地址。

超出范围

零页Y变址和绝对寻址的 STX 编码
对 zero_page 或 stx 的改动
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_037_STX_zero_page.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import lda, sta, ldx, stx


def stx_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    stx(cpu, addr)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x86: stx_zero_page,
}

查看测试代码test_037_STX_zero_page.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
from emulator.memory.fake_rom import FakeROM
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


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_stx_zero_page_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create in opcodes.py:
        def stx_zero_page(cpu):
            addr = zero_page(cpu)
            stx(cpu, addr)

    Then add:
        0x86: stx_zero_page
    """
    assert hasattr(opcodes, "stx_zero_page")
    assert callable(opcodes.stx_zero_page)
    assert list(inspect.signature(opcodes.stx_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x86] is opcodes.stx_zero_page


def test_opcode_86_stx_zero_page_stores_register_x():
    """
    Objective:
    86 10 means STX $10.
    Store register X into RAM $0010.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x86)
    rom.write(0x0001, 0x10)

    cpu.reset()
    cpu.x = 0x42
    cpu.step()

    assert bus.read(0x0010) == 0x42
    assert cpu.pc == 0x8002


def test_opcode_86_stx_zero_page_does_not_change_flags():
    """
    Objective:
    STX Zero Page stores X but does not update flags.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x86)
    rom.write(0x0001, 0x10)

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

    assert bus.read(0x0010) == 0x00
    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) != 0

测试函数 (3)

test_stx_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_86_stx_zero_page_stores_register_x
test_opcode_86_stx_zero_page_does_not_change_flags