032. Ldx 零页寻址

添加 LDX 零页寻址（$A6）。

第 32 / 356 · tests/chapter_01_cpu/test_032_LDX_zero_page.py

要更新的文件

emulator/cpu/opcodes.py

位置

opcodes.ldx_zero_page
opcodes.OPCODE_TABLE[$A6]

为什么需要这一步

在立即寻址的 LDX 之后，本课添加第一个基于内存的 LDX 编码。处理函数将既有的零页地址计算、一次总线读取和既有的 ldx 值操作组合起来。

完整示例实现

# emulator/cpu/opcodes.py
def ldx_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    ldx(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA6: ldx_zero_page,
}

重要不变式

$A6 映射到 ldx_zero_page
zero_page 获取一个操作数字节并返回一个位于 $00 页的地址
处理函数读取该地址，并把得到的值传给 ldx
整条指令使 PC 前进 2 字节，且 ldx 更新标志

常见误区

不要把零页地址直接传给 ldx；LDX 加载的是存储在该地址处的字节。

超出范围

变址和绝对寻址的 LDX 编码
对 zero_page 或 ldx 的改动
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_032_LDX_zero_page.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
def ldx_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    ldx(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA6: ldx_zero_page,
}

查看测试代码test_032_LDX_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


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_ldx_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create ldx_zero_page(cpu) and add 0xA6 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldx_zero_page")
    assert callable(opcodes.ldx_zero_page)
    assert list(inspect.signature(opcodes.ldx_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xA6] is opcodes.ldx_zero_page


def test_opcode_A6_ldx_zero_page_loads_register_x():
    """Objective: A6 10 means LDX $10, so X loads RAM[$0010]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA6)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x42)

    cpu.reset()
    cpu.step()

    assert cpu.x == 0x42
    assert cpu.pc == 0x8002

测试函数 (2)

test_ldx_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_A6_ldx_zero_page_loads_register_x