119. 零页,X AND

添加 AND 零页,X 操作码 ``0x35``。

第 119 / 356 · tests/chapter_01_cpu/test_119_AND_zero_page_x.py

在这一步中，在第 118 课的基础上，只在 `emulator/cpu/opcodes.py` 中添加变址零页的处理函数和表条目。

为什么需要这一步

这一步为零页 AND 添加了变址访问，并验证地址计算会在零页内回绕，然后内存值才会送入 AND 原语。

建议实现

def and_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x35: and_zero_page_x,
}

`emulator/cpu/addressing_modes.py::zero_page_x 计算 (base + cpu.x) & 0xFF。从概念上看，解析出的字节被读取一次，并传给 instructions.and_a：A 和 Z/N 发生变化，Carry/Overflow 和内存不变，PC 前进两个字节。基址 0xFE 加上 X 值 0x03 必须读取 $0001，而不是 $0101`。

误解澄清：变址不会改变数据本身，并且必须在总线读取之前完成回绕。本课不涉及的内容：绝对寻址到间接,Y 的 AND（第 120-124 课），以及对已有寻址辅助函数或指令语义的改动。

运行本课

uv run pytest tests/chapter_01_cpu/test_119_AND_zero_page_x.py -v

示例实现

建议实现

def and_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x35: and_zero_page_x,
}

查看测试代码test_119_AND_zero_page_x.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_and_zero_page_x_handler_exists_and_is_in_opcode_table():
    """Objective: create and_zero_page_x(cpu) and add 0x35 to OPCODE_TABLE."""
    assert hasattr(opcodes, "and_zero_page_x")
    assert callable(opcodes.and_zero_page_x)
    assert list(inspect.signature(opcodes.and_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x35] is opcodes.and_zero_page_x


def test_opcode_35_and_zero_page_x_reads_indexed_memory_value():
    """Objective: 35 20 with X=0x04 reads RAM[$0024]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x35)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0x0F)

    cpu.reset()
    cpu.x = 0x04
    cpu.a = 0xF3
    cpu.step()

    assert cpu.a == 0x03
    assert cpu.pc == 0x8002


def test_opcode_35_and_zero_page_x_wraps_zero_page_address():
    """Objective: base=0xFE and X=0x03 reads RAM[$0001]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x35)
    rom.write(0x0001, 0xFE)
    bus.write(0x0001, 0x0F)

    cpu.reset()
    cpu.x = 0x03
    cpu.a = 0xF3
    cpu.step()

    assert cpu.a == 0x03
    assert cpu.pc == 0x8002

测试函数 (3)

test_and_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_35_and_zero_page_x_reads_indexed_memory_value
test_opcode_35_and_zero_page_x_wraps_zero_page_address