137. 零页,X EOR

添加 EOR zero-page,X 操作码 ``0x55``。

第 137 / 356 · tests/chapter_01_cpu/test_137_EOR_zero_page_x.py

此步骤存在的原因

此步骤添加了变址零页 EOR,并验证 X 变址会在所寻址的值与累加器组合之前在页内回绕。

在此步骤中,继第 136 课之后,请在 `emulator/cpu/opcodes.py` 中准确添加以下内容:

def eor_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x55: eor_zero_page_x,
}

`emulator/cpu/addressing_modes.py::zero_page_x 获取基址,并在处理函数读取数据之前计算 (base + cpu.x) & 0xFF`。因此变址结果始终保持在零页之内。A 与 Z/N 可能改变;Carry/Overflow、内存、X 和 Y 保持不变;操作码加操作数使 PC 前进两个字节。

常见误解:诸如 `$FE + $03 之类的溢出会回绕为 $01,而不是 $0101`。不在本步骤范围内:绝对寻址和间接寻址的 EOR 模式(第 138-142 课)以及 BIT(第 143-145 课)。

运行本课

uv run pytest tests/chapter_01_cpu/test_137_EOR_zero_page_x.py -v

示例实现

此步骤存在的原因

def eor_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x55: eor_zero_page_x,
}

查看测试代码test_137_EOR_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_eor_zero_page_x_handler_exists_and_is_in_opcode_table():
    """Objective: create eor_zero_page_x(cpu) and add 0x55 to OPCODE_TABLE."""
    assert hasattr(opcodes, "eor_zero_page_x")
    assert callable(opcodes.eor_zero_page_x)
    assert list(inspect.signature(opcodes.eor_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x55] is opcodes.eor_zero_page_x


def test_opcode_55_eor_zero_page_x_reads_indexed_memory_value():
    """Objective: 55 20 with X=0x04 reads RAM[$0024]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x55)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0x0F)

    cpu.reset()
    cpu.x = 0x04
    cpu.a = 0xF0
    cpu.step()

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


def test_opcode_55_eor_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, 0x55)
    rom.write(0x0001, 0xFE)
    bus.write(0x0001, 0x0F)

    cpu.reset()
    cpu.x = 0x03
    cpu.a = 0xF0
    cpu.step()

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

测试函数 (3)

test_eor_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_55_eor_zero_page_x_reads_indexed_memory_value
test_opcode_55_eor_zero_page_x_wraps_zero_page_address