135. EOR 立即数

将 EOR immediate 暴露为操作码 ``0x49``。

第 135 / 356 · tests/chapter_01_cpu/test_135_EOR_immediate.py

此步骤存在的原因

立即寻址形式使 EOR 能够以字面量操作数执行,并建立起操作码层向共享异或原语的委托机制。

在此步骤中,继第 134 课引入 `or_e 之后,请将 or_e 添加到指令导入中,并在 emulator/cpu/opcodes.py` 中添加以下内容:

def eor_immediate(cpu: CPU):
    or_e(cpu, immediate(cpu))

OPCODE_TABLE = {
    ...
    0x49: eor_immediate,
}

`emulator/cpu/addressing_modes.py::immediate 调用 CPU.fetch_byte 并返回操作数值,同时使 PC 前进。处理函数不需要再进行第二次总线读取。instructions.or_e` 写入 A 并更新 Z/N;Carry/Overflow、内存、X 和 Y 保持不变,操作码加操作数使 PC 前进两次。

常见误解:立即寻址模式提供的是一个值,而不是一个需要解引用的地址。不在本步骤范围内:所有内存寻址的 EOR 处理函数(第 136-142 课)以及 BIT(第 143-145 课)。

运行本课

uv run pytest tests/chapter_01_cpu/test_135_EOR_immediate.py -v

示例实现

此步骤存在的原因

def eor_immediate(cpu: CPU):
    or_e(cpu, immediate(cpu))

OPCODE_TABLE = {
    ...
    0x49: eor_immediate,
}

查看测试代码test_135_EOR_immediate.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_eor_immediate_handler_exists_and_is_in_opcode_table():
    """Objective: create eor_immediate(cpu) and add 0x49 to OPCODE_TABLE."""
    assert hasattr(opcodes, "eor_immediate")
    assert callable(opcodes.eor_immediate)
    assert list(inspect.signature(opcodes.eor_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x49] is opcodes.eor_immediate


def test_opcode_49_eor_immediate_updates_accumulator():
    """Objective: 49 0F means EOR #$0F, so A becomes A ^ 0x0F."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x49)
    rom.write(0x0001, 0x0F)

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

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


def test_opcode_49_eor_immediate_updates_zero_flag():
    """Objective: equal operands produce result 0 and set Zero flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x49)
    rom.write(0x0001, 0xAA)

    cpu.reset()
    cpu.a = 0xAA
    cpu.step()

    assert cpu.a == 0x00
    assert (cpu.p & ZERO_FLAG) != 0
    assert (cpu.p & NEGATIVE_FLAG) == 0


def test_opcode_49_eor_immediate_updates_negative_flag():
    """Objective: result bit 7 sets Negative flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x49)
    rom.write(0x0001, 0x81)

    cpu.reset()
    cpu.a = 0x01
    cpu.step()

    assert cpu.a == 0x80
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0

测试函数 (4)

test_eor_immediate_handler_exists_and_is_in_opcode_table
test_opcode_49_eor_immediate_updates_accumulator
test_opcode_49_eor_immediate_updates_zero_flag
test_opcode_49_eor_immediate_updates_negative_flag