134. EOR 指令原语

实现与寻址方式无关的 EOR 行为。

第 134 / 356 · tests/chapter_01_cpu/test_134_instruction_or_e.py

此步骤存在的原因

将异或语义集中到一个面向数值的原语中,使得后续每一个 EOR 操作码都能共享完全相同的累加器和 Zero/Negative 标志位行为。

在此步骤中,完成所有 ORA 相关工作后,请在 `emulator/cpu/instructions.py` 中准确添加以下符号:

def or_e(cpu: CPU, value: int):
    result_8 = (cpu.a ^ value) & 0xFF

    # Flags:
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)

    cpu.a = result_8

面向数值的签名将 EOR 的语义与寻址方式分离开来。XOR 结果被掩码为八位,存入 A,并用于推导 Zero 和 Negative 标志位。由于 `or_e` 不会触及 Carry、Overflow、内存、X、Y 和 PC,它们保持不变。

常见误解:尽管辅助函数名为 `or_e,EOR 是异或(^),而不是或运算,value 也不是地址。不在本步骤范围内:在 emulator/cpu/opcodes.py 中导入 or_e`、每一个 EOR 操作码(第 135-142 课)以及 BIT(第 143-145 课)。

运行本课

uv run pytest tests/chapter_01_cpu/test_134_instruction_or_e.py -v

示例实现

此步骤存在的原因

def or_e(cpu: CPU, value: int):
    result_8 = (cpu.a ^ value) & 0xFF

    # Flags:
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)

    cpu.a = result_8

查看测试代码test_134_instruction_or_e.py

from emulator.cpu.instructions import or_e
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG, make_cpu


CARRY_FLAG = 1 << 0
OVERFLOW_FLAG = 1 << 6


def test_or_e_stores_exclusive_or_result_in_accumulator():
    """Objective: A becomes A ^ value."""
    cpu = make_cpu()
    cpu.a = 0b1100_1010

    or_e(cpu, 0b1010_1100)

    assert cpu.a == 0b0110_0110


def test_or_e_sets_zero_flag_when_result_is_zero():
    """Objective: when A ^ value is 0, Zero flag is set."""
    cpu = make_cpu()
    cpu.a = 0xAA

    or_e(cpu, 0xAA)

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


def test_or_e_sets_negative_flag_from_result_bit_7():
    """Objective: if result bit 7 is set, Negative flag is set."""
    cpu = make_cpu()
    cpu.a = 0x01

    or_e(cpu, 0x81)

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


def test_or_e_does_not_modify_carry_or_overflow_flags():
    """Objective: EOR only updates Z/N; Carry and Overflow are preserved."""
    cpu = make_cpu()
    cpu.a = 0xF0
    cpu.p |= CARRY_FLAG
    cpu.p |= OVERFLOW_FLAG

    or_e(cpu, 0x0F)

    assert cpu.a == 0xFF
    assert (cpu.p & CARRY_FLAG) != 0
    assert (cpu.p & OVERFLOW_FLAG) != 0

测试函数 (4)

test_or_e_stores_exclusive_or_result_in_accumulator
test_or_e_sets_zero_flag_when_result_is_zero
test_or_e_sets_negative_flag_from_result_bit_7
test_or_e_does_not_modify_carry_or_overflow_flags