125. ORA 指令原语

实现与寻址方式无关的 ORA 行为。

第 125 / 356 · tests/chapter_01_cpu/test_125_instruction_or_a.py

此步骤存在的原因

在操作码处理程序引入各具体寻址形式之前,单一的、面向值的 ORA 基元集中定义了包含或(inclusive-OR)的结果与标志位规则。

在此步骤中,在第 116-124 课完成 AND 之后,仅需在 `emulator/cpu/instructions.py` 中添加以下符号:

def or_a(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

这个面向值的 API 使 ORA 语义与寻址方式无关。八位结果存储在 A 中;Zero 反映结果是否等于零,Negative 反映结果的第 7 位。由于 `or_a` 不会触及 Carry、Overflow、内存、X、Y 及 PC,它们保持不变。

常见误解:ORA 表示按位包含或(inclusive OR),而不是异或(XOR),并且 `value 不是总线地址。不在本步骤范围内:将 or_a 导入 emulator/cpu/opcodes.py` 及所有 ORA 处理程序(第 126-133 课),以及第 134 课和第 143 课中引入的 EOR 与 BIT 符号。

运行本课

uv run pytest tests/chapter_01_cpu/test_125_instruction_or_a.py -v

示例实现

此步骤存在的原因

def or_a(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_125_instruction_or_a.py

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


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


def test_or_a_stores_bitwise_or_result_in_accumulator():
    """Objective: A becomes A | value."""
    cpu = make_cpu()
    cpu.a = 0b1100_0000

    or_a(cpu, 0b0000_1010)

    assert cpu.a == 0b1100_1010


def test_or_a_sets_zero_flag_when_result_is_zero():
    """Objective: when A | value is 0, Zero flag is set."""
    cpu = make_cpu()
    cpu.a = 0x00

    or_a(cpu, 0x00)

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


def test_or_a_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_a(cpu, 0x80)

    assert cpu.a == 0x81
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0


def test_or_a_does_not_modify_carry_or_overflow_flags():
    """Objective: ORA only updates Z/N; Carry and Overflow are preserved."""
    cpu = make_cpu()
    cpu.a = 0x00
    cpu.p |= CARRY_FLAG
    cpu.p |= OVERFLOW_FLAG

    or_a(cpu, 0x0F)

    assert cpu.a == 0x0F
    assert (cpu.p & CARRY_FLAG) != 0
    assert (cpu.p & OVERFLOW_FLAG) != 0

测试函数 (4)

test_or_a_stores_bitwise_or_result_in_accumulator
test_or_a_sets_zero_flag_when_result_is_zero
test_or_a_sets_negative_flag_from_result_bit_7
test_or_a_does_not_modify_carry_or_overflow_flags