143. 指令 bit

实现与寻址方式无关的 BIT 行为。

第 143 / 356 · tests/chapter_01_cpu/test_143_instruction_bit.py

此步骤存在的原因

BIT 的标志位语义不同寻常:Z 来自 A 与值的 AND 结果,而 N 和 V 复制操作数的相应位,因此一个共享原语就能防止各寻址处理函数出现偏差。

在此步骤中,完成各 EOR 课程后,请在 `emulator/cpu/instructions.py` 中准确添加以下符号:

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

    # Flags:
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((value & 0b1000_0000) != 0)
    cpu.flags.set_overflow_flag((value & 0b0100_0000) != 0)

BIT 将零测试与两个被复制的操作数位分离开来:Z 反映 `A & value`,而 N 和 V 反映值的第 7 位和第 6 位。 A、Carry、内存、X、Y 和 PC 保持不变;只有 Z/N/V 会被重写。

常见误解:BIT 既不会把 `A & value 存入 A,也不会从该 AND 结果推导出 N/V。超出范围:将 bit 导入 emulator/cpu/opcodes.py` 并接入零页与绝对寻址操作码属于第 144-145 课。

运行本课

uv run pytest tests/chapter_01_cpu/test_143_instruction_bit.py -v

示例实现

此步骤存在的原因

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

    # Flags:
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((value & 0b1000_0000) != 0)
    cpu.flags.set_overflow_flag((value & 0b0100_0000) != 0)

查看测试代码test_143_instruction_bit.py

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


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


def test_bit_sets_zero_flag_when_a_and_value_is_zero():
    """Objective: if A & value is 0, Zero flag is set."""
    cpu = make_cpu()
    cpu.a = 0b0000_1111

    bit(cpu, 0b1111_0000)

    assert (cpu.p & ZERO_FLAG) != 0


def test_bit_clears_zero_flag_when_a_and_value_is_not_zero():
    """Objective: if A & value is nonzero, Zero flag is cleared."""
    cpu = make_cpu()
    cpu.p |= ZERO_FLAG
    cpu.a = 0b0000_1111

    bit(cpu, 0b0000_0001)

    assert (cpu.p & ZERO_FLAG) == 0


def test_bit_sets_negative_flag_from_value_bit_7():
    """Objective: BIT copies value bit 7 into Negative flag."""
    cpu = make_cpu()
    cpu.a = 0x00

    bit(cpu, 0b1000_0000)

    assert (cpu.p & NEGATIVE_FLAG) != 0


def test_bit_sets_overflow_flag_from_value_bit_6():
    """Objective: BIT copies value bit 6 into Overflow flag."""
    cpu = make_cpu()
    cpu.a = 0x00

    bit(cpu, 0b0100_0000)

    assert (cpu.p & OVERFLOW_FLAG) != 0


def test_bit_clears_negative_and_overflow_when_value_bits_are_clear():
    """Objective: N and V follow value bits 7 and 6, including clearing them."""
    cpu = make_cpu()
    cpu.p |= NEGATIVE_FLAG
    cpu.p |= OVERFLOW_FLAG
    cpu.a = 0xFF

    bit(cpu, 0b0011_1111)

    assert (cpu.p & NEGATIVE_FLAG) == 0
    assert (cpu.p & OVERFLOW_FLAG) == 0


def test_bit_does_not_modify_accumulator_or_carry_flag():
    """Objective: BIT only updates Z/N/V; A and Carry are preserved."""
    cpu = make_cpu()
    cpu.a = 0b1010_1010
    cpu.p |= CARRY_FLAG

    bit(cpu, 0b1111_0000)

    assert cpu.a == 0b1010_1010
    assert (cpu.p & CARRY_FLAG) != 0

测试函数 (6)

test_bit_sets_zero_flag_when_a_and_value_is_zero
test_bit_clears_zero_flag_when_a_and_value_is_not_zero
test_bit_sets_negative_flag_from_value_bit_7
test_bit_sets_overflow_flag_from_value_bit_6
test_bit_clears_negative_and_overflow_when_value_bits_are_clear
test_bit_does_not_modify_accumulator_or_carry_flag