089. Asl a 指令

添加面向累加器的 ASL 行为。

第 89 / 356 · tests/chapter_01_cpu/test_089_instruction_asl_a.py

在这一步中，仅添加 asl_a，以测试 088 中的内存形式作为上下文，将 ASL 操作码映射留给测试 090-094。

生产代码位置与符号

emulator/cpu/instructions.py: `asl_a(cpu: CPU)`

本步骤存在的原因

ASL A 与内存 ASL 具有相同的位与标志规则，但其目标是累加器。使用单独的函数可以避免把 A 当作总线地址来处理。

实现建议

def asl_a(cpu: CPU):
    value = cpu.a
    result = cpu.a << 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_carry_flag((value & 0b1000_0000) != 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.a = result_8

重要不变量

Carry 接收 A 原本的第 7 位
A 被掩码为 8 位；Z 和 N 使用该掩码后的结果
不解析任何内存地址，也不修改任何总线位置
除 C、Z、N 之外的状态位均保持不变

常见误解

不要调用内存版的 asl(cpu, cpu.a)：累加器的值是数据，而不是地址，ASL A 不得触及内存。

不在本步骤范围内

导入 asl_a 并映射操作码 0x0A（测试 090）
内存操作码包装器及映射（测试 091-094）
周期计时

运行本课

uv run pytest tests/chapter_01_cpu/test_089_instruction_asl_a.py -v

示例实现

实现建议

def asl_a(cpu: CPU):
    value = cpu.a
    result = cpu.a << 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_carry_flag((value & 0b1000_0000) != 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.a = result_8

查看测试代码test_089_instruction_asl_a.py

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


CARRY_FLAG = 1 << 0


def test_asl_a_shifts_accumulator_left():
    """Objective: A value 0b0000_0011 becomes 0b0000_0110."""
    cpu = make_cpu()
    cpu.a = 0b0000_0011

    asl_a(cpu)

    assert cpu.a == 0b0000_0110


def test_asl_a_sets_carry_from_old_accumulator_bit_7():
    """Objective: old A bit 7 is moved into Carry before the 8-bit wrap."""
    cpu = make_cpu()
    cpu.a = 0b1000_0001

    asl_a(cpu)

    assert cpu.a == 0b0000_0010
    assert (cpu.p & CARRY_FLAG) != 0


def test_asl_a_clears_carry_when_old_accumulator_bit_7_was_clear():
    """Objective: Carry is cleared when original A did not have bit 7 set."""
    cpu = make_cpu()
    cpu.p |= CARRY_FLAG
    cpu.a = 0b0100_0000

    asl_a(cpu)

    assert cpu.a == 0b1000_0000
    assert (cpu.p & CARRY_FLAG) == 0


def test_asl_a_sets_zero_flag_when_result_is_zero():
    """Objective: A=0x80 shifts to 0x00 and sets Zero flag."""
    cpu = make_cpu()
    cpu.a = 0x80

    asl_a(cpu)

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


def test_asl_a_sets_negative_flag_from_result_bit_7():
    """Objective: A=0x40 shifts to 0x80, so Negative flag is set."""
    cpu = make_cpu()
    cpu.a = 0x40

    asl_a(cpu)

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


def test_asl_a_does_not_write_shifted_result_to_memory():
    """Objective: ASL A writes to A only; it must not treat A as a memory address."""
    cpu = make_cpu()
    cpu.a = 0x20
    cpu.bus.write(0x0020, 0x99)

    asl_a(cpu)

    assert cpu.a == 0x40
    assert cpu.bus.read(0x0020) == 0x99

测试函数 (6)

test_asl_a_shifts_accumulator_left
test_asl_a_sets_carry_from_old_accumulator_bit_7
test_asl_a_clears_carry_when_old_accumulator_bit_7_was_clear
test_asl_a_sets_zero_flag_when_result_is_zero
test_asl_a_sets_negative_flag_from_result_bit_7
test_asl_a_does_not_write_shifted_result_to_memory