088. Asl 指令

添加面向内存的 ASL 行为。

第 88 / 356 · tests/chapter_01_cpu/test_088_instruction_asl.py

在这一步中，添加可复用的内存指令。累加器行为是测试 089 的内容，操作码包装器从测试 090 开始。

生产代码位置与符号

emulator/cpu/instructions.py: `asl(cpu: CPU, addr: int)`

本步骤存在的原因

内存 ASL 是一个读-改-写操作。指令函数接收一个已解析的地址，以便后续的操作码处理函数可以共享同一套逻辑。

实现建议

def asl(cpu: CPU, addr: int):
    value = cpu.bus.read(addr)
    result = value << 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.bus.write(addr, result_8)

重要不变量

Carry 在结果被掩码之前接收旧的第 7 位
Zero 和 Negative 反映最终的 8 位结果
只读写所提供的那个地址；A 不受影响
除 C、Z、N 之外的所有状态位均保持不变

常见误解

Carry 并非来自移位结果的第 7 位；它捕获的是从原始值中移出的那一位。

不在本步骤范围内

针对累加器的 asl_a（测试 089）
操作码 0x0A 及内存寻址包装器（测试 090-094）
周期计时以及总线精确的哑写操作

运行本课

uv run pytest tests/chapter_01_cpu/test_088_instruction_asl.py -v

示例实现

实现建议

def asl(cpu: CPU, addr: int):
    value = cpu.bus.read(addr)
    result = value << 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.bus.write(addr, result_8)

查看测试代码test_088_instruction_asl.py

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


CARRY_FLAG = 1 << 0


def test_asl_memory_shifts_value_left_and_writes_result_back():
    """Objective: memory value 0b0000_0011 becomes 0b0000_0110."""
    cpu = make_cpu()
    cpu.bus.write(0x0020, 0b0000_0011)

    asl(cpu, 0x0020)

    assert cpu.bus.read(0x0020) == 0b0000_0110


def test_asl_memory_sets_carry_from_old_bit_7():
    """Objective: old bit 7 is moved into Carry before the 8-bit wrap."""
    cpu = make_cpu()
    cpu.bus.write(0x0020, 0b1000_0001)

    asl(cpu, 0x0020)

    assert cpu.bus.read(0x0020) == 0b0000_0010
    assert (cpu.p & CARRY_FLAG) != 0


def test_asl_memory_clears_carry_when_old_bit_7_was_clear():
    """Objective: Carry is cleared when the original value did not have bit 7 set."""
    cpu = make_cpu()
    cpu.p |= CARRY_FLAG
    cpu.bus.write(0x0020, 0b0100_0000)

    asl(cpu, 0x0020)

    assert cpu.bus.read(0x0020) == 0b1000_0000
    assert (cpu.p & CARRY_FLAG) == 0


def test_asl_memory_sets_zero_flag_when_result_is_zero():
    """Objective: 0x80 << 1 becomes 0x00 and sets Zero flag."""
    cpu = make_cpu()
    cpu.bus.write(0x0020, 0x80)

    asl(cpu, 0x0020)

    assert cpu.bus.read(0x0020) == 0x00
    assert (cpu.p & ZERO_FLAG) != 0
    assert (cpu.p & NEGATIVE_FLAG) == 0


def test_asl_memory_sets_negative_flag_from_result_bit_7():
    """Objective: 0x40 << 1 becomes 0x80, so Negative flag is set."""
    cpu = make_cpu()
    cpu.bus.write(0x0020, 0x40)

    asl(cpu, 0x0020)

    assert cpu.bus.read(0x0020) == 0x80
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0

测试函数 (5)

test_asl_memory_shifts_value_left_and_writes_result_back
test_asl_memory_sets_carry_from_old_bit_7
test_asl_memory_clears_carry_when_old_bit_7_was_clear
test_asl_memory_sets_zero_flag_when_result_is_zero
test_asl_memory_sets_negative_flag_from_result_bit_7