095. Lsr 指令

添加 LSR 内存指令行为。

第 95 / 356 · tests/chapter_01_cpu/test_095_instruction_lsr.py

在这一步中，只添加以内存为目标的 LSR 行为。累加器行为与操作码接入将在测试 096-101 中完成。

文件与符号

emulator/cpu/instructions.py: lsr

为什么需要这一步

在接入寻址模式处理函数之前，指令层必须先定义一次 LSR 的读取/修改/写回与标志位语义。这次改动仅涉及内存。

本步骤的建议实现

# emulator/cpu/instructions.py
def lsr(cpu: CPU, addr: int):
    value = cpu.bus.read(addr)
    result = value >> 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_carry_flag((value & 0x01) != 0)
    cpu.flags.set_negative_flag(False)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.bus.write(addr, result_8)

重要不变量

Carry 由原第 0 位替换得到，即使该位为 0 也会被清除
Zero 基于最终的 8 位结果，Negative 始终被清零
只精确读取并写回所提供的内存地址；A 保持不变
逻辑右移在第 7 位引入 0

常见误解

Carry 并非来自结果，Negative 也不是从原第 7 位复制而来。

本步骤不涉及

测试 096 中的累加器行为以及测试 097-101 中的操作码接入
周期精确的读取/修改/写回时序
与 asl_a 无关的清理工作

运行本课

uv run pytest tests/chapter_01_cpu/test_095_instruction_lsr.py -v

示例实现

本步骤的建议实现

# emulator/cpu/instructions.py
def lsr(cpu: CPU, addr: int):
    value = cpu.bus.read(addr)
    result = value >> 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_carry_flag((value & 0x01) != 0)
    cpu.flags.set_negative_flag(False)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.bus.write(addr, result_8)

查看测试代码test_095_instruction_lsr.py

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


CARRY_FLAG = 1 << 0


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

    lsr(cpu, 0x0020)

    assert cpu.bus.read(0x0020) == 0b0000_0011


def test_lsr_memory_sets_carry_from_old_bit_0():
    """Objective: old bit 0 is moved into Carry before the right shift."""
    cpu = make_cpu()
    cpu.bus.write(0x0020, 0b0000_0011)

    lsr(cpu, 0x0020)

    assert cpu.bus.read(0x0020) == 0b0000_0001
    assert (cpu.p & CARRY_FLAG) != 0


def test_lsr_memory_clears_carry_when_old_bit_0_was_clear():
    """Objective: Carry is cleared when the original value did not have bit 0 set."""
    cpu = make_cpu()
    cpu.p |= CARRY_FLAG
    cpu.bus.write(0x0020, 0b0000_0010)

    lsr(cpu, 0x0020)

    assert cpu.bus.read(0x0020) == 0b0000_0001
    assert (cpu.p & CARRY_FLAG) == 0


def test_lsr_memory_sets_zero_flag_when_result_is_zero():
    """Objective: 0x01 >> 1 becomes 0x00 and sets Zero flag."""
    cpu = make_cpu()
    cpu.bus.write(0x0020, 0x01)

    lsr(cpu, 0x0020)

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


def test_lsr_memory_always_clears_negative_flag():
    """Objective: LSR fills bit 7 with 0, so Negative must be cleared."""
    cpu = make_cpu()
    cpu.p |= NEGATIVE_FLAG
    cpu.bus.write(0x0020, 0x80)

    lsr(cpu, 0x0020)

    assert cpu.bus.read(0x0020) == 0x40
    assert (cpu.p & NEGATIVE_FLAG) == 0

测试函数 (5)

test_lsr_memory_shifts_value_right_and_writes_result_back
test_lsr_memory_sets_carry_from_old_bit_0
test_lsr_memory_clears_carry_when_old_bit_0_was_clear
test_lsr_memory_sets_zero_flag_when_result_is_zero
test_lsr_memory_always_clears_negative_flag