096. Lsr 累加器指令

添加 LSR 累加器指令行为。

第 96 / 356 · tests/chapter_01_cpu/test_096_instruction_lsr_a.py

在这一步中，在测试 095 的内存原语之后，添加以累加器为目标的 LSR 行为。

文件与符号

emulator/cpu/instructions.py: lsr_a

为什么需要这一步

累加器 LSR 与测试 095 共享移位和标志位逻辑，但数据目标不同，因此必须更新 cpu.a，而不能把该寄存器当作地址处理。

本步骤的建议实现

# emulator/cpu/instructions.py
def lsr_a(cpu: CPU):
    value = cpu.a
    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.a = result_8

重要不变量

原 A 的第 0 位替换 Carry
Zero 依据最终的 A 值，Negative 始终被清零
不会发生任何总线读取或写入
地址数值上等于原 A 的那处内存保持不变

常见误解

cpu.a 本身既是值也是目标，而不是要传给内存 lsr 的地址。

本步骤不涉及

测试 097 中操作码 0x4A 的接入
测试 098-101 中的内存寻址操作码
重构共享的 LSR 逻辑或添加周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_096_instruction_lsr_a.py -v

示例实现

本步骤的建议实现

# emulator/cpu/instructions.py
def lsr_a(cpu: CPU):
    value = cpu.a
    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.a = result_8

查看测试代码test_096_instruction_lsr_a.py

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


CARRY_FLAG = 1 << 0


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

    lsr_a(cpu)

    assert cpu.a == 0b0000_0011


def test_lsr_a_sets_carry_from_old_accumulator_bit_0():
    """Objective: old A bit 0 is moved into Carry before the right shift."""
    cpu = make_cpu()
    cpu.a = 0b0000_0011

    lsr_a(cpu)

    assert cpu.a == 0b0000_0001
    assert (cpu.p & CARRY_FLAG) != 0


def test_lsr_a_clears_carry_when_old_accumulator_bit_0_was_clear():
    """Objective: Carry is cleared when original A did not have bit 0 set."""
    cpu = make_cpu()
    cpu.p |= CARRY_FLAG
    cpu.a = 0b0000_0010

    lsr_a(cpu)

    assert cpu.a == 0b0000_0001
    assert (cpu.p & CARRY_FLAG) == 0


def test_lsr_a_sets_zero_flag_when_result_is_zero():
    """Objective: A=0x01 shifts to 0x00 and sets Zero flag."""
    cpu = make_cpu()
    cpu.a = 0x01

    lsr_a(cpu)

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


def test_lsr_a_always_clears_negative_flag():
    """Objective: LSR A fills bit 7 with 0, so Negative must be cleared."""
    cpu = make_cpu()
    cpu.p |= NEGATIVE_FLAG
    cpu.a = 0x80

    lsr_a(cpu)

    assert cpu.a == 0x40
    assert (cpu.p & NEGATIVE_FLAG) == 0


def test_lsr_a_does_not_write_shifted_result_to_memory():
    """Objective: LSR 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)

    lsr_a(cpu)

    assert cpu.a == 0x10
    assert cpu.bus.read(0x0020) == 0x99

测试函数 (6)

test_lsr_a_shifts_accumulator_right
test_lsr_a_sets_carry_from_old_accumulator_bit_0
test_lsr_a_clears_carry_when_old_accumulator_bit_0_was_clear
test_lsr_a_sets_zero_flag_when_result_is_zero
test_lsr_a_always_clears_negative_flag
test_lsr_a_does_not_write_shifted_result_to_memory