040. ldy 指令

添加核心 LDY 指令。

第 40 / 356 · tests/chapter_01_cpu/test_040_instruction_ldy.py

要更新的文件

emulator/cpu/instructions.py

位置

instructions.ldy, beside stx

为什么需要这一步

LDY 在任何 LDY 操作码接入之前先确立其值层面行为，由此开启 Y 寄存器加载指令家族。它沿用 LDA 和 LDX 的模式：把已解析好的值赋给目标寄存器，然后更新 Zero 和 Negative。

完整示例实现

# emulator/cpu/instructions.py
def ldy(cpu: CPU, value):
    cpu.y = value
    cpu._update_zero_and_negative_flags(cpu.y)

重要不变式

ldy 接收的是值，不是地址
Y 接收该值，而 A 和 X 保持不变
Zero 恰好在值为 $00 时置位，在其他情况下清零
Negative 反映第 7 位，当第 7 位为 0 时清零

常见误区

不要在 ldy 内部获取操作数或读取内存；未来的操作码处理函数会先解析出值，再调用这条指令。

超出范围

所有 LDY 操作码处理函数和操作码表项
STY 指令及其编码
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_040_instruction_ldy.py -v

示例实现

完整示例实现

# emulator/cpu/instructions.py
def ldy(cpu: CPU, value):
    cpu.y = value
    cpu._update_zero_and_negative_flags(cpu.y)

查看测试代码test_040_instruction_ldy.py

import inspect

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


def test_ldy_instruction_exists():
    """
    Objective:
    Create in instructions.py:
        def ldy(cpu, value):
            ...

    Example implementation:
        cpu.y = value
        cpu._update_zero_and_negative_flags(cpu.y)
    """
    assert hasattr(instructions, "ldy")
    assert callable(instructions.ldy)
    assert list(inspect.signature(instructions.ldy).parameters) == ["cpu", "value"]


def test_ldy_loads_value_into_register_y():
    """Objective: ldy(cpu, value) must put value inside cpu.y."""
    cpu = make_cpu()

    instructions.ldy(cpu, 0x42)

    assert cpu.y == 0x42


def test_ldy_sets_zero_flag_when_value_is_zero():
    """Objective: if LDY loads 0x00, Zero flag is set."""
    cpu = make_cpu()

    instructions.ldy(cpu, 0x00)

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


def test_ldy_clears_zero_flag_when_value_is_not_zero():
    """Objective: if LDY loads non-zero value, Zero flag is clear."""
    cpu = make_cpu()
    cpu.p |= ZERO_FLAG

    instructions.ldy(cpu, 0x01)

    assert cpu.y == 0x01
    assert (cpu.p & ZERO_FLAG) == 0


def test_ldy_sets_negative_flag_when_bit_7_is_one():
    """Objective: if LDY loads a value with bit 7 active, Negative flag is set."""
    cpu = make_cpu()

    instructions.ldy(cpu, 0x80)

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


def test_ldy_clears_negative_flag_when_bit_7_is_zero():
    """Objective: if LDY loads a value with bit 7 inactive, Negative flag is clear."""
    cpu = make_cpu()
    cpu.p |= NEGATIVE_FLAG

    instructions.ldy(cpu, 0x7F)

    assert cpu.y == 0x7F
    assert (cpu.p & NEGATIVE_FLAG) == 0

测试函数 (6)

test_ldy_instruction_exists
test_ldy_loads_value_into_register_y
test_ldy_sets_zero_flag_when_value_is_zero
test_ldy_clears_zero_flag_when_value_is_not_zero
test_ldy_sets_negative_flag_when_bit_7_is_one
test_ldy_clears_negative_flag_when_bit_7_is_zero