030. Ldx指令

添加核心LDX指令。

第 30 / 356 · tests/chapter_01_cpu/test_030_instruction_ldx.py

需要更新的文件

emulator/cpu/instructions.py

位置

instructions.ldx, beside lda and sta

为什么需要这一步

LDX开启了下一个加载指令家族，同时保留了LDA所确立的指令/操作码边界。核心指令接收一个已经解析好的值，将其存入X，并应用与 lda 相同的Zero/Negative规则。

完整示例实现

# emulator/cpu/instructions.py
def ldx(cpu, value) -> None:
    cpu.x = value
    cpu._update_zero_and_negative_flags(cpu.x)

重要不变量

ldx接收的是一个值，而不是地址
X接收该值，而A和Y保持不变
当值恰好为 $00 时置位Zero，否则清零
Negative镜像第7位，当第7位为零时清零

常见误解

不要在 ldx 内部获取操作数或读取总线；寻址与内存访问应由操作码处理函数在调用本指令之前完成。

不在本课范围内

所有LDX操作码处理函数以及操作码表条目
零页,Y与某个操作码的集成
后续的FlagsHandler重构

运行本课

uv run pytest tests/chapter_01_cpu/test_030_instruction_ldx.py -v

示例实现

完整示例实现

# emulator/cpu/instructions.py
def ldx(cpu, value) -> None:
    cpu.x = value
    cpu._update_zero_and_negative_flags(cpu.x)

查看测试代码test_030_instruction_ldx.py

import inspect

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


def test_ldx_instruction_exists():
    """
    Objective:
    Create in instructions.py:
        def ldx(cpu, value):
            ...

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


def test_ldx_loads_value_into_register_x():
    """Objective: ldx(cpu, value) must put value inside cpu.x."""
    cpu = make_cpu()

    instructions.ldx(cpu, 0x42)

    assert cpu.x == 0x42


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

    instructions.ldx(cpu, 0x00)

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


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

    instructions.ldx(cpu, 0x01)

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


def test_ldx_sets_negative_flag_when_bit_7_is_one():
    """Objective: if LDX loads a value with bit 7 active, Negative flag is set."""
    cpu = make_cpu()

    instructions.ldx(cpu, 0x80)

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


def test_ldx_clears_negative_flag_when_bit_7_is_zero():
    """Objective: if LDX loads a value with bit 7 inactive, Negative flag is clear."""
    cpu = make_cpu()
    cpu.p |= NEGATIVE_FLAG

    instructions.ldx(cpu, 0x7F)

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

测试函数 (6)

test_ldx_instruction_exists
test_ldx_loads_value_into_register_x
test_ldx_sets_zero_flag_when_value_is_zero
test_ldx_clears_zero_flag_when_value_is_not_zero
test_ldx_sets_negative_flag_when_bit_7_is_one
test_ldx_clears_negative_flag_when_bit_7_is_zero