031. Ldx 立即寻址

添加 LDX 立即寻址（$A2）。

第 31 / 356 · tests/chapter_01_cpu/test_031_LDX_immediate.py

要更新的文件

emulator/cpu/opcodes.py

位置

opcodes import of ldx
opcodes.ldx_immediate
opcodes.OPCODE_TABLE[$A2]

为什么需要这一步

测试 030 引入了值层面的 ldx 指令。本课通过获取字面操作数，并把寄存器和标志行为委托给这条既有指令，为它接入第一个机器码编码。

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import lda, sta, ldx


def ldx_immediate(cpu: CPU):
    ldx(cpu, immediate(cpu))


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA2: ldx_immediate,
}

重要不变式

$A2 映射到 ldx_immediate
immediate 恰好获取一个字节，因此整条指令使 PC 前进 2 字节
操作数字节本身被加载到 X；它不会被用作地址
ldx 仍负责更新 Zero 和 Negative

常见误区

不要按操作数的数值去读取内存。A2 42 将 $42 本身加载到 X。

超出范围

零页寻址、零页Y变址和绝对寻址的 LDX 编码
对 immediate 或 ldx 的改动
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_031_LDX_immediate.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import lda, sta, ldx


def ldx_immediate(cpu: CPU):
    ldx(cpu, immediate(cpu))


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA2: ldx_immediate,
}

查看测试代码test_031_LDX_immediate.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
from emulator.memory.fake_rom import FakeROM
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


def make_cpu_with_rom():
    rom = FakeROM()
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)
    bus = CpuBus(program_rom=rom)
    return CPU(bus), bus, rom


def test_ldx_immediate_handler_exists_and_is_in_opcode_table():
    """Objective: create ldx_immediate(cpu) and add 0xA2 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldx_immediate")
    assert callable(opcodes.ldx_immediate)
    assert list(inspect.signature(opcodes.ldx_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xA2] is opcodes.ldx_immediate


def test_opcode_A2_ldx_immediate_loads_register_x():
    """Objective: A2 42 means LDX #$42, so X becomes 0x42."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA2)
    rom.write(0x0001, 0x42)

    cpu.reset()
    cpu.step()

    assert cpu.x == 0x42
    assert cpu.pc == 0x8002


def test_opcode_A2_ldx_immediate_updates_flags():
    """Objective: LDX Immediate updates Zero and Negative flags."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA2)
    rom.write(0x0001, 0x80)

    cpu.reset()
    cpu.step()

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

测试函数 (3)

test_ldx_immediate_handler_exists_and_is_in_opcode_table
test_opcode_A2_ldx_immediate_loads_register_x
test_opcode_A2_ldx_immediate_updates_flags