042. Ldy 立即数

添加 LDY 立即数寻址（$A0）。

第 42 / 356 · tests/chapter_01_cpu/test_042_LDY_immediate.py

要更新的文件

emulator/cpu/opcodes.py

位置

opcodes import of ldy
opcodes.ldy_immediate
opcodes.OPCODE_TABLE[$A0]

为什么需要这一步

核心指令 ldy 已经负责寄存器和标志位行为。本课通过取出一个字面操作数并将该值传递给 ldy，来连接它的第一种编码。

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.addressing_modes import immediate
from emulator.cpu.instructions import ldy


def ldy_immediate(cpu: CPU):
    ldy(cpu, immediate(cpu))


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA0: ldy_immediate,
}

重要不变量

$A0 映射到 ldy_immediate
立即数寻址正好取出一个操作数字节，操作码之后 PC 前进一位
操作数本身被加载；它不会被当作内存地址处理
ldy 仍然负责更新 Zero 和 Negative

常见误区

不要用立即数字节的数值去读取总线。对于 A0 42，Y 会直接获得 $42。

范围之外

LDY 的零页、变址和绝对寻址编码
STY 操作码处理函数
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_042_LDY_immediate.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.addressing_modes import immediate
from emulator.cpu.instructions import ldy


def ldy_immediate(cpu: CPU):
    ldy(cpu, immediate(cpu))


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA0: ldy_immediate,
}

查看测试代码test_042_LDY_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_ldy_immediate_handler_exists_and_is_in_opcode_table():
    """Objective: create ldy_immediate(cpu) and add 0xA0 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldy_immediate")
    assert callable(opcodes.ldy_immediate)
    assert list(inspect.signature(opcodes.ldy_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xA0] is opcodes.ldy_immediate


def test_opcode_A0_ldy_immediate_loads_register_y():
    """Objective: A0 42 means LDY #$42, so Y becomes 0x42."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA0)
    rom.write(0x0001, 0x42)

    cpu.reset()
    cpu.step()

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


def test_opcode_A0_ldy_immediate_updates_flags():
    """Objective: LDY Immediate updates Zero and Negative flags."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA0)
    rom.write(0x0001, 0x80)

    cpu.reset()
    cpu.step()

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

测试函数 (3)

test_ldy_immediate_handler_exists_and_is_in_opcode_table
test_opcode_A0_ldy_immediate_loads_register_y
test_opcode_A0_ldy_immediate_updates_flags