045. Ldy 绝对寻址

添加 LDY 绝对寻址（$AC）。

第 45 / 356 · tests/chapter_01_cpu/test_045_LDY_absolute.py

要更新的文件

emulator/cpu/opcodes.py

位置

opcodes imports of absolute and ldy
opcodes.ldy_absolute
opcodes.OPCODE_TABLE[$AC]

为什么需要这一步

绝对寻址把 LDY 的能力扩展到零页之外。处理函数解码已有的小端 16 位操作数，读取该地址，并将加载到的值委托给 ldy。

完整示例实现

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


def ldy_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xAC: ldy_absolute,
}

重要不变量

$AC 映射到 ldy_absolute 并消耗两个操作数字节
absolute 先合并低字节，再合并高字节
处理函数读取有效地址，并将结果值传递给 ldy
执行总共前进三个字节，包括操作码在内

常见误区

不要颠倒 AC 00 02 的顺序；已有的绝对寻址辅助函数会把这些操作数字节解析为 $0200，而不是 $0002。

范围之外

绝对,X 寻址的 LDY
修改绝对寻址辅助函数
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_045_LDY_absolute.py -v

示例实现

完整示例实现

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


def ldy_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xAC: ldy_absolute,
}

查看测试代码test_045_LDY_absolute.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


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_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create ldy_absolute(cpu) and add 0xAC to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldy_absolute")
    assert callable(opcodes.ldy_absolute)
    assert list(inspect.signature(opcodes.ldy_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xAC] is opcodes.ldy_absolute


def test_opcode_AC_ldy_absolute_loads_register_y():
    """Objective: AC 00 02 means LDY $0200, so Y loads RAM[$0200]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xAC)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x42)

    cpu.reset()
    cpu.step()

    assert cpu.y == 0x42
    assert cpu.pc == 0x8003

测试函数 (2)

test_ldy_absolute_handler_exists_and_is_in_opcode_table
test_opcode_AC_ldy_absolute_loads_register_y