046. Ldy 绝对,X

添加 LDY 绝对,X 寻址（$BC）。

第 46 / 356 · tests/chapter_01_cpu/test_046_LDY_absolute_x.py

要更新的文件

emulator/cpu/opcodes.py

位置

opcodes imports of absolute_x and ldy
opcodes.ldy_absolute_x
opcodes.OPCODE_TABLE[$BC]

为什么需要这一步

本课通过复用绝对,X 地址解析来完成 LDY 操作码家族，然后读取有效地址并套用已有的 ldy 行为。

完整示例实现

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


def ldy_absolute_x(cpu: CPU):
    addr = absolute_x(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xBC: ldy_absolute_x,
}

重要不变量

$BC 映射到 ldy_absolute_x 并消耗两个操作数字节
加到解码出的 16 位基址上的是 X，而不是 Y
有效地址会先被读取，其值再传递给 ldy
ldy 根据加载到的字节更新 Zero 和 Negative

常见误区

LDY 绝对,X 不会用目的寄存器 Y 进行变址；该编码明确使用的是 X。

范围之外

STY 操作码处理函数
跨页周期惩罚
对 absolute_x 的修改

运行本课

uv run pytest tests/chapter_01_cpu/test_046_LDY_absolute_x.py -v

示例实现

完整示例实现

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


def ldy_absolute_x(cpu: CPU):
    addr = absolute_x(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xBC: ldy_absolute_x,
}

查看测试代码test_046_LDY_absolute_x.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_x_handler_exists_and_is_in_opcode_table():
    """Objective: create ldy_absolute_x(cpu) and add 0xBC to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldy_absolute_x")
    assert callable(opcodes.ldy_absolute_x)
    assert list(inspect.signature(opcodes.ldy_absolute_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xBC] is opcodes.ldy_absolute_x


def test_opcode_BC_ldy_absolute_x_loads_register_y():
    """Objective: BC 00 02 with X=0x04 reads RAM[$0204] into Y."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xBC)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0x42)

    cpu.reset()
    cpu.x = 0x04
    cpu.step()

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

测试函数 (2)

test_ldy_absolute_x_handler_exists_and_is_in_opcode_table
test_opcode_BC_ldy_absolute_x_loads_register_y