043. Ldy 零页

添加 LDY 零页寻址（$A4）。

第 43 / 356 · tests/chapter_01_cpu/test_043_LDY_zero_page.py

要更新的文件

emulator/cpu/opcodes.py

位置

opcodes imports of zero_page and ldy
opcodes.ldy_zero_page
opcodes.OPCODE_TABLE[$A4]

为什么需要这一步

与立即数寻址的 LDY 不同，零页寻址的 LDY 会将操作数解析为地址，读取该地址处的字节，然后将寄存器和标志位行为委托给 ldy。

完整示例实现

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


def ldy_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA4: ldy_zero_page,
}

重要不变量

$A4 映射到 ldy_zero_page 并消耗一个操作数字节
zero_page 返回的地址位于 $0000-$00FF 之间
处理函数执行一次数据读取，并将该数值（而非其地址）传递给 ldy
ldy 负责更新 Zero 和 Negative

常见误区

如果直接把 addr 传给 ldy，加载到的会是零页位置编号本身，而不是存储在那里的字节。

范围之外

零页,X 和绝对寻址的 LDY 编码
新的寻址模式辅助函数
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_043_LDY_zero_page.py -v

示例实现

完整示例实现

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


def ldy_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA4: ldy_zero_page,
}

查看测试代码test_043_LDY_zero_page.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_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create ldy_zero_page(cpu) and add 0xA4 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldy_zero_page")
    assert callable(opcodes.ldy_zero_page)
    assert list(inspect.signature(opcodes.ldy_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xA4] is opcodes.ldy_zero_page


def test_opcode_A4_ldy_zero_page_loads_register_y():
    """Objective: A4 10 means LDY $10, so Y loads RAM[$0010]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA4)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x42)

    cpu.reset()
    cpu.step()

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

测试函数 (2)

test_ldy_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_A4_ldy_zero_page_loads_register_y