033. Ldx 零页Y变址

添加 LDX 零页Y变址（$B6）。

第 33 / 356 · tests/chapter_01_cpu/test_033_LDX_zero_page_y.py

要更新的文件

emulator/cpu/opcodes.py

位置

opcodes.ldx_zero_page_y
opcodes.OPCODE_TABLE[$B6]

为什么需要这一步

测试 029 引入了带回绕的零页Y变址地址辅助函数。本课将它与一次内存读取和既有的 ldx 指令相结合，让该辅助函数第一次得到操作码层面的使用。

完整示例实现

# emulator/cpu/opcodes.py
def ldx_zero_page_y(cpu: CPU):
    addr = zero_page_y(cpu)
    value = cpu.bus.read(addr)
    ldx(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xB6: ldx_zero_page_y,
}

重要不变式

$B6 映射到 ldx_zero_page_y
变址使用的是 Y，而不是 X
基地址加 Y 的结果在总线读取之前回绕到 8 位
消耗一个操作数字节，且 X 和标志的更新由 ldx 负责

常见误区

LDX 的目标寄存器并不会决定变址寄存器：这个编码使用 Y，且 $FF,Y 在 Y=$01 时读取的是 $0000 而不是 $0100。

超出范围

修改 zero_page_y 或 ldx
绝对寻址的 LDX 编码
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_033_LDX_zero_page_y.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
def ldx_zero_page_y(cpu: CPU):
    addr = zero_page_y(cpu)
    value = cpu.bus.read(addr)
    ldx(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xB6: ldx_zero_page_y,
}

查看测试代码test_033_LDX_zero_page_y.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_ldx_zero_page_y_handler_exists_and_is_in_opcode_table():
    """Objective: create ldx_zero_page_y(cpu) and add 0xB6 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldx_zero_page_y")
    assert callable(opcodes.ldx_zero_page_y)
    assert list(inspect.signature(opcodes.ldx_zero_page_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xB6] is opcodes.ldx_zero_page_y


def test_opcode_B6_ldx_zero_page_y_loads_register_x():
    """Objective: B6 10 with Y=0x03 reads RAM[$0013] into X."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xB6)
    rom.write(0x0001, 0x10)
    bus.write(0x0013, 0x42)

    cpu.reset()
    cpu.y = 0x03
    cpu.step()

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


def test_opcode_B6_ldx_zero_page_y_wraps():
    """Objective: Zero Page,Y wraps inside page $00."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xB6)
    rom.write(0x0001, 0xFF)
    bus.write(0x0000, 0x37)

    cpu.reset()
    cpu.y = 0x01
    cpu.step()

    assert cpu.x == 0x37
    assert cpu.pc == 0x8002

测试函数 (3)

test_ldx_zero_page_y_handler_exists_and_is_in_opcode_table
test_opcode_B6_ldx_zero_page_y_loads_register_x
test_opcode_B6_ldx_zero_page_y_wraps