029. 零页Y寻址

添加零页,Y寻址方式。

第 29 / 356 · tests/chapter_01_cpu/test_029_zero_page_y_addressing.py

需要更新的文件

emulator/cpu/addressing_modes.py

位置

addressing_modes.zero_page_y

为什么需要这一步

后面的一些指令编码是用Y而不是X来索引零页操作数的。本课只引入这一可复用的地址计算，与测试017中的 zero_page_x 相对应，尚未与任何操作码关联。

完整示例实现

# emulator/cpu/addressing_modes.py
def zero_page_y(cpu) -> int:
    base = cpu.fetch_byte()
    address = (base + cpu.y) & 0xFF
    return address

重要不变量

只读取一个操作数字节
加到操作数上的是Y，而不是X
结果会回绕为八位，因而始终留在 $00 页内
该辅助函数只返回地址，不对该地址执行任何内存读取

常见误解

要对相加结果做掩码，而不仅仅是对操作数做掩码：$FF + $01 必须变为 $0000，而不是 $0100。

不在本课范围内

新增或修改操作码处理函数
LDX指令本身
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_029_zero_page_y_addressing.py -v

示例实现

完整示例实现

# emulator/cpu/addressing_modes.py
def zero_page_y(cpu) -> int:
    base = cpu.fetch_byte()
    address = (base + cpu.y) & 0xFF
    return address

查看测试代码test_029_zero_page_y_addressing.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import addressing_modes
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_zero_page_y_addressing_mode_exists():
    """
    Objective:
    Create in addressing_modes.py:
        def zero_page_y(cpu):
            ...

    Example implementation:
        base = cpu.fetch_byte()
        return (base + cpu.y) & 0xFF
    """
    assert hasattr(addressing_modes, "zero_page_y")
    assert callable(addressing_modes.zero_page_y)
    assert list(inspect.signature(addressing_modes.zero_page_y).parameters) == ["cpu"]


def test_zero_page_y_adds_y_to_base_address():
    """
    Objective:
    If the operand is 0x10 and Y is 0x03,
    zero_page_y(cpu) returns 0x0013.
    """
    cpu, _, rom = make_cpu_with_rom()

    cpu.reset()
    cpu.y = 0x03
    rom.write(0x0000, 0x10)

    addr = addressing_modes.zero_page_y(cpu)

    assert addr == 0x0013
    assert cpu.pc == 0x8001


def test_zero_page_y_wraps_inside_page_zero():
    """
    Objective:
    Zero Page,Y wraps inside page $00.

    Example:
    0xFF + Y where Y is 0x01 becomes 0x00, not 0x0100.
    """
    cpu, _, rom = make_cpu_with_rom()

    cpu.reset()
    cpu.y = 0x01
    rom.write(0x0000, 0xFF)

    addr = addressing_modes.zero_page_y(cpu)

    assert addr == 0x0000
    assert cpu.pc == 0x8001

测试函数 (3)

test_zero_page_y_addressing_mode_exists
test_zero_page_y_adds_y_to_base_address
test_zero_page_y_wraps_inside_page_zero