006. Fakerom

为 CPU 测试创建可写的 FakeROM 存储。

第 6 / 356 · tests/chapter_01_cpu/test_006_FakeROM.py

需要创建的文件

emulator/memory/fake_rom.py

位置

class FakeROM(MemoryDevice)

此步骤存在的原因

在真正的卡带解析功能就绪之前，指令测试需要确定的程序字节。FakeROM 将最大 $8000-$FFFF 的 PRG 窗口建模为 0x8000 个本地字节，并允许测试直接填充其内容。

完整示例实现

from dataclasses import dataclass, field

from emulator.memory.memory_device import MemoryDevice


@dataclass
class FakeROM(MemoryDevice):
    _data: bytearray = field(
        default_factory=lambda: bytearray(0x8000),
        init=False,
    )

    def read(self, addr: int) -> int:
        return self._data[addr]

    def write(self, addr: int, value: int) -> None:
        self._data[addr] = value

重要不变量

FakeROM 恰好存储 0x8000 字节
其地址是本地偏移 $0000-$7FFF
写入操作仅用于安排测试程序和数据

常见误解

FakeROM 不是真正的卡带映射器，它的写入方法并不意味着 NES 的 PRG ROM 是可写的。它只是受控的测试基础设施。

范围之外

将 FakeROM 映射到 CPU 地址
iNES 解析
存储体切换(bank switching)

运行本课

uv run pytest tests/chapter_01_cpu/test_006_FakeROM.py -v

示例实现

完整示例实现

from dataclasses import dataclass, field

from emulator.memory.memory_device import MemoryDevice


@dataclass
class FakeROM(MemoryDevice):
    _data: bytearray = field(
        default_factory=lambda: bytearray(0x8000),
        init=False,
    )

    def read(self, addr: int) -> int:
        return self._data[addr]

    def write(self, addr: int, value: int) -> None:
        self._data[addr] = value

查看测试代码test_006_FakeROM.py

from pathlib import Path
from emulator.memory.memory_device import MemoryDevice

def test_program_rom_class_exists():
    """
    We are going to create a Fake ROM for testing purposes
    It does not have any logic about how real cartridges work
    """
    assert Path("emulator/memory/fake_rom.py").exists()

def test_fake_rom_uses_memory_device_abstract_class():
    """
    Create a new FakeROM class inside fake_rom.py
    Ensure that it uses our abstract class MemoryDevice
    The methods read/write can be void on this test, just declare it
    """

    from emulator.memory.fake_rom import FakeROM

    assert FakeROM is not None

    rom = FakeROM()
    assert isinstance(rom, MemoryDevice)

def test_fake_rom_has_0x8000_bytes():
    """
    The CPU address range $8000-$FFFF is commonly used for PRG ROM.

    Look at: https://en.wikibooks.org/wiki/NES_Programming/Memory_Map
    NES has 2 Possible ROM Addresses 0x8000 and 0xC000

    Each one of 0x4000 Size, then the max size is 0x8000 [0x0000-0x7FFF]
    Lets implement a _data = bytearray(0x8000) inside our Fake ROM

    """

    from emulator.memory.fake_rom import FakeROM
    rom = FakeROM()
    assert hasattr(rom, "_data")
    assert isinstance(rom._data, bytearray)
    assert len(rom._data) == 0x8000

def test_write_read_fake_rom():
    """
    Remember: a real ROM will never implement Write methods,
    Our Fake ROM is for testing purposes and that's why we implement write
    """
    from emulator.memory.fake_rom import FakeROM
    fake_rom = FakeROM()

    for addr in range(0x0, 0x8000):
        test_value = addr & 0xFF

        fake_rom.write(addr, test_value)

        assert fake_rom.read(addr) == test_value

测试函数 (4)

test_program_rom_class_exists
test_fake_rom_uses_memory_device_abstract_class
test_fake_rom_has_0x8000_bytes
test_write_read_fake_rom