324. iNES 垂直镜像标志

从 iNES flags 6 第 0 位解码水平/垂直名称表镜像方式。

第 324 / 356 · tests/chapter_12_mirroring/test_324_ines_vertical_mirroring_flag.py

需要更新的文件

emulator/cartridge/ines.py

参考资料

https://www.nesdev.org/wiki/Mirroring#Nametable_Mirroring

该步骤存在的原因

在实现卷动视口之前,模拟器必须知道卡带如何将逻辑 PPU 名称表接线到物理名称表 RAM 上。

镜像不会在视觉上翻转图像,它控制的是四个逻辑名称表区域之间的内存映射:

$2000
$2400
$2800
$2C00

iNES flags 6 第 0 位选择基本的镜像模式:

bit 0 clear -> horizontal mirroring
bit 0 set   -> vertical mirroring

在当前的增量范围内,将其暴露为一个布尔属性

is_vertical_mirroring is False -> horizontal
is_vertical_mirroring is True  -> vertical

建议的实现示例

FLAGS6_VERTICAL_MIRRORING = 1 << 0


@dataclass(frozen=True)
class INesHeader:
    prg_rom_banks: int
    chr_rom_banks: int
    mapper_number: int
    has_trainer: bool
    flags_6: int
    flags_7: int

    @property
    def is_vertical_mirroring(self) -> bool:
        return (self.flags_6 & FLAGS6_VERTICAL_MIRRORING) != 0

为什么使用计算属性?flags_6 仍是唯一的信息来源,现有的 INesHeader 构造函数无需改动。这样可以保持旧版教程调用方和测试的兼容性。

未来的扩展

以后可以为四屏镜像新增一个 is_four_screen 属性,在检查 is_vertical_mirroring 之前先检查它。四屏存储和路由在此处未实现。

不在本步骤范围内

修改 Cartridge
修改 Mapper000
修改 PpuBus 的名称表映射
四屏名称表 RAM
卷动
商业 ROM 测试夹具

运行本课

uv run pytest tests/chapter_12_mirroring/test_324_ines_vertical_mirroring_flag.py -v

示例实现

建议的实现示例

FLAGS6_VERTICAL_MIRRORING = 1 << 0


@dataclass(frozen=True)
class INesHeader:
    prg_rom_banks: int
    chr_rom_banks: int
    mapper_number: int
    has_trainer: bool
    flags_6: int
    flags_7: int

    @property
    def is_vertical_mirroring(self) -> bool:
        return (self.flags_6 & FLAGS6_VERTICAL_MIRRORING) != 0

查看测试代码test_324_ines_vertical_mirroring_flag.py

from emulator.cartridge.ines import (
    FLAGS6_VERTICAL_MIRRORING,
    INES_MAGIC,
    INesHeader,
    parse_ines_header,
)


def make_header_bytes(flags_6: int = 0, flags_7: int = 0) -> bytes:
    """Build a minimal 16-byte iNES header with configurable flag bytes."""
    return bytes([
        *INES_MAGIC,
        1,
        1,
        flags_6,
        flags_7,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
    ])


def test_flags6_vertical_mirroring_constant_names_bit_zero():
    """
    Objective:
    Give iNES flags 6 bit 0 a domain-specific name instead of repeating a magic
    binary literal.
    """
    assert FLAGS6_VERTICAL_MIRRORING == 1 << 0


def test_ines_header_exposes_vertical_mirroring_property():
    """
    Objective:
    Mirroring metadata should be available directly from an INesHeader instance.
    """
    header = INesHeader(
        prg_rom_banks=1,
        chr_rom_banks=1,
        mapper_number=0,
        has_trainer=False,
        flags_6=0,
        flags_7=0,
    )

    assert hasattr(header, "is_vertical_mirroring")
    assert isinstance(header.is_vertical_mirroring, bool)


def test_flags6_bit_zero_clear_means_horizontal_mirroring():
    """
    Objective:
    False means horizontal mirroring; it does not mean mirroring is disabled.
    """
    header = parse_ines_header(make_header_bytes(flags_6=0b0000_0000))

    assert header.is_vertical_mirroring is False


def test_flags6_bit_zero_set_means_vertical_mirroring():
    """
    Objective:
    Setting flags 6 bit 0 requests vertical nametable mirroring.
    """
    header = parse_ines_header(
        make_header_bytes(flags_6=FLAGS6_VERTICAL_MIRRORING)
    )

    assert header.is_vertical_mirroring is True


def test_unrelated_flags6_bits_do_not_select_vertical_mirroring():
    """
    Objective:
    Trainer and mapper bits must not be confused with the mirroring bit.
    """
    unrelated_bits = 0b1010_0100
    assert (unrelated_bits & FLAGS6_VERTICAL_MIRRORING) == 0

    header = parse_ines_header(make_header_bytes(flags_6=unrelated_bits))

    assert header.is_vertical_mirroring is False


def test_vertical_mirroring_bit_can_coexist_with_other_flags():
    """
    Objective:
    Mirroring decoding should inspect bit 0 without discarding trainer or mapper
    information from the same flags byte.
    """
    flags_6 = 0b0010_0101

    header = parse_ines_header(make_header_bytes(flags_6=flags_6))

    assert header.is_vertical_mirroring is True
    assert header.has_trainer is True
    assert header.mapper_number == 2


def test_mirroring_property_does_not_change_ines_header_constructor_shape():
    """
    Objective:
    The property is computed from existing flags_6 metadata, so historical direct
    INesHeader construction remains valid.
    """
    header = INesHeader(1, 1, 0, False, 0, 0)

    assert header.prg_rom_banks == 1
    assert header.chr_rom_banks == 1
    assert header.is_vertical_mirroring is False

测试函数 (7)

test_flags6_vertical_mirroring_constant_names_bit_zero
test_ines_header_exposes_vertical_mirroring_property
test_flags6_bit_zero_clear_means_horizontal_mirroring
test_flags6_bit_zero_set_means_vertical_mirroring
test_unrelated_flags6_bits_do_not_select_vertical_mirroring
test_vertical_mirroring_bit_can_coexist_with_other_flags
test_mirroring_property_does_not_change_ines_header_constructor_shape