235. Ppu 总线的 vram 读写

为 PpuBus 添加基本的读写转发到 VRAM。

第 235 / 356 · tests/chapter_03_ppu_memory_and_graphics_data/test_235_ppu_bus_vram_read_write.py

要更新的文件

emulator/bus/ppu_bus.py

要实现的方法

PpuBus.read(addr: int) -> int
PpuBus.write(addr: int, value: int) -> None

为什么需要这一步

在 PPUADDR/PPUDATA 能够写入视频内存之前,PPU 需要一个类似总线的对象,能够接受 PPU 地址并执行内存访问。

在这第一步读写实现中,先不要测试 CHR 区域 $0000-$1FFF。CHR 将单独测试,因为它涉及卡带 mapper 的行为。

目前先使用非 CHR 地址,例如 $2000:

PpuBus.write($2000, $AA)
PpuBus.read($2000) -> $AA

非 CHR/默认区域的建议实现伪代码

def read(self, addr: int) -> int:
    addr = addr & PPU_ADDRESS_MASK
    return self.vram.read(addr)

def write(self, addr: int, value: int) -> None:
    addr = addr & PPU_ADDRESS_MASK
    self.vram.write(addr, value)

之后,这些方法会为以下部分增加代码

CHR 区域: $0000-$1FFF
名称表区域: $2000-$3EFF
调色板区域: $3F00-$3FFF

运行本课

uv run pytest tests/chapter_03_ppu_memory_and_graphics_data/test_235_ppu_bus_vram_read_write.py -v

示例实现

非 CHR/默认区域的建议实现伪代码

def read(self, addr: int) -> int:
    addr = addr & PPU_ADDRESS_MASK
    return self.vram.read(addr)

def write(self, addr: int, value: int) -> None:
    addr = addr & PPU_ADDRESS_MASK
    self.vram.write(addr, value)

查看测试代码test_235_ppu_bus_vram_read_write.py

from emulator.bus.ppu_bus import PpuBus


def test_ppu_bus_has_read_and_write_methods():
    """
    Objective:
    PpuBus exposes the stable read/write API that PPU will use later.
    """
    assert hasattr(PpuBus, "read")
    assert callable(PpuBus.read)
    assert hasattr(PpuBus, "write")
    assert callable(PpuBus.write)


def test_ppu_bus_reads_value_from_vram_for_non_chr_address():
    """
    Objective:
    PpuBus.read should return the value stored in VRAM for non-CHR addresses.
    """
    bus = PpuBus()
    bus.vram.write(0x2000, 0xAA)

    assert bus.read(0x2000) == 0xAA


def test_ppu_bus_writes_value_to_vram_for_non_chr_address():
    """
    Objective:
    PpuBus.write should forward non-CHR writes to VRAM.
    """
    bus = PpuBus()

    bus.write(0x2000, 0xBB)

    assert bus.vram.read(0x2000) == 0xBB


def test_ppu_bus_masks_addresses_before_accessing_vram():
    """
    Objective:
    PpuBus owns PPU address normalization.

    Example:
    $6000 masked with $3FFF becomes $2000.
    """
    bus = PpuBus()

    bus.write(0x6000, 0xCC)

    assert bus.read(0x2000) == 0xCC

测试函数 (4)

test_ppu_bus_has_read_and_write_methods
test_ppu_bus_reads_value_from_vram_for_non_chr_address
test_ppu_bus_writes_value_to_vram_for_non_chr_address
test_ppu_bus_masks_addresses_before_accessing_vram