237. Enrutamiento del área chr en el bus de la ppu

Enruta las lecturas del área CHR de PpuBus a través del mapper.

Lección 237 de 356 · tests/chapter_03_ppu_memory_and_graphics_data/test_237_ppu_bus_chr_area_routing.py

Archivo a actualizar

emulator/bus/ppu_bus.py

Constantes a añadir/usar

CHR_START = 0x0000
CHR_END = 0x1FFF

¿Qué es el área CHR? El rango de direcciones de la PPU $0000-$1FFF es el área de la pattern table. Contiene los bytes de gráficos de tiles usados más adelante para los píxeles de fondo y de sprites.

Dato importante de la NES

Esta área a menudo pertenece al cartucho, no a la VRAM interna.

CHR ROM cartridge:
    PPU reads pattern bytes from cartridge CHR ROM.

CHR RAM cartridge:
    PPU can write pattern bytes into cartridge CHR RAM.

Alcance actual

Por ahora, PpuBus debe detectar las direcciones del área CHR. Las lecturas deben usar mapper.read_chr(addr) cuando exista un mapper.

Deliberadamente no fijamos el comportamiento de escritura de CHR en esta prueba. Un paso futuro puede añadir mapper.write_chr(addr, value) para soporte de CHR RAM, y esta prueba no debería bloquear esa evolución.

Pseudocódigo de lectura sugerido

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

    if CHR_START <= addr <= CHR_END:
        if self.mapper is not None:
            return self.mapper.read_chr(addr)
        return self.vram.read(addr)

    return self.vram.read(addr)

Posible pseudocódigo de escritura actual

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

    if CHR_START <= addr <= CHR_END:
        if self.mapper is not None:
            raise ValueError("CHR writes are not supported yet")
        self.vram.write(addr, value)
        return

    self.vram.write(addr, value)

Posible pseudocódigo de escritura futuro

if CHR_START <= addr <= CHR_END and self.mapper is not None:
    self.mapper.write_chr(addr, value)
    return

Regiones futuras

$2000-$3EFF:
    currently backed by big VRAM, later nametable VRAM/mirroring

$3F00-$3FFF:
    currently backed by big VRAM, later palette RAM/mirroring

Ejecutar esta lección

uv run pytest tests/chapter_03_ppu_memory_and_graphics_data/test_237_ppu_bus_chr_area_routing.py -v

def read(self, addr: int) -> int: addr = addr & PPU_ADDRESS_MASK if CHR_START <= addr <= CHR_END: if self.mapper is not None: return self.mapper.read_chr(addr) return self.vram.read(addr) return self.vram.read(addr)

def write(self, addr: int, value: int) -> None: addr = addr & PPU_ADDRESS_MASK if CHR_START <= addr <= CHR_END: if self.mapper is not None: raise ValueError("CHR writes are not supported yet") self.vram.write(addr, value) return self.vram.write(addr, value)

from emulator.bus.ppu_bus import CHR_END, CHR_START, PpuBus class FakeMapper: """Small mapper test double for CHR read routing.""" def read_prg(self, addr: int) -> int: return 0xEA def read_chr(self, addr: int) -> int: if addr == 0x0000: return 0x11 if addr == 0x1FFF: return 0x22 return 0x33 def test_ppu_bus_declares_chr_area_constants(): """ Objective: Name the PPU pattern-table / CHR address range. """ assert CHR_START == 0x0000 assert CHR_END == 0x1FFF def test_ppu_bus_reads_chr_area_from_mapper_when_mapper_exists(): """ Objective: PPU $0000-$1FFF reads should go through mapper.read_chr when a mapper is attached. """ bus = PpuBus(mapper=FakeMapper()) assert bus.read(0x0000) == 0x11 assert bus.read(0x1FFF) == 0x22 def test_ppu_bus_falls_back_to_vram_for_chr_area_when_no_mapper_exists(): """ Objective: Without a mapper, the temporary big VRAM backing store can still serve CHR area accesses. Why: This keeps early PPU memory tests simple before cartridge CHR routing is connected everywhere. """ bus = PpuBus() bus.vram.write(0x0000, 0x44) assert bus.read(0x0000) == 0x44