242. PPUDATA escribe a través del bus de la PPU

Implementa las escrituras de PPUDATA ($2007) a través de PpuBus.

Lección 242 de 356 · tests/chapter_03_ppu_memory_and_graphics_data/test_242_ppudata_writes_through_ppu_bus.py

Referencia

https://www.nesdev.org/wiki/PPU_registers#PPUDATA

Archivo a actualizar

emulator/ppu/ppu.py

Método a actualizar

PPU.write_register(addr, value)

Por qué existe este paso

PPUDATA es el registro visible para la CPU en $2007. La CPU escribe en $2007, pero la dirección real de memoria de la PPU en la que se escribe no es la dirección de CPU $2007. El destino real es la dirección de VRAM interna de la PPU:

vram_addr

Esa dirección la establece PPUADDR ($2006) y después PPUDATA ($2007) escribe a través del

bus del lado de la PPU

PPU.write_register($2007, value)
    -> ppu_bus.write(vram_addr, value)
    -> increment vram_addr

Decisiones importantes de compatibilidad futura en este test

- Use non-CHR addresses such as $2000, not $0000-$1FFF.
  CHR writes will later involve mapper.write_chr / CHR RAM behavior.

- Test increment-by-1 only when PPUCTRL increment mode is clear.
  Later, PPUCTRL bit 2 may select increment-by-32. These tests should not
  block that future behavior.

Pseudocódigo de implementación sugerido para la etapa actual

case 0x2007:
    self.data = value
    self.ppu_bus.write(self.vram_addr, value)
    self.vram_addr = (self.vram_addr + 1) & 0x3FFF

Nota sobre la implementación futura

Más adelante, el incremento puede pasar a ser

increment = 32 if self.ctrl & VRAM_INCREMENT_BY_32 else 1
self.vram_addr = (self.vram_addr + increment) & 0x3FFF

Fuera del alcance

lecturas de PPUDATA
almacenamiento en búfer de lecturas de PPUDATA
excepciones de lectura de paleta
escrituras en CHR RAM
comportamiento de incremento de 32 de PPUCTRL

Ejecutar esta lección

uv run pytest tests/chapter_03_ppu_memory_and_graphics_data/test_242_ppudata_writes_through_ppu_bus.py -v

Implementación de ejemplo

Pseudocódigo de implementación sugerido para la etapa actual

case 0x2007:
    self.data = value
    self.ppu_bus.write(self.vram_addr, value)
    self.vram_addr = (self.vram_addr + 1) & 0x3FFF

Ver código del testtest_242_ppudata_writes_through_ppu_bus.py

from emulator.ppu.ppu import PPU


def set_ppuaddr(ppu: PPU, addr: int) -> None:
    """Set PPU.vram_addr through the public $2006 two-write interface."""
    ppu.write_register(0x2006, (addr >> 8) & 0xFF)
    ppu.write_register(0x2006, addr & 0xFF)


def test_ppudata_write_stores_value_through_ppu_bus_at_current_vram_addr():
    """
    Objective:
    Writing $2007 should write to PpuBus at the current internal PPU address.

    We use $2000 because it is outside the CHR area $0000-$1FFF, avoiding future
    mapper/CHR RAM write behavior.
    """
    ppu = PPU()
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0xAA)

    assert ppu.ppu_bus.read(0x2000) == 0xAA


def test_ppudata_write_preserves_data_as_last_written_value():
    """
    Objective:
    Keep the earlier simple `data` field useful as the last byte written to
    PPUDATA.

    Important:
    The real memory write goes through ppu_bus. The `data` field is only a simple
    observable last-written value at this tutorial stage.
    """
    ppu = PPU()
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0xBB)

    assert ppu.data == 0xBB


def test_ppudata_write_increments_vram_addr_by_one_when_ppuctrl_increment_mode_is_clear():
    """
    Objective:
    With the current/default PPUCTRL increment mode clear, writing $2007 advances
    vram_addr by 1.

    Future compatibility:
    If PPUCTRL bit 2 later selects increment-by-32, this test remains valid
    because PPUCTRL is left at its default value 0.
    """
    ppu = PPU()
    assert ppu.ctrl == 0
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0xCC)

    assert ppu.vram_addr == 0x2001


def test_ppudata_write_masks_value_through_ppu_bus_storage():
    """
    Objective:
    PPUDATA writes are byte writes.

    The value is already masked by PPU.write_register, and VRAM also stores only
    bytes. This test verifies the visible behavior without depending on where the
    masking happens internally.
    """
    ppu = PPU()
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0x123)

    assert ppu.ppu_bus.read(0x2000) == 0x23
    assert ppu.data == 0x23

Funciones de test (4)

test_ppudata_write_stores_value_through_ppu_bus_at_current_vram_addr
test_ppudata_write_preserves_data_as_last_written_value
test_ppudata_write_increments_vram_addr_by_one_when_ppuctrl_increment_mode_is_clear
test_ppudata_write_masks_value_through_ppu_bus_storage