261. Solicitud NMI de la PPU en VBlank

Implementar la solicitud de NMI del lado de la PPU en VBlank.

Lección 261 de 356 · tests/chapter_04_ppu_timing_and_vblank/test_261_ppu_nmi_request_on_vblank.py

Referencia

https://www.nesdev.org/wiki/PPU_registers#PPUCTRL
https://www.nesdev.org/wiki/PPU_registers#Vblank_NMI

Archivo a actualizar

emulator/ppu/ppu.py

Estado a añadir

nmi_requested: bool = False

Por qué existe este paso

Muchos juegos de NES no consultan PPUSTATUS en su bucle principal. En su lugar, activan NMI en el bit 7 de PPUCTRL y esperan que la PPU solicite una interrupción cuando comience el VBlank.

En este paso, implementa solo la señal del lado de la PPU

if VBlank starts and PPUCTRL bit 7 is set:
    ppu.nmi_requested = True

La CPU aún no consume la NMI. Eso es un paso de integración posterior.

Ejemplo de implementación sugerido

nmi_requested: bool = False
...
...

if self.scanline == PPU_VBLANK_START_SCANLINE:
    self.status |= VBLANK_STARTED

    if self.ctrl & CTRL_NMI_ENABLE:
        self.nmi_requested = True

Concepto importante

La solicitud de NMI es un evento producido cuando comienza el VBlank. No debe producirse continuamente en cada ciclo del VBlank.

Fuera de alcance

Manejo de interrupciones de la CPU
lectura del vector NMI
envío de PC/estado a la pila de la CPU
borrado de nmi_requested por integración CPU/sistema
activación de NMI durante un VBlank ya activo
temporización de NMI precisa a nivel de punto/ciclo

Ejecutar esta lección

uv run pytest tests/chapter_04_ppu_timing_and_vblank/test_261_ppu_nmi_request_on_vblank.py -v

Implementación de ejemplo

Ejemplo de implementación sugerido

nmi_requested: bool = False
...
...

if self.scanline == PPU_VBLANK_START_SCANLINE:
    self.status |= VBLANK_STARTED

    if self.ctrl & CTRL_NMI_ENABLE:
        self.nmi_requested = True

Ver código del testtest_261_ppu_nmi_request_on_vblank.py

from emulator.ppu.ppu import (
    CTRL_NMI_ENABLE,
    PPU,
    PPU_CYCLES_PER_SCANLINE,
    PPU_PRE_RENDER_SCANLINE,
    PPU_VBLANK_START_SCANLINE,
    VBLANK_STARTED,
)


def cycles_to_enter_scanline(scanline: int) -> int:
    """Return the number of PPU cycles needed to enter a scanline from reset."""
    return PPU_CYCLES_PER_SCANLINE * scanline


def test_ppu_nmi_requested_starts_false():
    """
    Objective:
    A new PPU should not start with a pending NMI request.
    """
    ppu = PPU()

    assert ppu.nmi_requested is False


def test_vblank_start_does_not_request_nmi_when_ppuctrl_nmi_bit_is_clear():
    """
    Objective:
    VBlank alone is not enough to request NMI. PPUCTRL bit 7 must be enabled.
    """
    ppu = PPU()
    assert (ppu.ctrl & CTRL_NMI_ENABLE) == 0

    ppu.step(cycles_to_enter_scanline(PPU_VBLANK_START_SCANLINE))

    assert (ppu.status & VBLANK_STARTED) != 0
    assert ppu.nmi_requested is False


def test_vblank_start_requests_nmi_when_ppuctrl_nmi_bit_is_set():
    """
    Objective:
    If PPUCTRL bit 7 is set, entering VBlank should request NMI.
    """
    ppu = PPU()
    ppu.write_register(0x2000, CTRL_NMI_ENABLE)

    ppu.step(cycles_to_enter_scanline(PPU_VBLANK_START_SCANLINE))

    assert (ppu.status & VBLANK_STARTED) != 0
    assert ppu.nmi_requested is True


def test_nmi_request_is_not_recreated_after_ppustatus_read_on_same_vblank_scanline():
    """
    Objective:
    NMI request is tied to the VBlank-start event. It should not be recreated just
    because the PPU remains on the VBlank start scanline.

    This test manually clears nmi_requested to simulate future CPU/system
    consumption, then steps a few cycles on the same scanline.
    """
    ppu = PPU()
    ppu.write_register(0x2000, CTRL_NMI_ENABLE)

    ppu.step(cycles_to_enter_scanline(PPU_VBLANK_START_SCANLINE))
    assert ppu.nmi_requested is True

    ppu.nmi_requested = False
    ppu.read_register(0x2002)
    ppu.step(10)

    assert ppu.scanline == PPU_VBLANK_START_SCANLINE
    assert ppu.nmi_requested is False


def test_pre_render_vblank_clear_does_not_implicitly_clear_pending_nmi_request():
    """
    Objective:
    For now, nmi_requested is a pending signal for CPU/system integration to
    consume. The PPU should not silently discard it when pre-render clears VBlank.

    A later CPU/system step can define how the request is consumed and cleared.
    """
    ppu = PPU()
    ppu.write_register(0x2000, CTRL_NMI_ENABLE)

    ppu.step(cycles_to_enter_scanline(PPU_VBLANK_START_SCANLINE))
    assert ppu.nmi_requested is True

    ppu.step(PPU_CYCLES_PER_SCANLINE * (PPU_PRE_RENDER_SCANLINE - PPU_VBLANK_START_SCANLINE))

    assert ppu.scanline == PPU_PRE_RENDER_SCANLINE
    assert (ppu.status & VBLANK_STARTED) == 0
    assert ppu.nmi_requested is True

Funciones de test (5)

test_ppu_nmi_requested_starts_false
test_vblank_start_does_not_request_nmi_when_ppuctrl_nmi_bit_is_clear
test_vblank_start_requests_nmi_when_ppuctrl_nmi_bit_is_set
test_nmi_request_is_not_recreated_after_ppustatus_read_on_same_vblank_scanline
test_pre_render_vblank_clear_does_not_implicitly_clear_pending_nmi_request