265. La consola consume la solicitud NMI de la PPU

Modelar Console como el coordinador que consume las solicitudes NMI de la PPU.

Lección 265 de 356 · tests/chapter_04_ppu_timing_and_vblank/test_265_console_consumes_ppu_nmi_request.py

Referencias

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

Archivo a crear

emulator/console.py

Por qué existe este paso

El emulador ahora tiene los dos mecanismos separados necesarios para el NMI de VBlank

PPU mechanism:
    PPU enters VBlank and sets ppu.nmi_requested = True

CPU mechanism:
    CPU.interrupt_nmi() pushes PC/status and jumps through $FFFA/$FFFB

Ahora necesitamos un pequeño coordinador que conecte esos mecanismos sin hacer que la CPU y la PPU dependan directamente entre sí.

¿Qué es Console? Console es la futura máquina NES emulada de nivel superior. Con el tiempo puede poseer y coordinar subsistemas como:

CPU
PPU
cartridge/mapper
controllers
APU/audio
frame stepping

En esta etapa, Console comienza siendo muy pequeño. Su primer trabajo es solo consumir una solicitud NMI de la PPU y llamar a CPU.interrupt_nmi().

Separación correcta de responsabilidades

PPU:
    produces nmi_requested

CPU:
    implements interrupt_nmi()

Console:
    connects the PPU signal to the CPU mechanism

Regla de arquitectura importante

No pongas la propiedad de la PPU dentro de la CPU. La CPU no debería preguntar a la PPU si hay una NMI pendiente. Eso crearía un acoplamiento oculto entre la ejecución de la CPU y el hardware de vídeo.

Ejemplo de implementación mínimo

from dataclasses import dataclass

from emulator.cpu.cpu import CPU
from emulator.ppu.ppu import PPU


@dataclass
class Console:
    cpu: CPU
    ppu: PPU

    def consume_nmi_if_requested(self) -> None:
        if not self.ppu.nmi_requested:
            return

        self.ppu.nmi_requested = False
        self.cpu.interrupt_nmi()

Forma futura, introducida en un paso de temporización posterior

class Console:
    ...

    def step(self) -> None:
        cpu_cycles = self.cpu.step()
        self.ppu.step(cpu_cycles * 3)
        self.consume_nmi_if_requested()

Este archivo no prueba Console.step(). La integración de temporización CPU/PPU se prueba como un paso separado después de que CPU.step() devuelva ciclos de instrucción.

Concepto erróneo común

Dado que la PPU causa NMI, puede parecer natural que la CPU posea la PPU. Evita eso. La CPU recibe señales de interrupción; no debería poseer el dispositivo de vídeo.

Fuera de alcance

Ratio de ciclos CPU/PPU
Console.step()
renderizado
controladores
APU/audio
latencia exacta de NMI

Ejecutar esta lección

uv run pytest tests/chapter_04_ppu_timing_and_vblank/test_265_console_consumes_ppu_nmi_request.py -v

Implementación de ejemplo

Ejemplo de implementación mínimo

from dataclasses import dataclass

from emulator.cpu.cpu import CPU
from emulator.ppu.ppu import PPU


@dataclass
class Console:
    cpu: CPU
    ppu: PPU

    def consume_nmi_if_requested(self) -> None:
        if not self.ppu.nmi_requested:
            return

        self.ppu.nmi_requested = False
        self.cpu.interrupt_nmi()

Forma futura, introducida en un paso de temporización posterior

class Console:
    ...

    def step(self) -> None:
        cpu_cycles = self.cpu.step()
        self.ppu.step(cpu_cycles * 3)
        self.consume_nmi_if_requested()

Ver código del testtest_265_console_consumes_ppu_nmi_request.py

from dataclasses import is_dataclass

from emulator.console import Console
from emulator.cpu.cpu import CPU
from emulator.ppu.ppu import PPU
from tests.helpers import make_cpu_with_rom


def write_nmi_vector(rom, addr: int) -> None:
    """
    Install the NMI vector in FakeROM for CPU.interrupt_nmi().

    CPU address $FFFA maps to FakeROM offset $7FFA because CpuBus maps
    $8000-$FFFF to program_rom offsets $0000-$7FFF.
    """
    rom.write(0x7FFA, addr & 0xFF)
    rom.write(0x7FFB, (addr >> 8) & 0xFF)


def make_console_with_fake_rom():
    """
    Build a coherent small console for NMI tests.

    Important:
    Use the PPU already owned by cpu.bus. That keeps this test close to the real
    emulator shape where CPU register accesses and Console coordination should be
    observing the same PPU instance.
    """
    cpu, bus, rom = make_cpu_with_rom()
    ppu = bus.ppu
    return Console(cpu=cpu, ppu=ppu), cpu, ppu, rom


def test_console_is_small_machine_coordinator_for_cpu_and_ppu():
    """
    Objective:
    Console starts as a small coordinator that stores CPU and PPU references.
    """
    assert is_dataclass(Console)

    console, cpu, ppu, _rom = make_console_with_fake_rom()

    assert console.cpu is cpu
    assert console.ppu is ppu
    assert isinstance(console.cpu, CPU)
    assert isinstance(console.ppu, PPU)


def test_console_exposes_nmi_consumption_method():
    """
    Objective:
    Console exposes the boundary method that connects PPU NMI requests to CPU NMI
    handling.
    """
    assert hasattr(Console, "consume_nmi_if_requested")
    assert callable(Console.consume_nmi_if_requested)


def test_console_does_nothing_when_ppu_has_no_nmi_request():
    """
    Objective:
    If ppu.nmi_requested is False, Console must not interrupt the CPU.
    """
    console, cpu, ppu, rom = make_console_with_fake_rom()
    write_nmi_vector(rom, 0xC000)
    cpu.pc = 0x8123
    cpu.s = 0xFD
    ppu.nmi_requested = False

    console.consume_nmi_if_requested()

    assert cpu.pc == 0x8123
    assert cpu.s == 0xFD
    assert ppu.nmi_requested is False


def test_console_consumes_ppu_nmi_request_and_interrupts_cpu():
    """
    Objective:
    When ppu.nmi_requested is True, Console calls CPU.interrupt_nmi() and clears
    the request.

    Example:
        PPU request: nmi_requested = True
        NMI vector:  $FFFA/$FFFB -> $C000
        CPU before:  PC = $8123
        CPU after:   PC = $C000
        PPU after:   nmi_requested = False
    """
    console, cpu, ppu, rom = make_console_with_fake_rom()
    write_nmi_vector(rom, 0xC000)
    cpu.pc = 0x8123
    cpu.s = 0xFD
    ppu.nmi_requested = True

    console.consume_nmi_if_requested()

    assert cpu.pc == 0xC000
    assert cpu.s == 0xFA
    assert ppu.nmi_requested is False


def test_console_consumes_each_ppu_nmi_request_only_once():
    """
    Objective:
    A single PPU NMI request should produce at most one CPU NMI.

    Why this matters:
    If Console forgot to clear ppu.nmi_requested, every coordinator check would
    interrupt the CPU again from the same stale request.
    """
    console, cpu, ppu, rom = make_console_with_fake_rom()
    write_nmi_vector(rom, 0xC000)
    cpu.pc = 0x8123
    cpu.s = 0xFD
    ppu.nmi_requested = True

    console.consume_nmi_if_requested()
    pc_after_first_consume = cpu.pc
    stack_after_first_consume = cpu.s

    console.consume_nmi_if_requested()

    assert cpu.pc == pc_after_first_consume
    assert cpu.s == stack_after_first_consume
    assert ppu.nmi_requested is False


def test_console_keeps_cpu_and_ppu_coupling_out_of_subsystems():
    """
    Objective:
    Document the desired dependency direction.

    Console may import CPU and PPU because it coordinates the machine. CPU should
    not need to import PPU just to handle NMI, and PPU should not call CPU directly.

    This test checks behavior through the public coordinator instead of adding a
    CPU -> PPU dependency.
    """
    console, cpu, ppu, rom = make_console_with_fake_rom()
    write_nmi_vector(rom, 0x9000)
    cpu.pc = 0x8000
    cpu.s = 0xFD

    ppu.nmi_requested = True
    console.consume_nmi_if_requested()

    assert cpu.pc == 0x9000
    assert ppu.nmi_requested is False

Funciones de test (6)

test_console_is_small_machine_coordinator_for_cpu_and_ppu
test_console_exposes_nmi_consumption_method
test_console_does_nothing_when_ppu_has_no_nmi_request
test_console_consumes_ppu_nmi_request_and_interrupts_cpu
test_console_consumes_each_ppu_nmi_request_only_once
test_console_keeps_cpu_and_ppu_coupling_out_of_subsystems