268. Console.step() продвигает PPU

Заставить Console.step() продвигать время PPU на основе тактов CPU.

Урок 268 из 356 · tests/chapter_04_ppu_timing_and_vblank/test_268_console_step_advances_ppu.py

Справочные материалы

https://www.nesdev.org/wiki/CPU
https://www.nesdev.org/wiki/PPU_rendering#Line-by-line_timing

Файл для обновления

emulator/console.py

Зачем нужен этот шаг

В эмуляторе теперь есть части, необходимые для связи времени CPU со временем PPU

CPU.step() returns base CPU cycles
PPU.step(cycles) advances PPU timing counters
Console.consume_nmi_if_requested() connects PPU NMI requests to CPU NMI

Console.step() — это то место, где эти части становятся одним шагом на уровне машины.

Что такое шаг на уровне машины? Шаг на уровне машины означает совместное продвижение нескольких эмулируемых микросхем согласно их тактовой взаимосвязи.

Минимальное правило синхронизации NES

1 CPU cycle = 3 PPU cycles

Минимальный пример

CPU executes NOP
NOP takes 2 CPU cycles
PPU advances 2 * 3 = 6 PPU cycles

Распространённое заблуждение

Не заставляйте CPU.step() вызывать PPU.step(). CPU не должен владеть видеосинхронизацией. Console владеет координацией между микросхемами.

Пример предлагаемой реализации

@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()

    def step(self) -> int:
        # This is base timing only. Later steps can add branch penalties,
        # NMI latency, DMA stalls, and other timing details.
        cpu_cycles = self.cpu.step()
        self.ppu.step(cpu_cycles * 3)
        self.consume_nmi_if_requested()
        return cpu_cycles

Важное ограничение

Это по-прежнему упрощённая синхронизация. CPU.step() сейчас возвращает только базовые такты.

Последующие улучшения могут учитывать

branch taken extra cycles
branch page-cross extra cycles
indexed addressing page-cross extra cycles
NMI latency
OAM DMA stalls

Выходит за рамки

динамические штрафы тактов CPU
задержка NMI с точностью до точки
OAM DMA
рендеринг пикселей
цикл pygame/фронтенда
ввод с контроллеров

Запустить этот урок

uv run pytest tests/chapter_04_ppu_timing_and_vblank/test_268_console_step_advances_ppu.py -v

Пример реализации

Пример предлагаемой реализации

@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()

    def step(self) -> int:
        # This is base timing only. Later steps can add branch penalties,
        # NMI latency, DMA stalls, and other timing details.
        cpu_cycles = self.cpu.step()
        self.ppu.step(cpu_cycles * 3)
        self.consume_nmi_if_requested()
        return cpu_cycles

Показать код тестаtest_268_console_step_advances_ppu.py

from emulator.console import Console
from emulator.ppu.ppu import (
    CTRL_NMI_ENABLE,
    PPU_CYCLES_PER_SCANLINE,
    PPU_VBLANK_START_SCANLINE,
)
from tests.helpers import load_program, make_cpu_with_rom


def write_nmi_vector(rom, addr: int) -> None:
    """Install the NMI vector in FakeROM for CPU.interrupt_nmi()."""
    rom.write(0x7FFA, addr & 0xFF)
    rom.write(0x7FFB, (addr >> 8) & 0xFF)


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

    Use cpu.bus.ppu so CPU bus register access and Console timing observe 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_exposes_step_method():
    """
    Objective:
    Console has a machine-level step method for CPU/PPU coordination.
    """
    assert hasattr(Console, "step")
    assert callable(Console.step)


def test_console_step_executes_one_cpu_instruction_and_returns_cpu_cycles():
    """
    Objective:
    Console.step() executes one CPU instruction and returns that instruction's CPU
    cycle count.

    Example:
        NOP is opcode $EA and returns 2 CPU cycles.
    """
    console, cpu, _ppu, rom = make_console_with_fake_rom()
    load_program(rom, 0x8000, [0xEA])
    cpu.pc = 0x8000

    cycles = console.step()

    assert cycles == 2
    assert cpu.pc == 0x8001


def test_console_step_advances_ppu_by_three_times_cpu_cycles():
    """
    Objective:
    Console.step() applies the NES clock ratio:

        1 CPU cycle = 3 PPU cycles

    Example:
        NOP takes 2 CPU cycles, so PPU advances 6 cycles.
    """
    console, cpu, ppu, rom = make_console_with_fake_rom()
    load_program(rom, 0x8000, [0xEA])
    cpu.pc = 0x8000
    ppu.cycle = 10

    cycles = console.step()

    assert cycles == 2
    assert ppu.cycle == 16


def test_console_step_advances_ppu_scanline_when_ppu_cycles_cross_scanline_boundary():
    """
    Objective:
    Console.step() should use PPU.step(), not manually increment ppu.cycle.
    Therefore normal PPU scanline wrapping still applies.

    Example:
        If PPU is 2 cycles before the end of a scanline and CPU executes NOP
        for 2 CPU cycles, the PPU advances 6 cycles total and enters the next
        scanline with 4 cycles remaining.
    """
    console, cpu, ppu, rom = make_console_with_fake_rom()
    load_program(rom, 0x8000, [0xEA])
    cpu.pc = 0x8000
    ppu.cycle = PPU_CYCLES_PER_SCANLINE - 2
    ppu.scanline = 7

    cycles = console.step()

    assert cycles == 2
    assert ppu.scanline == 8
    assert ppu.cycle == 4


def test_console_step_consumes_nmi_after_ppu_advancement():
    """
    Objective:
    If PPU stepping enters VBlank and requests NMI, Console.step() consumes that
    request after advancing PPU time.

    Setup:
        PPU is 6 cycles before entering scanline 241.
        CPU executes NOP for 2 CPU cycles.
        Console advances PPU by 2 * 3 = 6 cycles.
        PPU enters VBlank and requests NMI.
        Console consumes the request and CPU jumps to the NMI vector.
    """
    console, cpu, ppu, rom = make_console_with_fake_rom()
    load_program(rom, 0x8000, [0xEA])
    write_nmi_vector(rom, 0xC000)

    cpu.pc = 0x8000
    cpu.s = 0xFD
    ppu.write_register(0x2000, CTRL_NMI_ENABLE)
    ppu.scanline = PPU_VBLANK_START_SCANLINE - 1
    ppu.cycle = PPU_CYCLES_PER_SCANLINE - 6
    ppu.nmi_requested = False

    cycles = console.step()

    assert cycles == 2
    assert ppu.scanline == PPU_VBLANK_START_SCANLINE
    assert ppu.cycle == 0
    assert ppu.nmi_requested is False
    assert cpu.pc == 0xC000
    assert cpu.s == 0xFA


def test_console_step_does_not_create_cpu_to_ppu_coupling():
    """
    Objective:
    Document the boundary: Console coordinates CPU and PPU timing. CPU.step()
    returns cycles; it does not directly step the PPU.

    This test observes the effect through Console.step(), not by adding PPU logic
    inside CPU.
    """
    console, cpu, ppu, rom = make_console_with_fake_rom()
    load_program(rom, 0x8000, [0xEA, 0xEA])
    cpu.pc = 0x8000

    first_cycles = console.step()
    second_cycles = console.step()

    assert first_cycles == 2
    assert second_cycles == 2
    assert cpu.pc == 0x8002
    assert ppu.cycle == 12

Тестовые функции (6)

test_console_exposes_step_method
test_console_step_executes_one_cpu_instruction_and_returns_cpu_cycles
test_console_step_advances_ppu_by_three_times_cpu_cycles
test_console_step_advances_ppu_scanline_when_ppu_cycles_cross_scanline_boundary
test_console_step_consumes_nmi_after_ppu_advancement
test_console_step_does_not_create_cpu_to_ppu_coupling