261. VBlank 时 PPU 发起 NMI 请求

实现 VBlank 时 PPU 端的 NMI 请求。

第 261 / 356 · tests/chapter_04_ppu_timing_and_vblank/test_261_ppu_nmi_request_on_vblank.py

参考资料

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

需要更新的文件

emulator/ppu/ppu.py

需要添加的状态

nmi_requested: bool = False

此步骤存在的原因

许多 NES 游戏并不会在主循环中轮询 PPUSTATUS。相反,它们通过 PPUCTRL 的第 7 位启用 NMI,并期望 PPU 在 VBlank 开始时请求一次中断。

在这一步中,只实现 PPU 端的信号

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

CPU 目前还不会消费这个 NMI。那是之后的集成步骤要做的事情。

建议的实现示例

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

重要概念

NMI 请求是在 VBlank 开始时产生的事件,不应该在 VBlank 期间的每个周期都持续产生。

不在本步骤范围内

CPU 的中断处理
读取 NMI 向量
把 PC/状态寄存器压入 CPU 栈
由 CPU/系统集成来清除 nmi_requested
在 VBlank 已经处于激活状态时再次启用 NMI
逐点/逐周期精确的 NMI 时序

运行本课

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

示例实现

建议的实现示例

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

查看测试代码test_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

测试函数 (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