264. CPU 的 NMI 中断

实现 CPU 端的 NMI 中断机制。

第 264 / 356 · tests/chapter_04_ppu_timing_and_vblank/test_264_cpu_interrupt_nmi.py

参考资料

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

需要更新的文件

emulator/cpu/cpu.py

此步骤存在的原因

PPU 已经可以产生 nmi_requested 信号。在通过系统/主机协调器把这个信号接入 CPU 之前,CPU 本身必须先知道如何独立完成 NMI 序列。

什么是 NMI?NMI 意为不可屏蔽中断(Non-Maskable Interrupt)。在 NES 上,PPU 可以在 VBlank 时请求 NMI,从而让游戏代码运行其垂直消隐处理程序。

直观模型

NMI 就像一次硬件级的紧急跳转。CPU 暂停当前的执行路径,保存足够的状态以便之后返回,然后跳转到 NMI 向量中保存的地址。

机制模型

当 NMI 被接受时,CPU 会执行以下序列

1. Push PC high byte
2. Push PC low byte
3. Push status with:
       ONE_FLAG set
       B_FLAG clear
4. Set INTERRUPT_FLAG in CPU status
5. Read low byte from $FFFA
6. Read high byte from $FFFB
7. Set PC = high << 8 | low

重要区别

NMI 不会写入 $FFFA/$FFFB。CPU 只是读取这两个地址。在真实的 ROM 中,向量字节本来就存在于 PRG ROM 中。在这些测试里,FakeROM 让我们可以把这些字节作为测试搭建的一部分预先准备好。

实现示例

NMI_VECTOR_LOW = 0xFFFA
NMI_VECTOR_HIGH = 0xFFFB

class CPU:
    ...

    def interrupt_nmi(self) -> None:
        # Save the current PC so RTI can restore it later.
        pc_high = (self.pc >> 8) & 0xFF
        pc_low = self.pc & 0xFF
        self.push_stack(pc_high)
        self.push_stack(pc_low)

        # Hardware interrupts push status with B clear and bit 5 set.
        status_to_push = self.p | ONE_FLAG
        status_to_push &= ~B_FLAG
        self.push_stack(status_to_push)

        # After accepting an interrupt, set the interrupt-disable flag.
        self.p |= INTERRUPT_FLAG

        # NMI vector bytes are read from PRG space. NMI does not write them.
        low = self.bus.read(NMI_VECTOR_LOW)
        high = self.bus.read(NMI_VECTOR_HIGH)
        self.pc = low | (high << 8)

具体的运行时示例

FakeROM setup:
    $FFFA = $00
    $FFFB = $C0

CPU before NMI:
    PC = $8123
    S  = $FD

CPU.interrupt_nmi()

CPU after NMI:
    stack contains return PC/status
    PC = $C000

以 S = $FD、PC = $8123 为例的栈示例:

write $81 to $01FD, S becomes $FC
write $23 to $01FC, S becomes $FB
write status to $01FB, S becomes $FA

常见误解

硬件中断不会设置 B 标志位。BRK/PHP 压栈时状态字节会设置 B,但 NMI 压栈时状态字节的 B 是清零的。压入状态字节中的第 5 位 ONE_FLAG 仍然会被置位。

不在本步骤范围内

由 PPU 调用 CPU.interrupt_nmi()
清除 ppu.nmi_requested
精确的中断延迟/周期数
IRQ/APU/映射器中断
RTI 行为,这在 CPU 章节前面已经测试过

运行本课

uv run pytest tests/chapter_04_ppu_timing_and_vblank/test_264_cpu_interrupt_nmi.py -v

示例实现

实现示例

NMI_VECTOR_LOW = 0xFFFA
NMI_VECTOR_HIGH = 0xFFFB

class CPU:
    ...

    def interrupt_nmi(self) -> None:
        # Save the current PC so RTI can restore it later.
        pc_high = (self.pc >> 8) & 0xFF
        pc_low = self.pc & 0xFF
        self.push_stack(pc_high)
        self.push_stack(pc_low)

        # Hardware interrupts push status with B clear and bit 5 set.
        status_to_push = self.p | ONE_FLAG
        status_to_push &= ~B_FLAG
        self.push_stack(status_to_push)

        # After accepting an interrupt, set the interrupt-disable flag.
        self.p |= INTERRUPT_FLAG

        # NMI vector bytes are read from PRG space. NMI does not write them.
        low = self.bus.read(NMI_VECTOR_LOW)
        high = self.bus.read(NMI_VECTOR_HIGH)
        self.pc = low | (high << 8)

具体的运行时示例

FakeROM setup:
    $FFFA = $00
    $FFFB = $C0

CPU before NMI:
    PC = $8123
    S  = $FD

CPU.interrupt_nmi()

CPU after NMI:
    stack contains return PC/status
    PC = $C000

查看测试代码test_264_cpu_interrupt_nmi.py

from emulator.cpu import cpu as cpu_module
from emulator.cpu.flags_handler import (
    B_FLAG,
    CARRY_FLAG,
    DECIMAL_FLAG,
    INTERRUPT_FLAG,
    NEGATIVE_FLAG,
    ONE_FLAG,
    ZERO_FLAG,
)
from tests.helpers import make_cpu_with_rom


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

    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 test_cpu_declares_nmi_vector_constants():
    """
    Objective:
    Name the NMI vector addresses used by CPU.interrupt_nmi().
    """
    assert cpu_module.NMI_VECTOR_LOW == 0xFFFA
    assert cpu_module.NMI_VECTOR_HIGH == 0xFFFB


def test_interrupt_nmi_loads_pc_from_nmi_vector():
    """
    Objective:
    CPU.interrupt_nmi() reads $FFFA/$FFFB and jumps to that address.

    Example:
        $FFFA = $00
        $FFFB = $C0
        PC becomes $C000
    """
    cpu, _bus, rom = make_cpu_with_rom()
    write_nmi_vector(rom, 0xC000)
    cpu.pc = 0x8123
    cpu.s = 0xFD

    cpu.interrupt_nmi()

    assert cpu.pc == 0xC000


def test_interrupt_nmi_pushes_current_pc_high_then_low_to_stack():
    """
    Objective:
    NMI saves the current PC on the stack before jumping to the vector target.

    With PC=$8123 and S=$FD:
        $01FD receives $81
        $01FC receives $23
    """
    cpu, bus, rom = make_cpu_with_rom()
    write_nmi_vector(rom, 0xC000)
    cpu.pc = 0x8123
    cpu.s = 0xFD

    cpu.interrupt_nmi()

    assert bus.read(0x01FD) == 0x81
    assert bus.read(0x01FC) == 0x23


def test_interrupt_nmi_pushes_status_with_one_set_and_break_clear():
    """
    Objective:
    Hardware interrupts push a status byte with ONE_FLAG set and B_FLAG clear.

    This differs from BRK/PHP, which push B_FLAG set.
    """
    cpu, bus, rom = make_cpu_with_rom()
    write_nmi_vector(rom, 0xC000)
    cpu.pc = 0x8123
    cpu.s = 0xFD

    cpu.p = CARRY_FLAG | ZERO_FLAG | DECIMAL_FLAG | B_FLAG | NEGATIVE_FLAG

    cpu.interrupt_nmi()

    pushed_status = bus.read(0x01FB)

    assert pushed_status & CARRY_FLAG
    assert pushed_status & ZERO_FLAG
    assert pushed_status & DECIMAL_FLAG
    assert pushed_status & NEGATIVE_FLAG
    assert pushed_status & ONE_FLAG
    assert not (pushed_status & B_FLAG)


def test_interrupt_nmi_sets_interrupt_disable_flag_in_cpu_status():
    """
    Objective:
    After accepting NMI, the CPU sets the interrupt-disable flag in its live status
    register.
    """
    cpu, _bus, rom = make_cpu_with_rom()
    write_nmi_vector(rom, 0xC000)
    cpu.pc = 0x8123
    cpu.s = 0xFD
    cpu.p = 0x00

    cpu.interrupt_nmi()

    assert cpu.p & INTERRUPT_FLAG


def test_interrupt_nmi_consumes_three_stack_bytes():
    """
    Objective:
    NMI pushes PC high, PC low, and status. That consumes exactly three stack
    bytes.

    Example:
        S starts at $FD
        after three pushes, S is $FA
    """
    cpu, _bus, rom = make_cpu_with_rom()
    write_nmi_vector(rom, 0xC000)
    cpu.pc = 0x8123
    cpu.s = 0xFD

    cpu.interrupt_nmi()

    assert cpu.s == 0xFA


def test_interrupt_nmi_full_stack_layout_example():
    """
    Objective:
    Validate the complete NMI stack layout in one readable example.

    Given:
        PC = $ABCD
        S  = $80
        P  = ZERO_FLAG | B_FLAG

    Expected pushes:
        $0180 = $AB
        $017F = $CD
        $017E = status with ZERO_FLAG set, ONE_FLAG set, B_FLAG clear
    """
    cpu, bus, rom = make_cpu_with_rom()
    write_nmi_vector(rom, 0x9000)
    cpu.pc = 0xABCD
    cpu.s = 0x80
    cpu.p = ZERO_FLAG | B_FLAG

    cpu.interrupt_nmi()

    assert bus.read(0x0180) == 0xAB
    assert bus.read(0x017F) == 0xCD

    pushed_status = bus.read(0x017E)
    assert pushed_status & ZERO_FLAG
    assert pushed_status & ONE_FLAG
    assert not (pushed_status & B_FLAG)

    assert cpu.s == 0x7D
    assert cpu.pc == 0x9000
    assert cpu.p & INTERRUPT_FLAG

测试函数 (7)

test_cpu_declares_nmi_vector_constants
test_interrupt_nmi_loads_pc_from_nmi_vector
test_interrupt_nmi_pushes_current_pc_high_then_low_to_stack
test_interrupt_nmi_pushes_status_with_one_set_and_break_clear
test_interrupt_nmi_sets_interrupt_disable_flag_in_cpu_status
test_interrupt_nmi_consumes_three_stack_bytes
test_interrupt_nmi_full_stack_layout_example