190. BRK 隐含寻址

接入隐含寻址的 BRK 操作码 $00。

第 190 / 356 · tests/chapter_01_cpu/test_190_BRK_implied.py

前置条件:第 189 步已完成 `brk 及其所需的状态位行为。在本步骤中,在 emulator/cpu/opcodes.py` 中添加以下导入与直接映射:

from emulator.cpu.instructions import brk

OPCODE_TABLE = {
    # existing entries...
    0x00: brk,
}

操作码

0x00 -> BRK

目标

把操作码 0x00 添加到 OPCODE_TABLE 中。

学习指引

BRK 通常被列为隐含寻址指令,但它有一个特殊的长度:

opcode byte:       $00
padding byte:      one ignored byte after $00
total length:      2 bytes

这并不意味着 BRK 存在两个操作码表项。

官方操作码只有一个

0x00

BRK 之后的那个字节可以是任意值。CPU 会跳过它,并在该字节之后压入返回地址。

示例

$8000: 00    BRK opcode
$8001: AB    padding/signature byte, ignored by BRK
$8002: EA    next real instruction

在 CPU.step() 执行完 BRK 之后,压入的返回地址应为 $8002。

本步骤存在的原因

`CPU.step 取出 $00 并把 PC 前进一次;直接分发到 brk` 会处理隐含的填充字节,并完成所有栈操作与向量跳转工作。不变量:只有一个表项,且没有寻址方式的取值操作;填充字节的内容不影响 A 或目标地址;第 189 步中 BRK 的状态与栈规则保持不变。常见误解:BRK 在体系结构上的两字节长度,并不意味着操作码本身是两字节,也不意味着需要操作数解码的包装函数。

本步骤范围之外:RTI/$40 属于第 191-192 步。硬件中断分发、NMI、共用的栈辅助函数以及后续的操作码 API 都不属于本次改动。

运行本课

uv run pytest tests/chapter_01_cpu/test_190_BRK_implied.py -v

示例实现

from emulator.cpu.instructions import brk

OPCODE_TABLE = {
    # existing entries...
    0x00: brk,
}

查看测试代码test_190_BRK_implied.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
from emulator.cpu.instructions import brk
from emulator.memory.fake_rom import FakeROM


INTERRUPT_DISABLE_FLAG = 1 << 2
BREAK_FLAG = 1 << 4
ONE_FLAG = 1 << 5
STACK_BASE = 0x0100


def make_cpu_with_rom():
    rom = FakeROM()
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)
    rom.write(0x7FFE, 0x00)
    rom.write(0x7FFF, 0x90)
    bus = CpuBus(program_rom=rom)
    return CPU(bus), bus, rom


def test_brk_implied_is_in_opcode_table():
    """Objective: opcode 0x00 is the official BRK opcode."""
    assert opcodes.OPCODE_TABLE[0x00] is brk


def test_brk_instruction_signature_takes_only_cpu():
    """Objective: BRK dispatch does not need an addressing-mode argument."""
    assert list(inspect.signature(brk).parameters) == ["cpu"]


def test_opcode_00_brk_loads_pc_from_irq_brk_vector():
    """Objective: executing opcode 0x00 jumps to the vector stored at $FFFE/$FFFF."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x00)
    rom.write(0x0001, 0xAB)

    cpu.reset()
    cpu.p = 0x00
    cpu.step()

    assert cpu.pc == 0x9000


def test_opcode_00_brk_pushes_address_after_padding_byte():
    """
    Objective:
    BRK is treated as 2 bytes.

    Timeline:
        Before step: PC = $8000
        CPU.step fetches $00, so PC becomes $8001
        brk(cpu) skips padding byte and pushes $8002

    The padding byte can be $AB, $00, $FF, or anything else.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x00)
    rom.write(0x0001, 0xAB)

    cpu.reset()
    cpu.p = 0x00
    cpu.step()

    assert bus.read(STACK_BASE | 0xFD) == 0x80
    assert bus.read(STACK_BASE | 0xFC) == 0x02


def test_opcode_00_brk_pushes_status_with_break_flag_set():
    """Objective: opcode BRK pushes P with Break and ONE bits set."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x00)
    rom.write(0x0001, 0xAB)

    cpu.reset()
    cpu.p = 0x00
    cpu.step()

    pushed_status = bus.read(STACK_BASE | 0xFB)
    assert (pushed_status & BREAK_FLAG) != 0
    assert (pushed_status & ONE_FLAG) != 0


def test_opcode_00_brk_sets_interrupt_disable_and_clears_cpu_break_state():
    """
    Objective:
    After BRK finishes, Interrupt Disable is set, but Break is not kept as a
    persistent CPU status bit in this emulator model.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x00)
    rom.write(0x0001, 0xAB)

    cpu.reset()
    cpu.p = 0x00
    cpu.step()

    assert (cpu.p & INTERRUPT_DISABLE_FLAG) != 0
    assert cpu.flags.get_interrupt_disable_flag() is True
    assert cpu.flags.get_break_flag() is False
    assert cpu.flags.get_one_flag() is False


def test_opcode_00_brk_padding_byte_is_ignored_not_executed():
    """
    Objective:
    The byte after BRK is skipped by BRK. It is not executed as the next opcode.

    If the padding byte is $A9, that might look like LDA Immediate, but BRK must
    ignore it and jump to the interrupt vector instead.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x00)
    rom.write(0x0001, 0xA9)
    rom.write(0x0002, 0x42)

    cpu.reset()
    cpu.a = 0x00
    cpu.p = 0x00
    cpu.step()

    assert cpu.pc == 0x9000
    assert cpu.a == 0x00
    assert bus.read(STACK_BASE | 0xFC) == 0x02

测试函数 (7)

test_brk_implied_is_in_opcode_table
test_brk_instruction_signature_takes_only_cpu
test_opcode_00_brk_loads_pc_from_irq_brk_vector
test_opcode_00_brk_pushes_address_after_padding_byte
test_opcode_00_brk_pushes_status_with_break_flag_set
test_opcode_00_brk_sets_interrupt_disable_and_clears_cpu_break_state
test_opcode_00_brk_padding_byte_is_ignored_not_executed