177. Bmi 相对寻址

将 BMI 接入其相对寻址操作码。

第 177 / 356 · tests/chapter_01_cpu/test_177_BMI_relative.py

前置条件:步骤 176 已接通 BPL。在这一步中,向 `emulator/cpu/opcodes.py` 添加以下内容:

from emulator.cpu.instructions import bmi

def bmi_relative(cpu: CPU):
    offset = relative(cpu)
    bmi(cpu, offset)

OPCODE_TABLE[0x30] = bmi_relative

将导入项和映射项合并进文件已有的分组形式中。

这一步存在的原因

`relative 负责操作数解码,而步骤 169 的 instructions.bmi` 把“minus”解释为 Negative 置位,并应用该偏移量。

不变量:`0x30` 在跳转和未跳转两条路径上都会消费其操作数;Negative 置位时相对于操作数之后的 PC 分支,Negative 清零时 PC 停留在那里。不改变任何标志位或内存。常见误解:BMI 检测的是 Negative 状态位,而不是偏移量或任何寄存器在 Python 整数意义上的负值。

不在本步范围内:BVC 和 BVS 的操作码接线分别是步骤 178 和 179。间接寻址 JMP 从步骤 180 开始。

运行本课

uv run pytest tests/chapter_01_cpu/test_177_BMI_relative.py -v

示例实现

from emulator.cpu.instructions import bmi

def bmi_relative(cpu: CPU):
    offset = relative(cpu)
    bmi(cpu, offset)

OPCODE_TABLE[0x30] = bmi_relative

查看测试代码test_177_BMI_relative.py

import inspect

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


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


def test_bmi_relative_handler_exists_and_is_in_opcode_table():
    """Objective: create bmi_relative(cpu) and add 0x30 to OPCODE_TABLE."""
    assert hasattr(opcodes, "bmi_relative")
    assert callable(opcodes.bmi_relative)
    assert list(inspect.signature(opcodes.bmi_relative).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x30] is opcodes.bmi_relative


def test_opcode_30_bmi_relative_branches_when_negative_set():
    """Objective: BMI branches when Negative is set."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x30)
    rom.write(0x0001, 0x05)

    cpu.reset()
    cpu.flags.set_negative_flag(True)
    cpu.step()

    assert cpu.pc == 0x8007


def test_opcode_30_bmi_relative_does_not_branch_when_negative_clear():
    """Objective: BMI does not branch when Negative is clear."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x30)
    rom.write(0x0001, 0x05)

    cpu.reset()
    cpu.flags.set_negative_flag(False)
    cpu.step()

    assert cpu.pc == 0x8002

测试函数 (3)

test_bmi_relative_handler_exists_and_is_in_opcode_table
test_opcode_30_bmi_relative_branches_when_negative_set
test_opcode_30_bmi_relative_does_not_branch_when_negative_clear