173. Bcs 相对寻址

将 BCS 接入其相对寻址操作码。

第 173 / 356 · tests/chapter_01_cpu/test_173_BCS_relative.py

前置条件:步骤 172 已导入 `relative 并接通了 BCC。在这一步中,向 emulator/cpu/opcodes.py` 添加以下内容:

from emulator.cpu.instructions import bcs

def bcs_relative(cpu: CPU):
    offset = relative(cpu)
    bcs(cpu, offset)

OPCODE_TABLE[0xB0] = bcs_relative

将导入项和表项合并进已有的分组导入与表字面量中。

这一步存在的原因

寻址逻辑仍留在操作码层,而步骤 165 的 `instructions.bcs` 负责与之互补的 Carry 置位判断。

不变量:`0xB0 映射到 bcs_relative(cpu)`;带符号操作数始终会被消费;Carry 置位时从操作数之后的 PC 出发分支,Carry 清零时该 PC 保持不变。标志位和内存都不受影响。常见误解:BCS 意味着 Carry 置位,而不是 Carry 清零,并且该处理函数不应重复做符号转换。

不在本步范围内:BEQ/BNE/BPL/BMI/BVC/BVS 的操作码接线留待步骤 174-179;JMP 寻址是步骤 180。

运行本课

uv run pytest tests/chapter_01_cpu/test_173_BCS_relative.py -v

示例实现

from emulator.cpu.instructions import bcs

def bcs_relative(cpu: CPU):
    offset = relative(cpu)
    bcs(cpu, offset)

OPCODE_TABLE[0xB0] = bcs_relative

查看测试代码test_173_BCS_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_bcs_relative_handler_exists_and_is_in_opcode_table():
    """Objective: create bcs_relative(cpu) and add 0xB0 to OPCODE_TABLE."""
    assert hasattr(opcodes, "bcs_relative")
    assert callable(opcodes.bcs_relative)
    assert list(inspect.signature(opcodes.bcs_relative).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xB0] is opcodes.bcs_relative


def test_opcode_B0_bcs_relative_branches_when_carry_set():
    """Objective: BCS branches when Carry is set."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xB0)
    rom.write(0x0001, 0x05)

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

    assert cpu.pc == 0x8007


def test_opcode_B0_bcs_relative_does_not_branch_when_carry_clear():
    """Objective: BCS does not branch when Carry is clear."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xB0)
    rom.write(0x0001, 0x05)

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

    assert cpu.pc == 0x8002

测试函数 (3)

test_bcs_relative_handler_exists_and_is_in_opcode_table
test_opcode_B0_bcs_relative_branches_when_carry_set
test_opcode_B0_bcs_relative_does_not_branch_when_carry_clear