164. 指令 bcc

实现“进位清零时分支”行为。

第 164 / 356 · tests/chapter_01_cpu/test_164_instruction_bcc.py

为什么需要这一步

第 163 课已经返回了带符号的位移,并使 PC 停留在下一条指令处。BCC 只负责进位清零的判断以及目标地址的加法运算。

在这一步中,请在 `emulator/cpu/instructions.py::bcc` 中恰好添加以下实现:

def bcc(cpu: CPU, offset: int):
    if not cpu.flags.get_carry_flag():
        cpu.pc = (cpu.pc + offset) & 0xFFFF

该掩码保留了 CPU 的 16 位地址空间。

不变量:进位清零时,会把正数、零或负数的 `offset 以 0x10000` 为模加到 PC 上;进位置位时 PC 保持不变。标志位、除 PC 以外的寄存器以及内存均不受影响。常见误解:BCC 表示进位*清零*,并且该指令函数内部绝不能自行读取操作数或做符号转换。

超出范围:BCS/BEQ/BNE/BPL/BMI/BVC/BVS 属于第 165-171 课。将分支函数导入 `emulator/cpu/opcodes.py`、相对寻址处理函数以及操作码表项属于第 172-179 课。

运行本课

uv run pytest tests/chapter_01_cpu/test_164_instruction_bcc.py -v

示例实现

为什么需要这一步

def bcc(cpu: CPU, offset: int):
    if not cpu.flags.get_carry_flag():
        cpu.pc = (cpu.pc + offset) & 0xFFFF

查看测试代码test_164_instruction_bcc.py

from emulator.cpu.instructions import bcc
from tests.helpers import make_cpu


def test_bcc_branches_when_carry_flag_is_clear():
    """Objective: if Carry is clear, PC is changed by the signed offset."""
    cpu = make_cpu()
    cpu.pc = 0x8002
    cpu.flags.set_carry_flag(False)

    bcc(cpu, 0x05)

    assert cpu.pc == 0x8007


def test_bcc_does_not_branch_when_carry_flag_is_set():
    """Objective: if Carry is set, PC remains unchanged."""
    cpu = make_cpu()
    cpu.pc = 0x8002
    cpu.flags.set_carry_flag(True)

    bcc(cpu, 0x05)

    assert cpu.pc == 0x8002


def test_bcc_accepts_negative_signed_offsets():
    """Objective: offsets from relative(cpu) may be negative, such as -2."""
    cpu = make_cpu()
    cpu.pc = 0x8002
    cpu.flags.set_carry_flag(False)

    bcc(cpu, -2)

    assert cpu.pc == 0x8000


def test_bcc_wraps_pc_to_16_bits_when_branch_underflows():
    """
    Objective:
    PC must stay 16-bit after adding the offset.

    Example:
    0x0001 + (-2) should become 0xFFFF, not -1.

    Hint:
    Use `& 0xFFFF` after adding the offset.
    """
    cpu = make_cpu()
    cpu.pc = 0x0001
    cpu.flags.set_carry_flag(False)

    bcc(cpu, -2)

    assert cpu.pc == 0xFFFF


def test_bcc_wraps_pc_to_16_bits_when_branch_overflows():
    """
    Objective:
    PC must also wrap when the addition goes past 0xFFFF.

    Example:
    0xFFFF + 1 should become 0x0000.

    Hint:
    This is the same `& 0xFFFF` invariant.
    """
    cpu = make_cpu()
    cpu.pc = 0xFFFF
    cpu.flags.set_carry_flag(False)

    bcc(cpu, 1)

    assert cpu.pc == 0x0000

测试函数 (5)

test_bcc_branches_when_carry_flag_is_clear
test_bcc_does_not_branch_when_carry_flag_is_set
test_bcc_accepts_negative_signed_offsets
test_bcc_wraps_pc_to_16_bits_when_branch_underflows
test_bcc_wraps_pc_to_16_bits_when_branch_overflows