110. Instruction ror a 指令

实现针对累加器的 ROR。

第 110 / 356 · tests/chapter_01_cpu/test_110_instruction_ror_a.py

在本步骤中,以第 109 课中针对内存的 `ror 为前置条件,只添加 ror_a。操作码 0x6A` 推迟到第 111 课。

生产代码位置中的完整示例实现

`emulator/cpu/instructions.py::ror_a`::

def ror_a(cpu: CPU):
    value = cpu.a
    old_carry = int(cpu.flags.get_carry_flag())

    result = (value >> 1) | (old_carry << 7)
    result_8 = result & 0xFF

    # Set flags

    cpu.flags.set_carry_flag((value & 0x1) != 0)
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)

    cpu.a = result_8

本步骤存在的原因

累加器模式没有有效地址,因此需要一个具有与内存 ROR 相同的“经 Carry 旋转”语义的寄存器原语。

不变量:先采集旧的 Carry 并将其送入第 7 位;原始 A 的第 0 位成为新的 Carry;A 保持 8 位;Z/N 描述新的 A;内存不受影响。

常见误解:A 是寄存器值,而不是传给 `ror` 的地址。将其传入内存原语会旋转 RAM[A],而不是累加器本身。

不在本步骤范围内:`0x6A` 的分派是第 111 课,内存操作码寻址模式是第 112-115 课,时序/周期精度是之后的工作。

运行本课

uv run pytest tests/chapter_01_cpu/test_110_instruction_ror_a.py -v

示例实现

生产代码位置中的完整示例实现

def ror_a(cpu: CPU):
    value = cpu.a
    old_carry = int(cpu.flags.get_carry_flag())

    result = (value >> 1) | (old_carry << 7)
    result_8 = result & 0xFF

    # Set flags

    cpu.flags.set_carry_flag((value & 0x1) != 0)
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)

    cpu.a = result_8

查看测试代码test_110_instruction_ror_a.py

from emulator.cpu.instructions import ror_a
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG, make_cpu


CARRY_FLAG = 1 << 0


def test_ror_a_rotates_accumulator_right_with_old_carry_clear():
    """Objective: old Carry=0 means new A bit 7 becomes 0."""
    cpu = make_cpu()
    cpu.a = 0b0000_0110

    ror_a(cpu)

    assert cpu.a == 0b0000_0011
    assert (cpu.p & CARRY_FLAG) == 0


def test_ror_a_rotates_accumulator_right_with_old_carry_set():
    """Objective: old Carry=1 is inserted into result bit 7."""
    cpu = make_cpu()
    cpu.p |= CARRY_FLAG
    cpu.a = 0b0000_0010

    ror_a(cpu)

    assert cpu.a == 0b1000_0001
    assert (cpu.p & CARRY_FLAG) == 0


def test_ror_a_sets_carry_from_old_accumulator_bit_0():
    """Objective: old A bit 0 becomes the new Carry flag."""
    cpu = make_cpu()
    cpu.a = 0b0000_0011

    ror_a(cpu)

    assert cpu.a == 0b0000_0001
    assert (cpu.p & CARRY_FLAG) != 0


def test_ror_a_sets_zero_flag_when_result_is_zero():
    """Objective: A=0x01 with old Carry=0 becomes 0x00 and sets Zero."""
    cpu = make_cpu()
    cpu.a = 0x01

    ror_a(cpu)

    assert cpu.a == 0x00
    assert (cpu.p & ZERO_FLAG) != 0
    assert (cpu.p & NEGATIVE_FLAG) == 0
    assert (cpu.p & CARRY_FLAG) != 0


def test_ror_a_sets_negative_flag_from_result_bit_7():
    """Objective: old Carry=1 becomes result bit 7, so Negative is set."""
    cpu = make_cpu()
    cpu.p |= CARRY_FLAG
    cpu.a = 0x00

    ror_a(cpu)

    assert cpu.a == 0x80
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0


def test_ror_a_does_not_write_rotated_result_to_memory():
    """Objective: ROR A writes to A only; it must not treat A as a memory address."""
    cpu = make_cpu()
    cpu.a = 0x20
    cpu.bus.write(0x0020, 0x99)

    ror_a(cpu)

    assert cpu.a == 0x10
    assert cpu.bus.read(0x0020) == 0x99

测试函数 (6)

test_ror_a_rotates_accumulator_right_with_old_carry_clear
test_ror_a_rotates_accumulator_right_with_old_carry_set
test_ror_a_sets_carry_from_old_accumulator_bit_0
test_ror_a_sets_zero_flag_when_result_is_zero
test_ror_a_sets_negative_flag_from_result_bit_7
test_ror_a_does_not_write_rotated_result_to_memory