147. Cmp 立即寻址

添加 CMP immediate 操作码 ``0xC9``。

第 147 / 356 · tests/chapter_01_cpu/test_147_CMP_immediate.py

此步骤存在的原因

立即寻址形式首次通过 CPU 执行呈现 CMP,将 A 与一个字面字节进行比较,同时把所有比较标志位逻辑留在共享原语中。

在此步骤中,第 146 课已经添加了 `cmp。作为 CMP 的第一个操作码课程,请添加其导入,并在 emulator/cpu/opcodes.py` 中准确添加以下处理函数/表项:

from emulator.cpu.instructions import (..., cmp)

def cmp_immediate(cpu: CPU):
    cmp(cpu, immediate(cpu))

OPCODE_TABLE = {
    ...
    0xC9: cmp_immediate,
}

`addressing_modes.immediate` 直接返回取到的操作数值,因此不存在数据内存读取。 C/Z/N 可能改变;A、Overflow、X、Y 和内存保持不变;操作码加操作数使 PC 前进两个字节。

常见误解:立即寻址 CMP 不会将其操作数当作地址,也不会存储减法结果。超出范围:各内存寻址的 CMP 模式属于第 148-154 课;CPX 和 CPY 留在后续课程中。

运行本课

uv run pytest tests/chapter_01_cpu/test_147_CMP_immediate.py -v

示例实现

此步骤存在的原因

from emulator.cpu.instructions import (..., cmp)

def cmp_immediate(cpu: CPU):
    cmp(cpu, immediate(cpu))

OPCODE_TABLE = {
    ...
    0xC9: cmp_immediate,
}

查看测试代码test_147_CMP_immediate.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
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


CARRY_FLAG = 1 << 0


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_cmp_immediate_handler_exists_and_is_in_opcode_table():
    """Objective: create cmp_immediate(cpu) and add 0xC9 to OPCODE_TABLE."""
    assert hasattr(opcodes, "cmp_immediate")
    assert callable(opcodes.cmp_immediate)
    assert list(inspect.signature(opcodes.cmp_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xC9] is opcodes.cmp_immediate


def test_opcode_C9_cmp_immediate_compares_a_with_value():
    """Objective: C9 10 means compare A with 0x10."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xC9)
    rom.write(0x0001, 0x10)

    cpu.reset()
    cpu.a = 0x20
    cpu.step()

    assert cpu.a == 0x20
    assert (cpu.p & CARRY_FLAG) != 0
    assert cpu.pc == 0x8002


def test_opcode_C9_cmp_immediate_sets_zero_when_equal():
    """Objective: equal values set Zero and Carry."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xC9)
    rom.write(0x0001, 0x10)

    cpu.reset()
    cpu.a = 0x10
    cpu.step()

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


def test_opcode_C9_cmp_immediate_sets_negative_when_wrapped_result_has_bit_7():
    """Objective: A=0x01 compared with 0x02 gives wrapped result 0xFF."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xC9)
    rom.write(0x0001, 0x02)

    cpu.reset()
    cpu.a = 0x01
    cpu.step()

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

测试函数 (4)

test_cmp_immediate_handler_exists_and_is_in_opcode_table
test_opcode_C9_cmp_immediate_compares_a_with_value
test_opcode_C9_cmp_immediate_sets_zero_when_equal
test_opcode_C9_cmp_immediate_sets_negative_when_wrapped_result_has_bit_7