160. CPY 立即数寻址

将 CPY 立即数寻址暴露为操作码 ``0xC0``。

第 160 / 356 · tests/chapter_01_cpu/test_160_CPY_immediate.py

此步骤存在的原因

立即数寻址的 CPY 使用字面字节通过 CPU 执行暴露 Y 的比较操作,不需要额外的数据内存读取。

在此步骤中,在课程 159 创建该指令之后,在 `emulator/cpu/opcodes.py` 中做以下添加:

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

def cpy_immediate(cpu: CPU):
    cpy(cpu, immediate(cpu))

OPCODE_TABLE = {
    ...
    0xC0: cpy_immediate,
}

`emulator/cpu/addressing_modes.py::immediate 返回 CPU.fetch_byte()` 作为该值。CPY 只改变 C/Z/N;Y、内存和溢出标志保持不变,操作码加操作数使 PC 前进两个字节。

常见误解:立即数寻址方式并不指代总线地址,CPY 也不会把减法结果存入 Y。超出范围:CPY 零页和绝对寻址(课程 161-162)。

运行本课

uv run pytest tests/chapter_01_cpu/test_160_CPY_immediate.py -v

示例实现

此步骤存在的原因

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

def cpy_immediate(cpu: CPU):
    cpy(cpu, immediate(cpu))

OPCODE_TABLE = {
    ...
    0xC0: cpy_immediate,
}

查看测试代码test_160_CPY_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_cpy_immediate_handler_exists_and_is_in_opcode_table():
    """Objective: create cpy_immediate(cpu) and add 0xC0 to OPCODE_TABLE."""
    assert hasattr(opcodes, "cpy_immediate")
    assert callable(opcodes.cpy_immediate)
    assert list(inspect.signature(opcodes.cpy_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xC0] is opcodes.cpy_immediate


def test_opcode_C0_cpy_immediate_compares_y_with_value():
    """Objective: C0 10 means compare Y with 0x10."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xC0)
    rom.write(0x0001, 0x10)

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

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


def test_opcode_C0_cpy_immediate_sets_zero_when_equal():
    """Objective: equal values set Zero and Carry."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xC0)
    rom.write(0x0001, 0x10)

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

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


def test_opcode_C0_cpy_immediate_sets_negative_when_wrapped_result_has_bit_7():
    """Objective: Y=0x01 compared with 0x02 gives wrapped result 0xFF."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xC0)
    rom.write(0x0001, 0x02)

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

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

测试函数 (4)

test_cpy_immediate_handler_exists_and_is_in_opcode_table
test_opcode_C0_cpy_immediate_compares_y_with_value
test_opcode_C0_cpy_immediate_sets_zero_when_equal
test_opcode_C0_cpy_immediate_sets_negative_when_wrapped_result_has_bit_7