061. ADC 间接,Y

添加 ADC (间接),Y。

第 61 / 356 · tests/chapter_01_cpu/test_061_ADC_indirect_y.py

要更新的文件

emulator/cpu/opcodes.py

要添加/更新的符号

opcodes.adc_indirect_y and OPCODE_TABLE[0x71]

为什么需要这一步

最后一个 ADC 寻址变体将现有的 indirect_y 地址辅助函数与现有的基于数值的 adc 指令连接起来，用于操作码 $71。

完整示例实现

# emulator/cpu/opcodes.py
def adc_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    adc(cpu, value)

OPCODE_TABLE = {
    # ... existing entries ...
    0x71: adc_indirect_y,
}

重要不变量

indirect_y 取出单字节操作数并返回最终地址
处理函数恰好读取该地址处的数值一次
更新 A 和算术标志的是 adc 而不是处理函数
执行这条两字节指令会使 PC 前进两个字节

常见误解

不要把 indirect_y 返回的地址直接传给 adc；adc 接受的是存储在该地址处的字节。

范围之外

SBC 及其操作码处理函数
新的寻址模式辅助函数或重构
周期时序和跨页惩罚

运行本课

uv run pytest tests/chapter_01_cpu/test_061_ADC_indirect_y.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
def adc_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    adc(cpu, value)

OPCODE_TABLE = {
    # ... existing entries ...
    0x71: adc_indirect_y,
}

查看测试代码test_061_ADC_indirect_y.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_adc_indirect_y_handler_exists_and_is_in_opcode_table():
    """Objective: create adc_indirect_y(cpu) and add 0x71 to OPCODE_TABLE."""
    assert hasattr(opcodes, "adc_indirect_y")
    assert callable(opcodes.adc_indirect_y)
    assert list(inspect.signature(opcodes.adc_indirect_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x71] is opcodes.adc_indirect_y


def test_opcode_71_adc_indirect_y_adds_value_from_final_address():
    """Objective: 71 20 with Y=0x04 uses base pointer $0200 and adds RAM[$0204]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x71)
    rom.write(0x0001, 0x20)
    bus.write(0x0020, 0x00)
    bus.write(0x0021, 0x02)
    bus.write(0x0204, 0x05)

    cpu.reset()
    cpu.a = 0x10
    cpu.y = 0x04
    cpu.flags.set_carry_flag(False)
    cpu.step()

    assert cpu.a == 0x15
    assert cpu.pc == 0x8002

测试函数 (2)

test_adc_indirect_y_handler_exists_and_is_in_opcode_table
test_opcode_71_adc_indirect_y_adds_value_from_final_address