存储器（RAM 6264）扩展 可编程并行接口（8255）扩展 可编程计数/定时器（8253）扩展 A/D、D/A转换 可编程串行通信接口（8251）扩展 -Memory (RAM 6264) extended programmable parallel interface (8255) extended the programmable counter/timer (8253) Expansion A/D, D/A conversion programmable seri