MIT 6.S081 Lecture 9: Interrupts

Reading

• Read Chapter5
• read code
  

Interrupts

• HW wants attention now, like keyboard and network device

• 系统调用，traps，interrupts都使用相同的处理模式

  • save its work
  • process interrupt
  • resume its work

• interrrupts的特殊之处

  • asynchronous异步的
  • concurrency between cpu and device
  • program device

Where interrupts come from?

• 主板上的异步接口
  

• cpu架构
  

• PLIC 平台级中断控制器

• UART 通用异步收发器(Universal Asynchronous Receiver/Transmitter)

Driver manage device

• 经典的驱动架构
  

Programming Device

• memory mapped I/O

• ld/st read/write control register device

How does ‘$ ls’ work?

• device puts ‘$’ into uart

• uart generate interrupts when char has been sent

• keyboard connect to generate interrupts

RISC-V support for interrupts

• STE - one bit for E, S, T

• SSTATE - bit enable/disable

• SIP - interrrupt pending

• SCAUSE

• STVEC

Interrrupts and concurrency

• device and cpu run in parallel

  • producer / consumer parallelism

• interrupts stops the current program

  • user? OK
  • kernel? 必要时设置取消interrupts来保证原子性

• top of device and bottom of device run in parallel

  • use locks

Interrupts evolution

• polling(轮询)

  • 对于高速设备节省，如高性能网卡
  • 对于低速设备浪费性能，如键盘

• interrupts(中断)

• 动态切换polling和interrupts

• Tip: CS61C中关于设备访问的几种方式

  • polling相当于到店排队问好了没，好了我拿走
  • interrupts相当于准备好了通知你到店自取
  • DMA相当于外卖送货上门，你去拿就行了