CS 452/652 Winter 2023 - Lecture 9

Timer Interrupt

• timer compare register (CN reg vs. interrupt #)
  • rearm timer - new value (cf. real-time loop)
  • overflow? no problem...
• C0, C2 comparators used by VC? use C1 or C3
• edge vs. level: rearm timer by writing bit to CS
• also confirm timer interrupt to GIC via IAR

AwaitEvent

• matchmaking: task ↔ event
• task(s) waiting for interrupt to occur: store one or queue? → design decision
• interrupt without waiting task: buffer one or several? or failure?
  • excessive buffering is not helpful in any producer/consumer scenario
  • multiple same interrupts without waiting task indicate performance problem!
  • use task prioritity to make sure that hardware interrupts are handled promptly
• AwaitEvent() argument and return code → design decision

Interrupt Handling

1. user task executing
   [interrupt]
2. handler saves task state and switches to kernel
3. kernel makes task ready
4. kernel determines interrupt number
   • multiple interrupts pending: loop over IAR or one-shot processing
5. kernel processes interrupt
   • after completion of interrupt handler, interrupt would trigger again!
     → force status change or disable/mask interrupt
6. kernel switches to next ready task

Clock Server

• main loop of receiving and processing requests
• server never blocks ⇒ cannot call AwaitEvent()
• task blocking delegated to clock notifier
• server & notifier → general design pattern

Server Processing

• TIME: reply to client with current clock value
• DELAY: park client task (using tick & tid) in timer list
  • two variations: absolute vs. relative tick
• TICK: advance clock value and check parked tasks in timer list
  • reply to clients whose waiting times have completed
• timer list should probably be a sorted data structure

Clock Notifier

• main loop (at high priority)

  loop:
    AwaitEvent()
    Send() tick to clock server
  

Idle/Halt

• what to do when processor has not other work?
  • SETI? cryptomining? → often not appropriate!
  • maintenance, e.g., garbage collection → not relevant for CS 452 kernel
  • diagnosis, such as computation of idle fraction
  • save energy → low-power halt! (cf. "race-to-idle/sleep")
• implement as dedicated idle task at lowest priority

  for (;;) {
    while (maintenance()) yield();
    park();
  }
  

• ARMv8 instructions wfi (or wfe)