CS452 - Real-Time Programming - Fall 2008

Lecture 13 - Interrupts

Questions & Comments

Low-level primitive

This is what everything depends on

int AwaitEvent( int EventType );

What does it do?

Blocks on an interrupt of the type specified
When the interrupt occurs, may return one word of volatile data

Implementation notes

Event may already have occurred
AwaitEvent( ) can service the device.
- Then it must return the volatile data
Need to think of priorities when you partition the work
Kernel has interrupts turned off
- Therefore need IdleTask( ). Why?

How is it used?

Example: Clock Server

Provides a sleep prinitive

int Delay( int interval )

block and return later
Send to a server which
- maintains an ordered list of sleepers
- receives clock ticks
- on each tick checks the list and replies to each sleeper who is ready to wake up

Server must receive clock ticks

provided by a Notifier

Notifier code

InitializeHardware( );
FOREVER {
  tick = AwaitEvent( REAL_TIME_CLOCK );
  // check for errors
  error = Send( parentTid, .... );
  //check for errors
}

What does the kernel do?

Processor

INT pin
- checks it before executing each instruction
  if it is asserted initiates interrupt processing
runs one or more bus cycles to find the vector of the interrupt
puts CS, EIP, FLAGS of interrupted task on stack
retrieves new CS, EIP, FLAGS (with interrupts disabled) from IDT
begins executing ISR
when ISR is complete, IRET
- pops CS, EIP, FLAGS from stack
- puts them into the processor
- `interrupted' process continues

Software

gets volatile data from device
turns off source of interrupt
finds task waiting on the interrupt
makes it ready

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