CS452 - Real-Time Programming - Spring 2008

Lecture 20 - AwaitEvent

Questions & Comment

1. 16550 UART

AwaitEvent

Low-level primitive

int AwaitEvent( int EventType );

Implementation notes

1. Event may already have occurred
   • Should an event be buffered?
   • It depends on the type of event.
2. What does AwaitEvent( ) return?
   • errors
     1. no such event type
     2. time-out?

        but not for the timer event

   • volatile data
     • but what about UART buffers
     • specific example of the 'common type' problem.
3. AwaitEvent( ) can service the device.
   • Then it must return the volatile data
4. Need to think of priorities when you partition the work

What do we do when there is no task ready to be scheduled?

1. Kernel has interrupts turned off
   • Therefore need IdleTask( ). Why?
2. What does IdleTask( ) do?

   void IdleTask( ) {
     FOREVER ;
   }
       

   But in reality it could do anything

   • Search for extra-terrestrial intelligence
   • Calculate the digits of pi
   • Collect statistics to improve calibrations

Clock Server

Hardware

Intel decided exactly how you should use the timers when they designed the south bridge.

• Specifically, they cannot be ganged together.
• So the minimum rate at which the clock can tick is (timer clock frequency) / (2^16)

Interrupt controller we already discussed.

General description of clock and PIC included in 82801 documentation.

Software

What is the structure of the service regime?

• type of requests
• rate of requests
• service time per request

Who should initialize timer?

• We should try to localize everything hardware-specific.

Server

void ClockServer( ) {
  Tid requester;
  RegisterAs( "ClockServer" );
  notifier = Create( ... );
  if( notifier < 0 ) 
    // handle error
  FOREVER {
    length = Receive( *requester, request, requestlen );
    if ( length < 0 )
      // handle error
    else if (!( type = decode( request ) ) )
     // handle error
    else {
      switch( type ) {
      case TICK:
        time++;
        // service queues
        break;
      case XXX:
        // et cetera
    }
  }
}

Notifier

void TickNotifier( ) {
  server = WhoIs( "clockserver" );
  if ( server < 0 )
    //handle error
  FOREVER {
    error = AwaitInterrupt( TIMER );
    if ( error < 0 )
      //handle error
    error = Send( server, nullmessage );
    if ( error < 0 )
      //handle error
  }
}

All the error handling code should be telling you something. What?

Serial Server

Policy issues

1. One or two servers?
2. How should screen echo be handled?
   • and DELETE/BS, and line-editting, and ...
3. How should the buffers in the UART be handled?
4. Who will initialize the hardware?
5. What traffic do I expect through the server?
6. Should the be I/O units above the single character level?

As a general policy we would like to concentrate the hardware dependence

Hardware

Stubs

Software

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