CS452 - Real-Time Programming - Spring 2009

Lecture 18 - Serial IO: Programmer's Model

Practical Details

How many servers and notifiers?


	one server	two servers	four servers
one notifier	likely queue congestion in server		likely queue congestion in notifer
two notifiers	one per channel? one per direction?	how should they be paired?
four notifiers	certain queue congestion in server	likely queue congestion in server	best performance, most tasks

How should we handle a terminal?

Issues

Line editing
- undefined intermediate states
- well-defined trigger
Echo
Either
- one server per channel,
- passing characters from one notifier to the other
Or
- courier connecting receive server to transmit server
- What characteristics does the terminal have? termcap follies

Many other issues come up below as we consider possible task structures.

Task Structure

Servers and Attendent Tasks

Proprietor with a Notifier

Proprietor `owns' a service

Think of the owner at the counter of an old-fashioned store
Somebody has to get stuff from the back of the store, and put it back

Proprietor Code

Initialize

notifierPid = Create( notifier );     //Should notifier code name be hard coded?
Send( notifierTid, MyTid( ), ... );   //On return noifier is known to be okay
RegisterAs( );                        //On return requests can begin.

Work

FOREVER {
  requesterTid = Receive( request );
  switch ( request.type ) {
  case NOTIFIER:
    Reply( notifier );
    updateDataStructures( );
    for each queued client {
      if( serviceable( client ) ) {
        provideService( client );
        Reply( client );
      }
    }
    break;
  case CLIENT:
    if ( serviceable( requester ) ) {
      provideService( requester );
      Reply( requesterTid );
    else enQueue( requester );
  }
}

Notifier Code

Initialize

Notes

Notifier is usually of higher priority than server
- Notice the early reply in the proprietor
When, and by whom, do interrupts get turned on in the CPU?
- Presumably the Notifier
  - configures the hardware
  - turns on interrupts in the hardware
  - enables interrupts in the VIC
Who coordinates hardware ownership?

Using a Courier

Return to: