CS452 - Real-Time Programming - Fall 2008

Lecture 5 - Kernel, Tasks

Questions & Comments

1. Lab will be ready some time during the week-end for: Assignment 2 (aka 1)

Kernel of a Real-time Operating System

Diagram

What Does the Kernel Provide

Tasks

• Provide applications with modularity, which is a higher level at which they can be organized
• Consist of
  • instructions, common to all tasks
  • global state, common to all tasks
  • local state, different state in different tasks
• How tasks work together
  • synchronization
  • communication
  • combined into one mechanism: message passing

Communication

Communication has two sides

1. sharing information
2. handshaking

We use Send/Receive/Reply (SRR) to do both.

1. Send blocks
2. Receive blocks: is synchronous with the call to send
3. Reply doesn't block: is synchronous with the return from send

Diagram

Synchronization

1. Between tasks
   • Coordination of execution in co-operating tasks
   • Uses SRR
2. With internal events
   • Real-time by synchronizing with a real-time clock: e.g. clock server
   • Ordering execution: e.g. name server, bounded buffer
   • Uses SRR
3. With external events
   • interrupts

Kernel Structure

The kernel is just a function like any other, but which runs forever.

kernel( ) {
  initialize( );
  FOREVER {
    request = getNextRequest( );
    handle( request );
  }
}

This hides all the interesting stuff inside

int getNextRequest( ) {
  active = schedule( ); //active is a pointer to a TD
  nextRequest = activate( active ); //the active task doesn't change
  return nextRequest);
}

What's inside activate( active )?

1. transfer of control to the active task
2. execution to completion of the active task
   • `to completion' means until the active task sends a request to the kernel
3. transfer of control back to the kernel
4. getting the request

The hard part to get right is `transfer of control'

• which we call a context switch

How I Get Started Writing a Context Switch

1. Start with a program that calls a function

void func( ) {
  printf( "func: running\n" );
}

void main( ) {
  printf( "main: beginning\n" );
  func( );
  printf( "main: ending\n" );
}

1. Compile and run it.
2. Compile with the -S flag and look at the assembly code.

2. Find out how to put a PC into interrupt vector n

1. Add to the assembly code a line that puts the address of the first instruction of func( ) into the interrupt vector
2. Compile the assembly code and run it.

3. Change to calling func( ) using an interrupt

1. Replace the instruction in main that calls func( ) to int n
2. Replace the ret instruction in func( ) with iret.

4. You have just written a context switch, now you need to dress it up.

1. Add stuff to main( ) so that it has context
2. Save the context of main( ) on the stack just before int n.
3. Restore the context of main( ) from the stack just after int n.

5. Add a return value from func( ) and pick it up in main( ).

To go beyond this we need to think about tasks.

Tasks

Kernel maintains a task descriptor (TD) for each created task. That is, to create a task the kernel must allocate a TD and initialize it. The TD normall contains

1. The task's stack pointer, which points to a private stack containing
   • PC
   • other registers
   • local variables

   all ready to be reloaded whenever the task next runs.

2. Possibly the return value for when the task is next activated
3. The task's parent
4. The task's state

Possible states of the task

1. Active: running or about to run
2. Ready: can run if scheduled
3. Blocked: waiting for something to happen

Diagram

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