CS452 - Real-Time Programming - Spring 2008

Lecture 3 - Old-style RT Programming Methods

Busy Waiting

This is used to synchronize with an external event, with minimal response time.

#define FOREVER for( ; ; )
FOREVER {
   while( !ready( ) ) ;
   do-it( );
}

Aside: The nature of of I/O devices

• Two read/write registers
  1. Command/status
     • Give a command when you write it
     • Get device status when you read it
  2. Data
     • Put data when you write it
     • Get data when you read it
• What you do for input
  1. Read status register until the ready bit sets
  2. Read the data register and process the data

Worst case response time

• execution time for while( !ready( ) )

The NOP technique

Maybe this isn't fast enough for you. Here's an "improvement".

L1:
  NOP
L2:
  NOP
...
L1000:
  NOP
L1001:
  LDR  R0, [R3]
  STR  R0, [R4, #4]!
  BX   R5

Worst case response time

• execution time of NOP
• usually one clock cycle

Polling Loops

Polling loops allow you to manage more than one condition/activity pair at the same time.

The basic polling loop

FOREVER {
  if( c1 ) a1;
  if( c2 ) a2;
  ...
  if( cN ) aN;
}

Worst case response time

• \sum_n execution time of if( cn ) + execution time of an

Testing more than once

Suppose you want a better response time for a1. Then try the loop

FOREVER {
  if( c1 ) a1;
  if( c2 ) a2;
  if( c1 ) a1;
  if( c3 ) a3;
  ...
  if( c1 ) a1;
  if( cN ) aN;
}

Worst case response time for a1

• execution time for a1 + \max_n execution time for if( cn ) an

Breaking into pieces

Suppose the response time is still too long, because the execution of one action, say a2, is too long. Then you can break a2 into two parts

FOREVER {
  if( c1 ) a1;
  if( c2 ) { a2.1; half-done = TRUE }
  if( c1 ) a1;
  if( halfdone ) a2.2;
  ...
}

This is strarting to get a little complicated and we haven't said anything about inter-action communication

Interrupt Service Routines (ISRs)

We can use interrupts to get rid of polling

Interrupts are exceptions

1. Mask interrupts
2. Set link register
3. Save CSR
4. Transfer control

Interrupt vectors

ISRs

Like function calls without arguments or return values.

1. Save state of interrupted task
2. Turn off source of interrupt
3. Do the work
4. Restore state of interrupted task
5. Return to next instruction of interrupted task

Contrast RT to non-RT

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