CS452 - Real-Time Programming - Spring 2012

Lecture 2 - Polling Loops

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Practical Details: pdf

What is real-time programming?

Actually real-world programming, which means

What is important for real-time?

  1. Throughput
  2. Response time

In cs452 we take guaranteed response time as the defining quality of real-time computation.


Timers

How does one keep time in a computer?

Polling Loops

Busy Waiting

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

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

or in another form

FOREVER {
   if ( ready( ) ) do-it( );
}

Sometimes you only want to do the thing once, as you do when putting a character on a serial line.

#define UART1_BASE        0x808c0000
#define UART_DATA_OFFSET        0x00    // low 8 bits
#define UART_FLAG_OFFSET        0x18    // low 8 bits
#define TXFF_MASK               0x20    // Transmit buffer full

        flags = (int *)( UART1_BASE + UART_FLAG_OFFSET );
        data = (int *)( UART1_BASE + UART_DATA_OFFSET );
        while( ( *flags & TXFF_MASK ) ) ;
        *data = c;

Note. The volatile keyword.

Worst case response time

From the time that the ready bit sets until the first instruction of do-it is executed

The problem with busy-waiting

What if the CPU has to two things at once?

E.g.,

  1. collect bytes coming in a serial port
  2. maintain a clock

Unless the rate of bytes coming in and rate of clock ticks are identical

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

What you put into an action matters a lot.

Suppose you put busy-wait I/O to the train controller into an action

Will you catch it in your testing?

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

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( half-done ) { a2.2; half-done = FALSE; }
  ...
}

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


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