Chapter 1 Sample Code Real-Time Programming Two Types of ...

• A typical real-time multi-tasking program
main( )
{
...
subroutine1( ... );
...
subroutine2( ... );
...
subroutine3( ... );
...
}
Cyclic Executive
execute
every
10 ms
execute
whenever
the CPU
is available
give up
CPU time
main( )
{
...
create( task1, 10, ... );
create( task2, 20, ... );
create( task3, -1, ... );
...
} ....
task2()
{ ....
if( ...) suspend( );
...
}
Multi-tasking
1.04 OS_Enter_Critical() and
OS_Exit_Critical()
#define OS_CRITICAL_METHOD 2
#if OS_CRITICAL_METHOD == 1
#define OS_ENTER_CRITICAL() asm CLI
#define OS_EXIT_CRITICAL() asm STI
#endif
#if OS_CRITICAL_METHOD == 2
#define OS_ENTER_CRITICAL() asm {PUSHF; CLI}
#define OS_EXIT_CRITICAL() asm POPF
#endif
• Disabling interrupts affects interrupt latency, so be
carefully
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7
Task Scheduling (which one can be serviced
by CPU)
• Round-robin and time-slicing
– What is the difference?
Task No
1
2
3
Task No
1
2
3
(a)
(b)
Time
Time
Round-robin
Time-sliced
(More fairness)
a quantum
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1.07 Example 1
• Consist 13 tasks
– Two internal tasks: the idle task and a task that determines CPU
usage
– Create 11 tasks: the TaskStart() task is created by main()
– Other 10 tasks is create by TaskStart() task. This 10 tasks are based
on the same code
void main (void)
{
PC_DispClrScr(DISP_FGND_WHITE + DISP_BGND_BLACK); (1)
OSInit(); (2)
PC_DOSSaveReturn(); (3)
PC_VectSet(uCOS, OSCtxSw); // INT 80H 200HZ (4)
RandomSem = OSSemCreate(1); (5)
OSTaskCreate(TaskStart, (6)
(void *)0,
(void *)&TaskStartStk[TASK_STK_SIZE-1],
0);
OSStart(); (7)
}
– OSInit () creates two tasks : an idle task, which executes when no
other task is ready to run and a statistic task, which computes CPU
usage
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