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

Two Types of Real-time System
Chapter 1 Sample Code
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• Must finish operations by deadlines.
– Hard real time: missing deadline causes failure.
• Tasks have to be performed not only correctly but
on time
– Soft real time: missing deadline results in degraded
performance.
• Tasks are performed by the system as fast as
possible
• Most real-time systems have a combination of
SOFT and HARD requirements.
• Most real-time systems are embedded
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Real-Time Programming
Multi-task Programming
Single Task:
Multi Task:
Subroutine 1
Task A
Task C
Main routine
•••
Task B
Task D
Subroutine n
Subroutine
No
1
2
3
Task
No
1
2
3
(a) Sequential subroutine calls (cyclic executive)
(b) Multi-tasking
subroutine 3 finished
calling subroutine 2
task 1 suspended
resume task 3
time
time
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• 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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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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Listing 1.3 Example #1, TaskStart()
void TaskStart (void *data)
{
#if OS_CRITICAL_METHOD == 3 /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr;
#endif
char s[100];
INT16S key;
pdata = pdata; (1) /* Prevent compiler warning */
TaskStartDispInit(); (2) /* Initialize the display */
OS_ENTER_CRITICAL(); (3)
PC_VectSet(0x08, OSTickISR); (4)
PC_SetTickRate(200); (5)
OS_EXIT_CRITICAL(); (6)
OSStatInit(); (7)
TaskStartCreateTasks()
(8) Create 10 identical tasks;
for (;;) {
TaskStartDisp();
// Display the number of tasks created;
if (key was pressed) {
if (key pressed was the ESCAPE key) {
PC_DOSReturn();
}
}
Delay for 1 Second;
}
}
Listing 1.4 Example #1 TaskStartCreateTasks()
static void TaskStartCreateTasks (void)
{
INT8U i;
for (i = 0; i < N_TASKS; i++) { /* Create N_TASKS identical tasks */
TaskData[i] = '0' + i; /* Each task will display its own letter */
OSTaskCreate(Task,
(void *)&TaskData[i],
&TaskStk[i][TASK_STK_SIZE - 1],
i + 1);
}
}
If you run code in an embedded application, you should always enable the ticker within the first task
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Example #1 Determining the PC’s speed
void OSStatInit (void)
{
OSTimeDly(2); (1)
OS_ENTER_CRITICAL();
OSIdleCtr = 0L; (2)
OS_EXIT_CRITICAL();
OSTimeDly(OS_TICKS_PER_SEC); (3)
OS_ENTER_CRITICAL();
OSIdleCtrMax = OSIdleCtr; (4)
OSStatRdy = TRUE; (5)
OS_EXIT_CRITICAL();
}
• OSIdleCtrMax contains the largest value of the OSIdleCtl
• Use OSStatTask() to compute the CPU utilization, which
executes every second
Listing 1.5 Example #1 Task that displays a number
at random locations on the screen
void Task (void *data)
{
UBYTE x;
UBYTE y;
UBYTE err;
for (;;) {
OSSemPend(RandomSem, 0, &err); (1)
x = random(80); (2)
y = random(16);
OSSemPost(RandomSem); (3)
PC_DispChar(x, y + 5, *(char *)data, DISP_FGND_LIGHT_GRAY); (4)
OSTimeDly(1); (5)
}
}
• TaskStart () creates all the 10 identical tasks, and no
context switch occurs because TaskStart() has a priority of
0 (the highest priority)
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