Programming Embedded Systems in C and C++

A Decent Embedded Operating System 111

Scheduler

Because I use an ordered linked list to maintain the ready list, the scheduler is

easy to implement. It simply checks to see if the running task and the highest-

priority ready task are one and the same. If they are, the scheduler’s job is done.

Otherwise, it will initiate a context switch from the former task to the latter. Here’s

what this looks like when it’s implemented in C++:

/**********************************************************************

*

* Method: schedule()

*

* Description: Select a new task to be run.

*

* Notes: If this routine is called from within an ISR, the

* schedule will be postponed until the nesting level

* returns to zero.

*

* The caller is responsible for disabling interrupts.

*

* Returns: None defined.

*

**********************************************************************/

void

Sched::schedule(void)

{

Task * pOldTask;

Task * pNewTask;

if (state != Started) return;

//

// Postpone rescheduling until all interrupts are completed.

//

if (interruptLevel != 0)

{

bSchedule = 1;

return;

}

//

// If there is a higher-priority ready task, switch to it.

//

if (pRunningTask != readyList.pTop)

{

pOldTask = pRunningTask;

pNewTask = readyList.pTop;

pNewTask->state = Running;

pRunningTask = pNewTask;

if (pOldTask == NULL)

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