Programming Embedded Systems in C and C++

A Decent Embedded Operating System 113

void

main(void)

{

os.start();

// This point will never be reached.

} /* main() */

Because this is an important piece of code, let me reiterate what you are looking

at. This is an example of the application code you might write as a user of

ADEOS. You begin by including the header file adeos.h and declaring your tasks.

After you declare the tasks and call os.start, the task functions taskAfunction and

taskBfunction will begin to execute (in pseudoparallel). Of course, taskB has the

highest priority of the two (200), so it will get to run first. However, as soon as it

relinquishes control of the processor for any reason, the other task will have a

chance to run as well.

The other situation in which the ADEOS scheduler will not perform a context

switch is during interrupt processing. The operating system tracks the nesting level

of the current interrupt service routine and allows context switches only if the

nesting level is zero. If the scheduler is called from an ISR (as it is during the timer

tick), the bSchedule flag is set to indicate that the scheduler should be called

again as soon as the outermost interrupt handler exits. This delayed scheduling

speeds up interrupt response times throughout the system.

Context Switch

The actual process of changing from one task to another is called a context switch.

Because contexts are processor-specific, so is the code ...