Programming Embedded Systems, 2nd Edition

Other Functionality

We have now covered the basic mechanisms that are commonly found in most real-time operating systems. Several other features may be included, depending on the operating system, to perform various other useful operations. Some of the other mechanisms commonly found in real-time operating systems are:

Event flags

These allow a task to wait for multiple events to occur before unblocking. Either all of the events must occur (the events are ANDed together) or any of the events may occur (the events are ORed together).

Condition variables

These are similar to a counting semaphore where a task signals another task to wake up; however, unlike a semaphore, if no task is currently waiting on the condition variable when it is signaled, the signal is lost.

Spinlocks

These are similar to a mutex and are typically used in symmetric multiprocessing (SMP) systems. Like a mutex, a spinlock is a binary flag that a task attempts to claim. If the flag is not set, the task is able to obtain the spinlock. If the flag is set, the task will spin in a loop, constantly checking to see when the flag is not set. This might seem wasteful (and it can be); however, it is assumed that the spinlock is only held for a very short period of time, and this CPU must wait for software running on the other CPU to progress first anyway.

Counters and alarms

A counter keeps track of the number of times a specific event has occurred. An alarm is used in conjunction with a counter to wake up a task (so that ...