RTEMS Motorola MC68xxx Applications Supplement
An RTEMS context switch is defined as the act of taking the CPU from the currently executing task and giving it to another task. This process involves the following components:
RTEMS defines the hardware state of a task to include the CPU's data registers, address registers, and, optionally, floating point registers.
Context switch time is often touted as a performance measure of real-time executives. However, a context switch is performed as part of a directive's actions and should be viewed as such when designing an application. For example, if a task is unable to acquire a semaphore and blocks, a context switch is required to transfer control from the blocking task to a new task. From the application's perspective, the context switch is a direct result of not acquiring the semaphore. In this light, the context switch time is no more relevant than the performance of any other of the executive's subroutines which are not directly accessible by the application.
In spite of the inappropriateness of using the context switch time as a performance metric, RTEMS context switch times for floating point and non-floating points tasks are provided for comparison purposes. Of the executives which actually support floating point operations, many do not report context switch times for floating point context switch time. This results in a reported context switch time which is meaningless for an application with floating point tasks.
The actual context switch times are reported in the Timing Data chapter of this supplement.
RTEMS Motorola MC68xxx Applications Supplement
Copyright © 1988-2004 OAR Corporation