RTEMS 4.6.99.3 On-Line Library

PSIM Timing Data Context Switch

RTEMS PowerPC Applications Supplement

10.4: Context Switch

The RTEMS processor context switch time is 214 instructions on the PSIM benchmark platform when no floating point context is saved or restored. Additional execution time is required when a TASK_SWITCH user extension is configured. The use of the TASK_SWITCH extension is application dependent. Thus, its execution time is not considered part of the raw context switch time.

Since RTEMS was designed specifically for embedded missile applications which are floating point intensive, the executive is optimized to avoid unnecessarily saving and restoring the state of the numeric coprocessor. The state of the numeric coprocessor is only saved when an FLOATING_POINT task is dispatched and that task was not the last task to utilize the coprocessor. In a system with only one FLOATING_POINT task, the state of the numeric coprocessor will never be saved or restored. When the first FLOATING_POINT task is dispatched, RTEMS does not need to save the current state of the numeric coprocessor.

The following table summarizes the context switch times for the PSIM benchmark platform:

No Floating Point Contexts	214
Floating Point Contexts
restore first FP task	255
save initialized, restore initialized	140
save idle, restore initialized	140
save idle, restore idle	290

RTEMS PowerPC Applications Supplement