RTEMS Hewlett Packard PA-RISC Applications Supplement
The RTEMS processor context switch time is RTEMS_NO_FP_CONTEXTS microsections for the HP-7100 when no floating point context switch is saved or restored. Saving and restoring the floating point context adds additional time to the context switch procedure. 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. On many processors, 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. As discussed in the Register Usage section, on the HP-7100 the every task is considered to be floating point registers and , as a rsule, every context switch involves saving and restoring the state of the floating point unit.
The following table summarizes the context switch times for the HP-7100 processor:
no times are available for the HP-7100
RTEMS Hewlett Packard PA-RISC Applications Supplement
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