RTEMS Porting Guide
The CPU_USE_DEFERRED_FP_SWITCH macro is set based upon the answer to the following question: Should the saving of the floating point registers be deferred until a context switch is made to another different floating point task? If the floating point context will not be stored until necessary, then this macro should be set to TRUE. When set to TRUE, the floating point context of a task will remain in the floating point registers and not disturbed until another floating point task is switched to.
If the CPU_USE_DEFERRED_FP_SWITCH is set to FALSE, then the floating point context is saved each time a floating point task is switched out and restored when the next floating point task is restored. The state of the floating point registers between those two operations is not specified.
There are a couple of known cases where the port should not defer saving the floating point context. The first case is when the compiler generates instructions that use the FPU when floating point is not actually used. This occurs on the HP PA-RISC for example when an integer multiply is performed. On the PowerPC, the printf routine includes a save of a floating point register to support printing floating point numbers even if the path that actually prints the floating point number is not invoked. In both of these cases, deferred floating point context switches can not be used. If the floating point context has to be saved as part of interrupt dispatching, then it may also be necessary to disable deferred context switches.
Setting this flag to TRUE results in using a different algorithm for deciding when to save and restore the floating point context. The deferred FP switch algorithm minimizes the number of times the FP context is saved and restored. The FP context is not saved until a context switch is made to another, different FP task. Thus in a system with only one FP task, the FP context will never be saved or restored.
The following illustrates setting the CPU_USE_DEFERRED_FP_SWITCH macro on a processor family such as the M68K or i386 which can use deferred floating point context switches.
#define CPU_USE_DEFERRED_FP_SWITCH TRUE
RTEMS Porting Guide
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