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Using the FPU in Interrupt HandlersRTKernel-32 maintains an FPU context for each thread when a Floating Point driver such as Fltpii.lib has been linked and the thread was created with flag TF_MATH_CONTEXT. However, no FPU context is maintained for interrupt handlers. Thus, interrupt handlers should not use the FPU. Instead, interrupt handlers should delegate floating point calculations to a thread. If an interrupt handler must nevertheless use the FPU (which is not recommended), it must save and restore the FPU context. This can be done as in this example:
#include <Rtk32.h>
#include <Rtkflt.h>
...
typedef struct { ... } DeviceContext;
...
void * FPUContext; // could be part of DeviceContext
...
int RTKAPI IntHandler(void * P) // high level interrupt handler
{
DeviceContext * Context = (DeviceContext*) P; // get device context
if (Context->StatusRegister != 0) // any interrupt condition set?
{
_rtkFLTSave(FPUContext); // save FPU registers
_fpreset(); // reset FPU
... // do calculations here
_rtkFLTRestore(FPUContext); // restore FPU registers
return 1; // done
}
else
return 0; // this int is not for me
}
...
int main(void)
{
RTKernelInit(0); // must start the kernel before using the FPU driver
FPUContext = calloc(_rtkFLTDataSize(), 1); // must be zero initialized!
RTInstallSharedIRQHandlerEx(IRQ, IntHandler, &Context);
// program device to start sending interrupts here
...
}
A separate FPU context for each interrupt handler - which needs the FPU - is required. It is required to zero-initialize the FPU context once before it is used for the first time. Function _rtkFLTDataSize must not be called before RTKernelInit. Please note that the value returned by _rtkFLTDataSize is CPU dependent and will increase with future CPU models, even when the driver Fltpii.lib is not updated. RTKernel-32 will not call the same interrupt handler recursively. It is thus sufficient to maintain exactly one global FPU context per interrupt handler. Saving/resetting/restoring the FPU context in interrupt handlers consumes a lot of CPU time, degrading the overall real-time performance of the application. No ISR of any device driver shipped with On Time RTOS-32 makes use of the FPU.
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