FloatPoint in ISR

Joel Sherrill <joel@OARcorp.com> joel.sherrill at OARcorp.com
Mon Apr 25 18:41:19 UTC 2005

Till Straumann wrote:
> Manuel Pedro Coutinho wrote:
>> Hello again!
>> I'm now in need of float point operations inside an interuption 
>> handler in the pc386 BSP. I've seen that the initial task has to have 
>> float point but apparently this isn't enough... All my tasks now have 
>> float point attributes but the ISR just blocks after performing the 
>> first float point operation.
>> Can float point operations be done inside an ISR? and if so, can 
>> anyone tell me how to allow this? 
> An ISR is not operating in any task context - therefore it doesn't 
> matter if tasks are FP enabled.
> RTEMS has to save the register context before dispatching an ISR and for 
> sake of performance
> the FP registers are usually *not* saved. In order to detect illegal FP 
> usage by ISRs, the FPU
> should be disabled during execution of ISRs (I'm more familiar with PPC 
> than with the i386 BSP
> - a problem with the PPC BSPs I recently discovered was just that the FP 
> context was not saved
> but the FPU was not disabled either --> corruption by an ISR was hard to 
> detect. I suspect the
> i386 BSP is better in that regard and possibly disables the FPU prior to 
> dispatching to your ISR.)
> That said, here's what you have to do if you want to use the FPU from an 
> ISR:
> ISR {
> enable_FPU();
> save_caller_saved_FP_registers();
> your_fp_routine();
> restore_caller_saved_FP_registers();
> disable_FPU();
> }
> Call your 'real' routine as a subroutine from the 'wrapper'
> so you only have to save/restore the 'caller-saved' FP registers
> (according to the x86 ABI - don't know by heart) any others are
> been taken care of by the compiler.

You should be able to use the internal RTEMS FP save/restore
routines to help out here.

FWIW I have seen the same discussion and similar example code in
more than one RTOS manual.  It is a relatively common issue.  In 
general, FP operations are too heavy to allow them for all ISRs
but for the case where you need the FPU, it becomes your ISRs
responsibility to be a good citizen.

> -- Till
>> Many Thanks
>> Manuel Coutinho
>> _________________________________________________________________
>> MSN Busca: fácil, rápido, direto ao ponto.  http://search.msn.com.br

Joel Sherrill, Ph.D.             Director of Research & Development
joel at OARcorp.com                 On-Line Applications Research
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