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From:	Charles Sandmann <sandmann AT clio DOT rice DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Hardware interrupt stack and task switching
Date:	Fri, 03 Oct 2003 09:42:22 CDT
Organization:	Rice University, Houston, TX
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> I can do task level switching, but it seems like one cannot do task
> switching from a hardware interrupt routine. 

Not directly.  This is outlined in the DPMI spec.

> So my question: is the DPMI host allocating stacks for each HW ISR? 

Yes - it does this for stability.  When a HW interrupt comes in, it
needs to service the interrupt, even if the current user stack is
corrupt, or paged out to disk, or too small.  Same thing for exceptions.

> If that is so, how can I do task switching from the ISR? Is it at
> all possible?

Yes, you can with some tricks.  The DJGPP library has a feature which
allows you to convert a hardware interrupt into a fake exception.  It
does this by invalidating the DS selector limit - so the next access
to the stack or memory will allow you to task swap.  The interval
timer works this way (that's how we can support profiling and wakeup
timers).

You can either look at the signals support to do this in a portable
way, or examine the source for the library routines (djdev203.zip)
to see how to do the conversion from interrupt to exception yourself.

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