From:	John Fine <johnfine AT erols DOT com>
Newsgroups:	alt.os.development,alt.os.assembly,comp.lang.asm.x86,comp.os.msdos.djgpp,alt.lang.asm
Subject:	Re: ISR with DJGPP in a kernel.
Date:	8 Jan 2000 15:17:11 GMT
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To:	djgpp AT delorie DOT com
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Reply-To:	djgpp AT delorie DOT com

Groman wrote:

> Now I want to create a handler for 0(the timer) in C...
> now, how exactly do I do that... I mean it jumps out of the code right in
> the middle of the function...and starts another one?
> 
> that doesn't sound right...

  It is very tricky to write an ISR in DJGPP C.  It isn't worth the
trouble.

> shoud I do the handler in assembly that calls C functions then?

  Yes.  That is the best way.  Remember, before calling the C routine,
the assembler routine must save the registers that aren't saved by C
routines and must set DS and ES equal to SS (I'm assuming you can set up
the interrupt conditions so that SS will be correct on entry to the
interrupt.  That is almost automatic as long as the ISR is in ring 0 and
you have only one ring 0 stack segment per TSS).

> or is there a way to turn an ordinary DJGPP function
> into a handler...

  I'm fairly sure there isn't.  Some Borland C compilers support
keywords to declare a routine as an ISR;  Gnu C doesn't.  You could put
some inline ASM at the start and end of the C routine, but it is very
hard to coordinate with the entry and epilog code inserted by the
compiler.
-- 
http://www.erols.com/johnfine/
http://www.geocities.com/SiliconValley/Peaks/8600/

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