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From:	Charles Sandmann <sandmann AT clio DOT rice DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: locking for HW ints/RMCB
Date:	Mon, 18 Sep 1995 21:05:11 CDT
Organization:	Rice University, Houston, Texas
Lines:	14
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To:	djgpp AT sun DOT soe DOT clarkson DOT edu
Dj-Gateway:	from newsgroup comp.os.msdos.djgpp

> How can you make sure that the stack in an assembly HW int handler is 
> locked. You can keep the code and data within boundaries, but the stack 
> is accessed before you get to change the stack registers (ie to save 
> CS:EIP), or does this one use of the stack not matter (page almost always 
> will be in memory as the program itself is using it)?

All Hardware interrupts and RMCBs are handled on a locked 4Kb stack provided
by the DPMI provider (so SS:ESP won't be in the DJGPP address space).  The
_go32_dpmi ... wrappers swap onto a local 64K stack in the DJGPP address
space so SS=DS and GCC's code won't get confused (then jump back on exit).

So, if you write in assembler, you know that DS != SS and can handle this,
also knowing you must minimize stack use (with only 4K available for all
nested interrupts/execeptions).

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