From: Chris Frolik <frolikcc AT indy DOT net>
Newsgroups: comp.os.msdos.djgpp
Subject: Real-mode pointers in real-mode interrupt
Date: Wed, 27 Aug 1997 22:16:49 -0500
Organization: IndyNet - Indys Internet Gateway (info AT indy DOT net)
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I have a question which I believe is beyond the scope of the FAQ, as I
have read through the relevant sections.  What I am asking may not be
possible in DJGPP or protected mode, but if anyone knows how, I'd be
grateful if they gave me some hints on how to do this.

My problem is using real-mode pointers in a real-mode interrupt. 
Basically, I am writing a library for IPX in DJGPP, and it requires the
use of interrupt 0x7A, which is hooked by the packet driver.  If I
could, I'd just re-write the interrupt, but since that isn't possible, I
have to use it as it is.  Here is what I am trying to do:

I have a structure (S) which holds (among other things) a pointer to
more data (P).  The interrupt requires that ES:SI points to this
structure.  I can do this using __dpmi_int just fine.  The problem here
is with the pointer (P) inside that struct (S).  Interrupt 0x7A expects
that this will be a real-mode pointer to more data, but of course I
don't think it is possible to get the segment:offset of a pointer or
variable in DJGPP.  So, I thought about doing what the FAQ says --
placing my data in conventional memory.  I can put the struct (S) there
and have the ES:SI in the _dpmi_regs struct point to this conventional
memory.  But, it is not enough just to put my struct in conventional
memory, because the pointer in that struct still points to something in
DJGPP's flat address space.  Since the interrupt expects this to be a
real-mode pointer, and the FAQ says that conventional memory uses only
20-bit addresses, I am stuck.  If anyone knows how I can get this to
work, please let me know.  Thanks,

-Chris