Xref:	news2.mv.net comp.os.msdos.djgpp:5552 comp.os.msdos.programmer:26024
From:	Shaune Beattie <shaune AT beattie DOT demon DOT co DOT uk>
Newsgroups:	comp.os.msdos.programmer,comp.os.msdos.djgpp
Subject:	Help please! protected mode "TSR"'s
Date:	Sun, 30 Jun 1996 19:58:43 +0100
Lines:	59
Message-ID:	<31D6CE63.353587C0@beattie.demon.co.uk>
NNTP-Posting-Host:	beattie.demon.co.uk
MIME-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Hi,
Basically I want to write a program that traps a HW interrupt while a
DPMI32 program is running.
Obviously writting a normal "real" mode TSR is no good as interrupts are
passed to the protected mode first (and the program in question will
also be hooking the same interrupt I want to).

If I could write a protected mode TSR as described in the DPMI 1.0 spec,
then what I would do is in my initialisation function, hook both the
hardware interupt i want, and also int 31H..
this way when the program also tried to hook the same interrupt i can
make sure i reinsert my interupt handler so that it gets called first,
optionally chaining on to the handler that the program has installed.

however the program will be running under a dpmi v0.9 host.. and hence I
can't use the 1.0 spec functions..

so somehow I need to kludge it...

my thought is that if I install a normal real mode tsr on an real mode
interrupt that I know the target program will also call.. then I can
then make calls to the dpmi host that has been loaded to run the target
program, hopefully then what I can do is call the int 31H routines to
obtain the real mode to protected mode switch routine.. call it.. then
allocate some protected mode memory, allocate a descriptor and point it
at that memory, setting it to be executable, 32bit etc.
then copy the rest of the real mode tsr routine (which is actually the
32bit protected mode program) into that memory, finally calling into
it..
that program will then hook the protected mode interupts as normal etc.


first of all, this is pretty horendous.. any easier ways of doing it?
secondly what are the chances of it working?
can I call the real to protected mode switch routine whilst in a
interrupt service routine? is this at all valid? what about the rest of
the int 31H functions.. are these valid to call whilst in a real mode
interupt service routine?

do i actually need to bother with the copy.. or can i simply create a
descriptor pointing at the rest of my code whereever it happens to be in
dos memory?

if some of this is completely off track it is because i only started
writting ms-dos programms 2 weeks ago :-) before then I spent all my
time in the isolated worlds of full 32bit programs and didnt have to
worry about all this stuff.
so one really stupid question.. what signifies the difference between 32
and 16 bit code? once my code calls the real to protected mode switch..
so it is running in protected mode.. is there anything stopping me
simply executing 32bit code straight away? and hence ignore all the
allocating memory and copying parts? or do I need to create a descriptor
that marks the memory as being 32bit code to make the 16->32bit switch?

I hope i explained at least some of this well enough for someone to give
me any hints

thanks for any help
Shaune

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