From: Neil Stone <Neil_Stone AT CyberServices DOT Com>
Newsgroups: comp.os.msdos.djgpp
Subject: Hooking int 21h and other stuff
Date: Mon, 24 Mar 1997 01:26:19 +0000
Organization: [not set]
Lines: 22
Message-ID: <3335D83A.2CC9@CyberServices.Com>
NNTP-Posting-Host: n70i056.c1r2.pol.co.uk
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

I have been trying to hook the DOS interrupt, 21h, but have been
unsuccessful in my attempts.  I have been able to hook other interrupts
of interest, keyboard, timer, etc.  But no matter what I do my computer
always crashes when hooking the DOS one.  Are then any special
precautions I should take, what registers do I need to save and restore
before calling the old ISR?  I haven't actually tried this in DJGPP, I
tried it at college - I'm 16, using TC 3.0 but I don't think DJGPP would
be much different.  From a file I read, not sure which one, I was told
it would need to be done in assembler, I don't know much about this but
am willing to learn.  As the DOS is 16-bit I think my ISR would have to
be 16-bit, i.e. real mode, as well.  How do I do this?  Does it need to
be locked? 

For real mode ISR's in general, can you use the e* registers, eax, ebx,
etc., as long as you don't access memory outside conventional.  Also, if
I have a DOS segment:offset address how do I get a selector:offset and
how do I use this selector to access memory.

Please give all answers in a not to technical form preferable with
commented source code.  Thanks in advance.

Gareth Wilson