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
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

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