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From: lbr@holos0.uucp (Len Reed)
Newsgroups: comp.os.msdos.programmer
Subject: Re: stealing an interrupt
Message-ID: <1990Dec19.172359.4140@holos0.uucp>
Date: 19 Dec 90 17:23:59 GMT
References: <90351.150210TOMIII@MTUS5.BITNET> <37017@cup.portal.com>
Organization: Holos Software, Inc., Atlanta, GA
Lines: 50

In article <37017@cup.portal.com> ekalenda@cup.portal.com (Edward John Kalenda) writes:
>Thomas Dwyer write:
=> Hi there.  Would some kind soul please tell me why my machine hangs when
=> I run this TSR?  What am I doing wrong?
=
=The first problem that comes to mind is that your interrupt handling
=routine is doing a jump to a location pointed to by an area in your
=data segment. You should use JMP cs:[oldint] since the data segment
=at the time the interrupt occurs will be that of the program running,
=not your TSR. You're probobly jumping off into space somewhere based on
=the value of ds:[oldint] in the current program's frame of reference.

This analysis is correct but the solution is a little dangerous.  I say
this because I assume you're going to add code rather than leave your
do-nothing skeleton in place. :-)

Rather than putting the explicit "cs:" prefix in you should ensure that
the in-effect "assumes" always match what's in the segment registers.
Having done a lot of driver and TSR programming I can assure you that
using explicit prefix overrides to save coding an "assume" will
eventually lead to problems.  Explicitly marking your assumes informs
the assembler *and* the next programmer what you thought was in the
segment registers.

In your case, before the interrupt handler you should say:
	assume cs:code, ds:nothing, es:nothing, ss:nothing

The assembler will then add the segment prefix for you.  If it's not
addressable you should get an assembly-time error.

BTW, why do you embed your pointer to the saved handler inside your
procedure?  Just say

	; [first part omitted here]
	mov ax,3100h	; terminate and stay resident
	int 21h
	;NOT REACHED

   even			; word-align the data for efficiency
old_int dd ?

	assume cs:code, ds:nothing, es:nothing, ss:nothing
new_int proc far
	jmp [old_int]
new_int endp
-- 
Len Reed
Holos Software, Inc.
Voice: (404) 496-1358
UUCP: ...!gatech!holos0!lbr