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From: ftpam1@acad3.alaska.edu (MUNTS PHILLIP A)
Newsgroups: comp.lang.pascal
Subject: Re: Problems with timer interrupts
Message-ID: <1991Jan8.034315.15222@ims.alaska.edu>
Date: 8 Jan 91 03:43:15 GMT
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In article <1991Jan6.150656.8281@odin.diku.dk>, ballerup@diku.dk (Per Goetterup) writes...

[much deleted]

>Also, when should I call the old service routine from within my new one? -
>First, last or in the middle?

     Unless you run your program from another that uses INT 1CH, there won't
BE any old routine.  The vector points to an IRET in ROM on my machine (286
with DOS 3.3) unless changed by the user program.

     If you are simply running one program from DOS, just point the vector
to an interrupt procedure in the TP6 program.  You should reset the vector in
an exit routine (Programmer's Guide p. 247), not just at the end of the main
program.  Otherwise, abnormal program exit (like HALT, run time error, or
^C) will hash DOS for you by leaving vectors pointing into never never land
(where your program used to be).

     If you are nesting programs that use INT 1CH, you will need to carefully
consider what you want to happen before chaining the INT 1CH interrupt
procedures.  Should you decide to chain, you should do it at the end of the
interrupt procedure, after saved registers have been unstacked (but leave flags
and return address) and before the final IRET, which should be replaced by a
long jump to the original service procedure.  This all requires a lot of
inline machine code to get the stack in the right state before the long jump.

     If you think this is bad, just wait until you try ^C and/or BREAK handlers.

Philip Munts N7AHL
NRA Extremist, etc.
University of Alaska, Fairbanks