🚀 go-pugleaf

In <eoq.25.0@vthnw.cvm.msu.edu> eoq@vthnw.cvm.msu.edu (Edward Quillen) writes:

>In article <14339@mindlink.bc.ca> Charlie_Hutchins@mindlink.bc.ca (Charlie Hutchins) writes:
>>From: Charlie_Hutchins@mindlink.bc.ca (Charlie Hutchins)
>>Subject: Why can't I hook INT 21h???
>>Date: 17 Aug 92 17:41:03 GMT
>>Okay, maybe someone could help me here.  I am trying to write a TSR
>>that hooks interrupt 21h but whenever I try and hook that vector I
>>get an invalid command.com error and my system crashes.  I am using
>>TC++ 3.0 and I have tryed both using their set vect routine as
>>well as manually changing the interrupt vector table but in both
>>cases the same thing happens...crash!  Even if I write an ISR for
>>INT 21h that simply returns control to the original ISR the system
>>crahes.  For example, even this crashes my system:
>>
>>#include <dos.h>
>>#include <stdio.h>
>>
>>void interrupt new21handler(void);
>>void interrupt (*old21handler)();
>>
>>void main(void)
>>{
>>        disable();
>>        old21handler=getvect(0x21);
>>        setvect(0x21, new21handler);
>>        enable();
>>        keep(0, 4000);
>>}
>>
>>void interrupt new21handler(void)
>>{
>>        (old21handler)();
>>}
>>
>>Anyone know what's wrong here??? As far as I can see this program should
>>basically do nothing but take up memory but instead, it crahes the system when
>>run.  Any info would be greatly appreciated.
>>
>>...Charlie Hutchins

>Please post replies to this ? because I've always wanted to hook INT 21h
>but have never been able to. Thanks!!!

>+++++++++++++++++++++++++++++++++++++++++++++++++
>+++ ED (Edward Quillen)   (517) 336-1293      +++
>+++ Vet Teaching Hosp.    System Admin.       +++
>+++ Email: quillen@cps.msu.edu                +++



Two things to consider:
1.  Beware of the "version" of your compiler...for example Microsoft 6.0 
exhibits an improper coding of the "GETVECT" routine and returns the correct
offset but the RONG segment!!!! (By the bye...MS6.AX revision seems to do 
this OK.

2. You might also try using "_far" in front of the routine prototypea and 
instances.

Luck,
tob

>+++++++++++++++++++++++++++++++++++++++++++++++++