Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!uoregon!lth
From: lth@uoregon.uoregon.edu (Lars Thomas Hansen)
Newsgroups: comp.sys.ibm.pc
Subject: Re: problems trying to intercept msdos function requests
Message-ID: <3932@uoregon.uoregon.edu>
Date: 23 Feb 89 01:12:08 GMT
References: <3204@ttrdc.UUCP> <788@ethz.UUCP>
Reply-To: lth@drizzle.UUCP (Lars Thomas Hansen)
Organization: University of Oregon, Computer Science, Eugene OR
Lines: 23

In article <788@ethz.UUCP> zu@bernina.UUCP (Urs Zurbuchen) writes:
>Why do you use an interrupt vector to store the old vector, anyway. Just
>get the old vector from DOS (AH=35h, AL=number, INT 21h) save it
>somewhere in your program's memory space. Then set your entry point via
>DOS (AH=25h,AL=number,INT 21h). You will then hopefully get called :-)
>
>To chain to the original interrupt handler issue a far JMP (or CALL, but
>I'd prefer JMP) indirect via the vector you stored in your program's
>work space.

Have you considered that the DOS routine called via the FAR JMP/CALL will
issue an IRET (which pops three words off the stack; FLAGS, IP and CS) and
not a RET (which pops only IP and CS)?

I think this is a sticky issue. The scheme of putting the original interrupt
vector somewhere and doing a software interrupt to get at it might actually
be safer.


--lars

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