Path: utzoo!attcan!uunet!mcvax!kth!sunic!dkuug!iesd!kjeld
From: kjeld@iesd.dk (Kjeld Flarup)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MS-DOS Redirectors:  How do they work?
Message-ID: <1916@iesd.dk>
Date: 18 May 89 07:13:40 GMT
References: <1989May17.191901.16666@ateng.ateng.com>
Reply-To: kjeld@iesd.dk (Kjeld Flarup)
Organization: Dept. of Comp. Sci., Aalborg University, Denmark
Lines: 29

In article <1989May17.191901.16666@ateng.ateng.com> chip@ateng.ateng.com (Chip Salzenberg) writes:
>I've become interested in the use of "redirectors" in extending MS-DOS's
>idea of a file system.  Essentially, there are two ways to make MS-DOS
>network:
>
>    A network SHELL intercepts interrupt 21H, notices requests for access to
>    virtual drives, and performs the requested action without MS-DOS ever
>    knowing what happened.
>    A network REDIRECTOR intercepts an MS-DOS interrupt intended
>    specifically for network extensions.  This interrupt is called at
>    strategic points in the processing of file requests, and gives the
>    network extension a chance to do its thing.
>So:  Does anyone out there know where redirectors hook in?  Even a solid
>lead on an interrupt number will gain my eternal gratitude.

The BIOS interrupt 013H gives direct acces to the direct disksystem. Instead
of working with filenames you here works with absolute sectors and tracks.
However you should not use this if you want to do network things, because 
the DOS does some critical FAT access here.
	Instead you should keep at int 021H and put a layer above that. It is
not a problem to make a program belive i got an virtual drive if it uses file
handles. However if the program uses the old DOS 1.0 file control blocks you
may have some hard work to do. Anyway you must have a tecnical reference over
MS-DOS.
	I am my self fooling around with writing a multitasking level over
DOS and here you really get into hard work.

Kjeld Flarup Christensen | "I'am now twentyseven times older than the universe
kjeld@iesd.dk            | itself." Marvin the depressed Robot.