Path: utzoo!attcan!uunet!husc6!rutgers!att!whuts!homxb!houxv!osh
From: osh@houxv.UUCP (D.OSHINSKY)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Help, window library needed
Summary: undocumented global variable "_video"
Keywords: window, Turbo C
Message-ID: <816@houxv.UUCP>
Date: 14 Nov 88 17:02:07 GMT
References: <386@iemisi.UUCP> <42724@yale-celray.yale.UUCP>
Organization: AT&T Bell Laboratories, Holmdel
Lines: 34

In article 23090, Joel Spolsky writes:

> I understand there is an undocumented variable that can be used to
> keep Turbo-C (in directvideo mode) from trying to prevent snow (i.e.,
> how can you tell Turbo-C not to wait for Horiz. refresh to end before
> it writes directly to the screen). Does anybody know how to do this?

Article 23150, by Jerry Berlin, contains the following reply:

> This variable is described on page 52 of the Turbo C Addendum to
> version 1.5 and is named directvideo.

The directvideo variable controls whether output goes directly to video RAM
(directvideo = 1, the default) or through ROM BIOS calls (directvideo = 0).
The version 1.5 documentation does not describe how to control snow
removal when directvideo = 1.

There is an undocumented structure called _video which contains a field
which controls snow removal.  If you look in a MAP file produced by the
linker, you will see this global variable listed.  The following code works
with Turbo C versions 1.5 or 2.0:

	extern char _video;

	*(&_video+10) = X;   /* X=1 turns on snow removal, X=0 turns it off */

Beware of the fact that this may break in future versions of Turbo C
if the offset of the "snow" field changes.

				David Oshinsky
				AT&T Bell Laboratories
				Holmdel, NJ
				UUCP:  att!houxv!osh
				Phone: (201)949-0037