Path: utzoo!attcan!utgpu!watserv1!maytag!watstat.waterloo.edu!dmurdoch
From: dmurdoch@watstat.waterloo.edu (Duncan Murdoch)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Detecting direct screen writes on an 8088 for use in remote access
Keywords: remote access
Message-ID: <1990Dec18.025133.14539@maytag.waterloo.edu>
Date: 18 Dec 90 02:51:33 GMT
References: <6982@uceng.UC.EDU>
Sender: daemon@maytag.waterloo.edu (Admin)
Organization: University of Waterloo
Lines: 15

In article <6982@uceng.UC.EDU> dsims@uceng.UC.EDU (david l sims) writes:
>Programs like Carbon Copy & PC-Anywhere let the remote user run programs
>that use direct screen writes. How do CC & PCA do it on an 8088? (I've
>heard that the 80386 can set "hardware breakpoints", but I digress.)

I don't know how those ones do it, but Desqview does the same thing when
you do direct writes to its video buffer, and has a patent (!) on its
algorithm.  I don't know how DV does it either, but from the look of it,
it checks very frequently near the cursor, less frequently away from it.

Another idea is to use error-correcting code methods:  sophisticated variants 
on keeping row and column checksums might make it fast to detect changes
without comparing every byte. 

Duncan Murdoch