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From: rws@EXPO.LCS.MIT.EDU
Newsgroups: comp.windows.x
Subject: Quarterdeck patent: ado about nothing
Message-ID: <8905241219.AA00789@expire.lcs.mit.edu>
Date: 24 May 89 12:19:53 GMT
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Organization: The Internet
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There were headlines a couple of weeks ago about "Quarterdeck Patents Windows".
I suspect I'm not the only one who was curious.  I have a copy of the patent
(thanks to Karen Kramaric at the MIT Technology Licensing Office).  It was
apparently first filed by Quarterdeck Office Systems on May 2, 1984, and then
a revised version was filed on March 17, 1988.  The Inventor is Gary W. Pope.

The patent deals with a particular mechanism for having (multiple) application
programs avoid the overhead of making "operating environment calls" (read
"system calls") to write into the video display.  Each application is given a
"Pseudo Screen Buffer" that it can write into as direct memory without
interference.  Using a timer interrupt, the system periodically compares these
buffers against a "Previous Image Buffer", which contains a copy of the actual
screen contents.  Where they differ, an update is done to the Previous Image
and the display.  An algorithm for making this "efficient", based on various
heuristics, is described.  Overlapping windows are handled with a Screen Map:
each cell of the Screen Map contains the id of the top-most window overlaying
that cell, and updates to a particular cell in the Previous Image and display
are conditional on the Pseudo Screen Buffer id matching the id in the Screen
Map.  Overlap support is clearly assuming character-cell graphics; some of the
patent hints at "bits" in addition to characters, but most of the language in
the patent deals with "character" output.  There is also some stuff about
calculating memory locations "cleverly" to make some computations efficient,
which appears to be dependent on a very specific memory layout.

I can't see anything in the patent that should cause anyone I know to worry.