Return to BSD News archive
Xref: sserve comp.org.eff.talk:9485 misc.int-property:593 comp.unix.bsd:6528
Path: sserve!manuel.anu.edu.au!munnari.oz.au!constellation!mimbres.cs.unm.edu!ogicse!uwm.edu!caen!sdd.hp.com!think.com!barmar
From: barmar@think.com (Barry Margolin)
Newsgroups: comp.org.eff.talk,misc.int-property,alt.suit.att-bsdi,comp.unix.bsd
Subject: Backing Store (was Re: Patents: What they are. What they aren't. Other factors.)
Message-ID: <1bht2vINNkq3@early-bird.think.com>
Date: 14 Oct 92 19:40:15 GMT
Article-I.D.: early-bi.1bht2vINNkq3
References: <id.6S0U.TRE@ferranti.com> <1992Oct13.055638.23596@netcom.com> <1992Oct14.033523.13036@u.washington.edu>
Distribution: inet
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 33
NNTP-Posting-Host: telecaster.think.com
In article <1992Oct14.033523.13036@u.washington.edu> tzs@stein.u.washington.edu (Tim Smith) writes:
>If Pike's backing store is what I'm guessing (storing off-screen images of
>the parts of windows that are obscured by other windows), then if it's so
>obvious, how come Apple, Commodore, Microsoft, and many others overlooked
>this technique? The 128K Mac was rather pressed for memory -- I would have
>expected them to come up with any "obvious" methods to save RAM.
Huh? It doesn't "save RAM", it *uses* RAM to reduce communication and/or
image regeneration. Many early window system implementors knew about
backing store but didn't use it because they didn't have the memory to
spare. Now that memory is cheap, it's an obvious solution whose time has
come.
My complaint with AT&T regarding the backing store patent is that they've
threatened suits against window system implementors that don't actually use
Pike's technique. I've read the patent, and it claims a very specific
implementation of backing store, using linked lists of obscured regions.
The linked lists make it efficient to find the appropriate region to
display when an obscuring window goes away, or to update the backing store
of a window as the window changes.
AT&T threated MIT with patent infringement suits over X, but when I looked
in the sources to the MIT sample server I couldn't find any comparable data
structures. I think its backing store implementation simply keeps a copy
of the entire window. I even found a comment that referred to Pike's
paper, and the comment said that it wasn't using his technique (if it gave
a reason, I think it was a technical reason, not a patent reason).
--
Barry Margolin
System Manager, Thinking Machines Corp.
barmar@think.com {uunet,harvard}!think!barmar