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From: cjmchale@cs.tcd.ie (Ciaran McHale)
Newsgroups: comp.windows.x
Subject: Re: PC X servers / backing store
Message-ID: <1991Mar5.165633.17593@cs.tcd.ie>
Date: 5 Mar 91 16:56:33 GMT
References: <9103032356.AA00215@devnull.Eng.Sun.COM> <952@boing.UUCP>
Organization: DSG, Dept. of Comp. Sci., Trinity College, Dublin.
Lines: 44

In <952@boing.UUCP> dale@boing.UUCP (Dale Luck) writes:

>In article <9103032356.AA00215@devnull.Eng.Sun.COM>
>dshr@eng.sun.COM (David Rosenthal) writes:
>
>>This is a very important point to understand.  If we had been prepared to
>>*require* the server to maintain backing store for windows and to make
>>a number of other similar decisions during the design of X,  we would have
>>ended up with a completely different design which would have
>>been faster and simpler but which would have consumed much more resource.
>>It would,  for example,  have been improbable that an X-terminal or a
>>PC X-server would have been a viable product had we taken this route.
>
>I couldn't agree with you less on this last statement. Case in point:
>The Amiga guarentees backing store with it's SMART_REFRESH windows.
>The programmer is able to choose whether they can handle expose
>events or not. SIMPLE_REFRESH windows always generate expose events
>whenever it is uncovered. SMART_REFRESH windows do not.
>If the Amiga was able to do this in a 16k layer library and a 64k
>graphics library (nearly all written in C) then it could be done
>in the X Window system as well.

The problem isn't in writing code to support the
unconditionaly maintainence of backing store; the problem is
that you can't be sure the X server has enough main memory
to do so. I don't know much about the Amiga but in the X
window system a window can be quite large: 2^16 wide by 2^16 high.
(that's 2^9 Megabytes for a mono window if my maths is correct.)
I don't think it is reasonable to expect an X server to maintain
backing store for windows that big. I would imagine that the
Amiga either (i) puts an upper limit on how big a SMART_REFRESH
window can be, or (ii) runs out of memory for large
SMART_RESFRESH windows.

X *could* have done this. But it didn't because the
designers thought that it might be a bit limiting.


Ciaran.
-- 
Ciaran McHale		"Verbosity says it all"			      ____
Department of Computer Science, Trinity College, Dublin 2, Ireland.   \  /
Telephone: +353-1-772941 ext 1538	FAX: +353-1-772204	       \/
Telex: 93782 TCD EI			email: cjmchale@cs.tcd.ie