Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!gatech!udel!new
From: new@udel.EDU (Darren New)
Newsgroups: comp.windows.x
Subject: Re: Contents of windows
Message-ID: <14706@louie.udel.EDU>
Date: 4 May 89 23:52:06 GMT
References: <4009@ficc.uu.net> <8904281155.AA02450@expire.lcs.mit.edu>
Sender: usenet@udel.EDU
Reply-To: new@udel.EDU (Darren New)
Organization: University of Delaware
Lines: 32

In article <8904281155.AA02450@expire.lcs.mit.edu> rws@EXPO.LCS.MIT.EDU writes:
>
>    That's an interesting comparison, since the Amiga windowing system does
>    guarantee a backing-store for all windows. If Intuition can do it, why
>    not X?
>
>How does Intuition do it?  Does it pop up a message: "Please insert floppy
>number 137, I need to save/retrieve some backing-store bits" ?  They must be
>trading off something somewhere.  For example, do they guarantee backing-store
>regardless of how big you later make the window?  Does that mean they
>preallocate max-sized memory chunks for every window created?  Is there a
>small limit to the number of windows that can be created?  Or does the
>"guarantee" mean the system will crash unexpectedly at some point?

There are three kinds of Amiga windows. 
"Simple" windows are like X-windows, which need to be refreshed
when exposed.  "Smart" windows need only to refresh a window
when it is made larger. When the window is covered and then
revealed, the revealed portion is displayed from
the "backing store" which is actually main memory.
(I.e., never swap to disk.) "Super-Bitmap" windows
have preallocated bitmaps when opened, and the portion
visible "through" the window is copied to the screen
memory. When a Smart window is overlapped, the obscured
potion is saved to newly allocated memory.
The other two obviously don't need on-the-fly allocation.
If the memory can't be allocated, as far as I know all that
happens is that you position the window and it fails to
move. I.e., attempts to move windows when there is 
insufficient memory result in the move request being
ignored/cancelled.
Disclaimer: I just work here. -- Darren