Path: utzoo!attcan!uunet!husc6!ukma!sean
From: sean@ms.uky.edu (Sean Casey)
Newsgroups: comp.graphics
Subject: Re: Amiga or PC-AT ?
Message-ID: <10216@s.ms.uky.edu>
Date: 5 Sep 88 15:31:55 GMT
References: <1820006@hpuamsa.UUCP> <46900021@uxe.cso.uiuc.edu> <31386@clyde.ATT.COM> <588@rtg.cme-durer.ARPA> <1874@iscuva.ISCS.COM>
Reply-To: sean@ms.uky.edu (Sean Casey)
Organization: The Leaning Tower of Patterson Office @ The Univ. of KY
Lines: 34

In article <1874@iscuva.ISCS.COM> jimc@iscuva.ISCS.COM (Jim Cathey) writes:
>>If one puts up five windows up on a Sun/3, and then removes the first,
>>the bank of RAM allocated for the first window will not be accessable
>>until the second, third, fourth, and fifth windows are removed.
>
>I thought that was what MMU's were for.  (You know, bunches of teeny
>little page-sized 'segments' that come and go as they please?)  Or am I
>missing the point?

Most graphics hardware expects bitplanes to be in contiguous memory. This is
because most graphics hardware refreshes by DMA or dual-ported memory,
bypassing the MMU for speed.

If a process on a Sun isn't reclaiming memory, it's probably just a feature
of the way that Unix allocates memory. Although a Unix user space process
can be physically spread all over the place, to the user, and for most
purposes it's a contiguous set of memory locations, zero through n. You get
more memory through a call to something like brk(2) or sbrk(2). The memory
is added to the end of the process space. Likewise, you can only free memory
from the end of the process space.

Thus if a window is in the middle of the process space (and I'm assuming
it's implemented as a user mode page) then unmallocing the storage will not
reduce the process size, and thus not cause any memory to be freed.

If your Unix implementation is sensible, it will realize sooner or later
that the page isn't being used and swap it out.

Sean
-- 
***  Sean Casey                        sean@ms.uky.edu,  sean@ukma.bitnet
***  (Looking for his towel)           {backbone|rutgers|uunet}!ukma!sean
***  U of K, Lexington Kentucky, USA   Internet site? "talk sean@g.ms.uky.edu"
***  ``With a name like Renderman, you know it's good jam.''