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From: guy@rlgvax.UUCP (Guy Harris)
Newsgroups: net.micro.68k
Subject: Re: 68K MMU is good for something
Message-ID: <1508@rlgvax.UUCP>
Date: Sat, 31-Dec-83 00:50:08 EST
Article-I.D.: rlgvax.1508
Posted: Sat Dec 31 00:50:08 1983
Date-Received: Sun, 1-Jan-84 02:27:06 EST
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Organization: CCI Office Systems Group, Reston, VA
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<PLEASE fix this bug, people!>

	The Mororola MMU is, however, the best chip for the Buddy
	System, a good algorithm for segment allocation on a uniprocessor.

Good, that is, *if* you have lots of physical memory and/or lots of 68451s.
If you want to allocate segments of arbitrary size (not an uncommon thing
to do on UNIX), you have to find a set of powers of 2 which add to some
number greater than or equal to the size of the segment.  If you do this
the "fast" way, by allocating segments for each 1 bit in the binary
representation of the segment size, you end up with a lot of large lumps
which *have* to sit on high-power-of-2 boundaries.  If you try more clever
schemes, it takes more CPU time to figure out the segment sizes (although
this needs only be done when the segment size is set) and it takes more
segmentation registers.

	Guy Harris
	{seismo,ihnp4,allegra}!rlgvax!guy