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From: rod@isi.edu (Rodney Doyle Van Meter III)
Newsgroups: comp.arch
Subject: Re: Adding fire to the segmentation flamefest...
Keywords: segments: do they really suck?
Message-ID: <17629@venera.isi.edu>
Date: 18 Apr 91 17:02:31 GMT
References: <9234@lkbreth.foretune.co.jp> <45180@super.ORG> <9244@lkbreth.foretune.co.jp> <2275@cluster.cs.su.oz.au>
Reply-To: rod@venera.isi.edu (Rodney Doyle Van Meter III)
Distribution: comp
Organization: Information Sciences Institute, Univ. of So. California
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I came in late on this discussion, so forgive me if if this has been
mentioned.

I don't know what became of it, but in 1984 Jim Kajiya and William
Dally of Caltech proposed a segmented architecture with floating point
addresses. The exponent determines where in the remaining bits the
division between segment number and offset into the segment occurs.
The exponent is also considered to be part of the segment number, so
an exponent of 1 and segment fraction of 0 is different from an
exponent of 2 and a segment fraction of 0.

They give as an example MULTICS addressing. MULTICS had 36-bit
addresses partitioned as an 18-bit segment number and an 18-bit
offset. This is awkward if you want segments bigger than a quarter
meg, or if you need more than a quarter million segments.

With their architecture, 5 bits out of 36 would be the exponent,
indicating how the other 31 bits are partitioned between segment and
exponent. This gives the ability to address up to 8 billion segments,
and segments can be up to 2 billion words long.

This work of theirs was part of a proposed object-oriented machine.
They expecet one segment per object. Access between segments will have
to be very fast, unlike some other machines.

Check out Caltech Technical Report 5168:TR:84 if you're interested.

			--Rod