Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!labrea!aurora!eos!ames!hao!husc6!tut.cis.ohio-state.edu!ut-sally!utah-cs!utah-gr!uplherc!sp7040!obie!wsccs!terry
From: terry@wsccs.UUCP (terry)
Newsgroups: comp.arch
Subject: Re: More than 32 bits needed where?
Message-ID: <178@wsccs.UUCP>
Date: 23 Feb 88 03:13:49 GMT
References: <235@unicom.UUCP> <28200089@ccvaxa> <3104@watcgl.waterloo.edu> <1333@vaxb.calgary.UUCP>
Lines: 53
Summary: 48 bits is a kludge

In article <1333@vaxb.calgary.UUCP>, radford@calgary.UUCP (Radford Neal) writes:
> Why is everyone assuming that if you want greater than 32 address bits
> you need 64?

	Because it's orthogonal if I want to grab the memory in ( GOD *FORBID*)
Intel style chunks.  Besides, if I'm silly, I may want to multiplex by putting
it together like so:

   ----   ----   ----     32  \ 64
___|  |___|  |___|  |___      /
			  data  address

> My guess is that 48 (or maybe even 40) bit addresses will be used.

	48?  48?  Is this some throwback to the old VOS systems from Harris
that had 24?  Porting to and fro from such an architecture is pure pain,
regardless of how much time the company has been building the OS.

> If you assume that memory is dominated by such addresses, or
> other quantities held in the same word size, going to 64 costs 33% more
> than using 48. Note that for the systems mentioned, memory must surely 
> be a large part of the total cost.

	Why, simply becuase I have the ability to access more memory without
some kludge of a paging algorythm, does my 64 meg cost me more?

> Now I can see the flames coming on this one... How increadibly short
> sighted! History shows that we *always* need more address bits... Please
> calculate the amount of memory addressable with 48 bits, then ask whether
> systems with this much memory, if they ever appear, will have anything
> like a current machine architecture anyway...

	There are *already* chips capable of addressing a Terabyte.  And
they look just like they look... in other words, semi-standard architecture.

> Of course, there are other reasons for wanting long addresses, than just
> access to physical memory.

	You bet!  What if I, for instance, wanted a seperate byte address
for every character in every book in the library of congress; Or maybe I
want to do interesting thing with a distributed system over an ethernet.

	The main point is to plan ahead.  As to your argument about
whether or not the architecture will be recognizable, so what?  How are
we going to develop new architectures without the capability to play
around with them in the first place?  You have to start somewhere...


| Terry Lambert           UUCP: ...!decvax!utah-cs!century!terry              |
| @ Century Software       or : ...utah-cs!uplherc!sp7040!obie!wsccs!terry    |
| SLC, Utah                                                                   |
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