Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!sco!seanf
From: seanf@sco.COM (Sean Fagan)
Newsgroups: comp.arch
Subject: Re: looking for >32-bit address space
Message-ID: <2534@scolex.sco.COM>
Date: 10 Apr 89 20:07:27 GMT
References: <1032@myrias.UUCP> <1989Apr3.164538.277@utzoo.uucp> <2516@scolex.sco.COM> <1989Apr7.194933.4861@utzoo.uucp>
Reply-To: seanf@scolex.UUCP (Sean Fagan)
Organization: The Santa Cruz Operation, Inc.
Lines: 37

In article <1989Apr7.194933.4861@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>In article <2516@scolex.sco.COM> seanf@scolex.UUCP (Sean Fagan) writes:
>>The 8086 doesn't bank-switch.  The Apple //e bank switches.  The 8086 uses
>>segments, which are similar, but, IMHO, better...
>Either way, a complete address is larger than the
>"natural" pointer size, and you can't use it directly -- you have to
>feed the segment/bank part into the machinery separately, and *then*
>use the rest of the address.
>*Real* segments, as on the Multics machines, are part of the normal
>pointer format, and to access something in a segment, you just indirect
>through the pointer.

I do admit that Intel segments are, um, unpleasant (at least, on the 8086).
On the '286 and '386, they are better, and, in fact, on the '386, they're
quite pleasant (as segments go...).  I've never played with Multics, but I
have played on a Cyber 180-state machine, which seems to have been modeled
after a Multics machine.  The segments there are *very* similar to the way
the '386 does it, except that you can use more registers for segment
registers on the Cyber (and, I presume, Multics).

>One can argue about the merits of breaking the
>linear address space up into segments, but the point is that normal
>pointer manipulation is not penalized in either efficiency or simplicity.

Neither is it on the '386.

>The real problem with >32 bit addresses is that avoiding the dreaded
>Intel Pointer Syndrome requires a 64-bit word.  Which costs more than
>a lot of people want to pay just now.

I still maintain that segments are possible a Good Thing, although I don't
like some implementations.

-- 
Sean Eric Fagan  | "An acid is like a woman:  a good one will eat
seanf@sco.UUCP   |  through your pants." -- Mel Gibson, Saturday Night Live
(408) 458-1422   | Any opinions expressed are my own, not my employers'.