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Path: utzoo!utcsri!greg
From: greg@utcsri.UUCP (Gregory Smith)
Newsgroups: comp.arch
Subject: Re: 64 Vs 32
Message-ID: <4366@utcsri.UUCP>
Date: Sun, 15-Mar-87 12:58:18 EST
Article-I.D.: utcsri.4366
Posted: Sun Mar 15 12:58:18 1987
Date-Received: Sun, 15-Mar-87 15:35:25 EST
References: <3810013@nucsrl.UUCP> <985@rpics.RPI.EDU> <755@ames.UUCP>
Reply-To: greg@utcsri.UUCP (Gregory Smith)
Organization: CSRI, University of Toronto
Lines: 48
Summary: 

In article <755@ames.UUCP> fouts@orville.UUCP (Marty Fouts) writes:
>In article <985@rpics.RPI.EDU> yerazuws@rpics.RPI.EDU (Crah) writes:
>
>>Yep.  Consider- it's now a standard configuration for VAX 8800's
>>to come with 512 megs of memory (I've got the part number around
>>somewhere).  A VAX has 32 bits- so if we assume (*) that all 32
>>can be used as memory address, a VAX (or other 32-bit processor)
>>can have AT MOST 4 GIG of memory.
>>	
>c
>Although this is true, historically when the limit is first reached,
>processors aren't redesigned to use larger address spaces, but rather
>work arounds are found.  When 16 bits wasn't enough address space for
>micros, "bank selection" was utilized.  There are machines around[...]

Even if you have 128 GIG of physical memory, it is still reasonable
to restrict user processes to 4 GIG of virtual memory. Some PDP-11's
have a 22-bit physical address space and all have a 16-bit virtual
address space ( is the 22-bit figure right? ). A vax running a process
in PDP-11 mode gives it only a 64K byte virtual address space.

The point of all this being that you can still get by with 32-bit user
registers and data width - you just have to widen the MMU. 

Another question: If you get to the point where you are using more
than 2 GIG of virtual space on your 32-bit machine, and thus have
'negative' addresses, how many compiler (and other) bugs are suddenly
going to show up? You had to be careful with comparisons on the
PDP-11, and I've seen some sloppy code for 32-bit machines which assumes
that pointers are always 'positive'.

>Besides, with 4Mb memory chips, 4Gb is still (not counting ECC) 8
>THOUSAND chips; so we aren't likely to see very many machines with
>that many memory parts, soon.

I have used an 11/34 with 256K, made of 2Kx1 static rams. That's 1024
chips, (which probably used much more power per chip than a 1M chip)
and they were all on a single hex UNIBUS card, and not very densely,
either. A double-sided board.

>I still hold out for ten years, if at all.
Maybe 10 years for the 64-bit data path on a chip, but not for >32 bits
address in an external MMU chip.

-- 
----------------------------------------------------------------------
Greg Smith     University of Toronto      UUCP: ..utzoo!utcsri!greg
Have vAX, will hack...