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From: bzs@world.std.com (Barry Shein)
Newsgroups: comp.arch
Subject: Re: 64-bit addresses
Message-ID: <1990Mar2.232735.6071@world.std.com>
Date: 2 Mar 90 23:27:35 GMT
References: <WAYNE.90Feb27094326@dsndata.uucp> <1786@gannet.cl.cam.ac.uk>
Organization: The World @ Software Tool & Die
Lines: 74


>On IBM 3090s (probably the mainframe that the previous poster had in mind)
>"expanded storage" is RAM which is not directly addressable, but is moved
>in 4Kbyte chunks to and from directly addressable storage by synchronous
>instructions. It is used by MVS and VM as a very-high-speed paging device;
>microcode assists can do the transfers without a software page fault in
>some circumstances.
>
>If someone would like to explain what the economic advantages are to 
>providing extra storage in this form, I would be interested to listen.
>
>
>Chris Thompson

In the first place, the 370 is basically limited to a 24-bit (16MB)
address space. Several instructions used the high eight bits of
addresses to store other things and, probably more importantly, so did
applications (such as type fields for pointers.)

They also use a base+displacement addressing scheme which usually (one
can in theory code around it) requires one, and sometimes more,
registers to be dedicated to base addressing. The displacement is in
the instruction and is limited to 12-bits (4K.)

A few years ago IBM came up with XA which added some new instructions
(e.g. BASR) replacing the ones which used the hi eight bits in
addresses and declared the machine now safe, from their side, for
using the full 32 bits of address. Of course, that didn't change the
applications, so you had to have well behaved applications or be
willing to rewrite them (usually not a horrendous rework, but
definitely a chore.)

At that point of course they're out of juice, and about all one can do
is what was described, use any more memory as a fast place to store
things while they're not in use rather than going to disk. I wonder
why they didn't re-invent the separate I&D space of the PDP-11?
Perhaps DEC holds a patent on that scheme, it would at least double
the address space (although it's likely that it's almost all needed in
the data space, I doubt many people have gigabytes of code that needs
to be resident.)

The major economic advantage is not having to move to a non-370
architecture.

The 370 goes back to the 360 which goes back to the early 1960's.

Needless to say it's not an environment that was driven by
portability, a lot of applications are written in assembler, not to
mention that even higher level language applications can be full of
tons of System/370 specific stuff (JCL and all that.) And this is a
world with megalines of code, you don't just sit down and rewrite it
all if you can avoid it.

The 3090 still provides fairly awesome I/O for the price, this is the
class of machines used on those terabyte databases people like
Mastercard or J.C. Penney's have to manage.

It's not a cost-effective compute server if you were starting out
today with fresh code (of course, even in the compute server world
there are people who have tons of code locked into the 370
architecture.) It probably still is a cost-effective database
transaction machine for very large databases and other similar
data-intensive tasks. People have told me that some Crays are
competitive, but few other machines (note that Amdahls are 370 clones,
so they're in the same ballpark.) It hardly has any place in an
academic computing environment even though some diehards maintain them
against all reason.

So, there's reasons to keep kicking that dead whale down the beach.
-- 
        -Barry Shein

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