Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!cwjcc!ukma!gatech!ncsuvx!mcnc!rti!bcw
From: bcw@rti.UUCP (Bruce Wright)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 80x86 numbering (was: 80486)
Summary: IBM mainframe architecture still 360-like
Message-ID: <2661@rti.UUCP>
Date: 19 Dec 88 07:01:46 GMT
References: <15892@iuvax.cs.indiana.edu>
Organization: Research Triangle Institute, RTP, NC
Lines: 47

In article <15892@iuvax.cs.indiana.edu>, bobmon@iuvax.cs.indiana.edu (RAMontante) writes:
> IBM made big bucks with downloadable microcode in their mainframes;
> they kept the S/360 architecture alive for years after they got better
> hardware, by emulating a 360 whenever a customer wanted to run an old
> program.  Maybe not so fast, except for the fact the the new iron would
> run so much faster than an original 360 anyway.

IBM mainframes are still running an architecture extremely similar to the
S/360.  A few additional instructions, and the entire 32 bits of a base
register can be used as a memory address (instead of only 24 as on the
S/360), but close enough so that most well-written assembler programs
(or even binaries) would run in native mode on the latest machines.

You are almost certainly thinking about 1401 emulation mode.  The early
S/360 and S/370 machines could emulate an old IBM 1401 machine;  I think
this emulation mode has been removed from the latest machines (but I
haven't had any reason to keep track so I don't know this for certain).

> If the '486 can be re-microprogrammed, perhaps the 8080 will ride again!
> (Or a 4004?  32-to-64 bit bus, 4-bit words, riiiiggghhhhttttttt.)
> Or more radically, Intel can make a generic engine that it can program
> as an 80x86 (for the obsolescence freaks), a 680x0 (for the
> addressing-mode freaks), another S/360 (for the IBM freaks) . . .

This has of course always been possible - you just have to remask the
ROM for the microcode.  IBM has a chipset for the S/360 based I think
on the 68k.  One point (which you may be trying to make) is that it is
not clear that changing the personality of the processor has much utility
unless you have old programs to run.  Most of the users running 1401
emulation mode were doing so because they hadn't rewritten all their code
to run in S/360 native mode (sometimes they no longer HAD the source!!) -
in fact there were several companies that made a very nice living converting
1401 binaries to S/360 COBOL code.  Anyway, after experiences like that I
don't think that there are very many companies that take such a cavalier
attitude towards their critical program sources.

If it is really interesting to be able to run, say, S/360 code on your
PC, why didn't the XT/370 and similar products go anywhere?  (this machine
could run both PC and S/360 applications on the same machine).  Would making
the chip dynamically reconfigurable really make that much difference?  I
can see that there would be a few selected applications where it would be
nice (I can even think of a couple of times it might have some marginal
utility in some of the off-the-wall type projects I sometimes get involved
in), but for the VAST run of PC-type and minicomputer applications I just
don't see where the percentage is.  Maybe I'm just getting old.

						Bruce C. Wright