Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!rutgers!ames!amdcad!sun!gorodish!guy
From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.lang.c,comp.arch
Subject: Re: Was the 360 badly-designed?  (was Re: Compatibility with EBCDIC)
Message-ID: <26295@sun.uucp>
Date: Sat, 22-Aug-87 02:31:37 EDT
Article-I.D.: sun.26295
Posted: Sat Aug 22 02:31:37 1987
Date-Received: Sun, 23-Aug-87 11:37:51 EDT
References: <855@tjalk.cs.vu.nl> <2683@hoptoad.uucp> <916@haddock.ISC.COM> <26291@sun.uucp>
Sender: news@sun.uucp
Followup-To: comp.arch
Lines: 22
Xref: mnetor comp.lang.c:3824 comp.arch:1865

> From what I have heard, one reason for the IBM 360's success was forsight
> on the part of the designers.  They decided that the machine should have
> 24 address bits.  At the time, 16 MBytes seemed like a heck of a lot of
> memory.  At the time, 16 MBytes *was* a heck of a lot of memory.

Unfortunately, one reason for what I presume was a big effort on the part of
IBM (can you say XA?) was *lack* of foresight on the part of the designers;
they decided that the machine should have 24 address bits.  Unfortunately, this
not only applied to things such as memory buses, it applied to effective
address formation.  As a result, everybody stuffed things into the upper 8 bits
of pointers, since they weren't used; when they ran out of the 16MB virtual
address space (by that time, it had virtual memory), they had to introduce a
mode bit to permit old 24-bit-addressing applications and new 31-bit-addressing
applications to run together.

From reading some of the XA documentation, it seems they planned for further
expansion, by using - wait for it - segmentation; it appears to have some
similarities to that provided by a machine nearer the bottom of their product
line.  :-)
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com