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From: doug@terak.UUCP (Doug Pardee)
Newsgroups: net.arch
Subject: Re: I like segmented architectures
Message-ID: <603@terak.UUCP>
Date: Tue, 11-Jun-85 11:22:51 EDT
Article-I.D.: terak.603
Posted: Tue Jun 11 11:22:51 1985
Date-Received: Sun, 16-Jun-85 00:46:10 EDT
References: <276@spar.UUCP> <5653@utzoo.UUCP> <206@ganehd.UUCP>
Organization: Terak Corporation, Scottsdale, AZ, USA
Lines: 33

> The iNTEL segments
> are just big versions of the 4k addressing problem on the IBM mainframes 
> except on the IBM you can use 24bit pointers.

Once again I must point out that the IBM 360/370/30xx architecture is
*not* segmented.  Addresses run linearly from 0 to 16M-1 for non-XA
systems, 0 to 2G-1 for XA.

What *was* botched was that there is no PC-relative addressing mode for
branching.  Since there is also no direct addressing mode available,
this means that in order to branch, you first have to load a register
with the address of the destination.  Or, the more common approach, keep
a register loaded with a procedure code address and then use the
(register + offset) addressing mode.  The offset is limited to 0-4095,
giving rise to the erroneous claims that the IBM architecture has 4K
segments.

The "proper" way around this is to limit your routines to 4096 bytes in
length, and when you call a subroutine you should load its address into
a register to address it rather than using the (register + offset) mode.
Then the subroutine can use that register to address all of its own
procedure code using the (register + offset) addressing mode.

The reason that this is important is that while the Intel architecture
is a nuisance for large amounts of data but the procedure code is still
quite manageable, the IBM is the other way around.  Many IBM systems
process unbelievable quantities of in-memory data (that's why they had
to expand the addressing to allow each process to have more than 16 Mb
of data).  Arrays of 1 Gb in size are "no sweat" on an IBM XA machine.
But addressing branch destinations requires planning.
-- 
Doug Pardee -- Terak Corp. -- !{ihnp4,seismo,decvax}!noao!terak!doug
               ^^^^^--- soon to be CalComp