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From: mac%uvacs@uvacs.UUCP
Newsgroups: net.arch
Subject: Re: ORed indexes
Message-ID: <411@uvacs>
Date: Tue, 29-Apr-86 20:08:00 EDT
Article-I.D.: uvacs.411
Posted: Tue Apr 29 20:08:00 1986
Date-Received: Sun, 11-May-86 15:47:07 EDT
References: <5100066@ccvaxa>
Lines: 25

> How would you react to an instruction set that had no indexed addressing
> mode, but did have a mode where a register was ORed into the address?
> 
> I can vaguely recall having seen such a beast - is anyone familiar with it?

IBM 7090?  3-bit index register field, 3 index registers.  I believe the
registers selected were OR'd together, then ADDed to the offset.

> ....  Losing indexing for array accesses isn't too bad, since
> just about every expensive array operation can be written using
> pointers instead of indexes (post-increment doesn't require a
> carry to perform a memory address).

No semi-random access by subscripts?  Your machine is to be
programmed exclusively in C or assembler, never Fortran.

> So what can you do? Adding an index register in slows down the rest of your
> instruction set,

Even when you don't index?

> .... and, if memory is cheap, you might be willing to pad out a
> lot more of your structures, to get a speed increase.

or use a separate add instruction.