Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!ucbvax!hplabs!hpcc01!aspen!huck
From: huck@aspen.IAG.HP.COM (Jerry Huck)
Newsgroups: comp.arch
Subject: Re: IBM S/360 FP (was Re: RS6000 Multiply/Accumulate instruction)
Message-ID: <1360002@aspen.IAG.HP.COM>
Date: 21 Mar 90 22:06:18 GMT
References: <3060@wtkatz.oakhill.UUCP>
Organization: HP Information Architecture Group - Cupertino, CA
Lines: 20

In an response by lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) he
writes:
>In article <3060@wtkatz.oakhill.UUCP> chinds@oakhill.UUCP 
>	(Chris Hinds) writes:
>>In last weeks COMPCOM presentation of the RS6000, the
>>IBM speaker was asked if the new, fast fp multiply/
>>accumulate instruction was IEEE 754 compatible.  The
>>reply was 'no.'
>
>The machine doesn't have a multiply instruction, nor does it have an
>add instruction. It has a four-argument multiply-and-add instruction.
>One does multiplies, or adds, by the judicious use of nil arguments.

What are nil arguments?  One approach might be to provide the constant
one and zero into the multiplier and adder but that has problems with
handling signed zeros.  How does the HW know which operation to simply
"pass" an argument?  sub-op bits?, reserved register spec?

Thanks,
Jerry (huck@iag.hp.com)