Xref: utzoo comp.arch:7778 comp.lang.fortran:1678 comp.misc:4614
Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-lcc!ames!ncar!tank!shamash!raspail!hmt
From: hmt@raspail.UUCP (Hemi Thaker)
Newsgroups: comp.arch,comp.lang.fortran,comp.misc
Subject: Re: Quadruple-Precision Floating Point ?
Summary: hardware implementations of 128 bit
Keywords: CDC,Cyber180,floating-point
Message-ID: <1131@raspail.UUCP>
Date: 6 Jan 89 16:33:55 GMT
References: <8561@alice.UUCP> <3688@s.cc.purdue.edu> <285@loligo.fsu.edu> <2303@garth.UUCP>
Organization: Control Data Corporation, Arden Hills, MN
Lines: 32



>In article <2334@tekcae.CAX.TEK.COM>,
>kurtk@tekcae.CAX.TEK.COM (Kurt Krueger) writes:
>>Lots of deleted stuff about CDC doing double precision (128 bit) floating
>>arithmetic with two seperate instructions ....
>>
>>The new 180 series machines DO have a real full double precision 
>>(128 bit) machine instruction set for the four basic arithmetic ops.
>
>Even the top-of-the-line C180 model 99_ perform their 128-bit floating-point
>operations in microcode.  The 64-bit floating-point is done in hardware.
>
>Not exactly the choice that one would expect from a company with 
>the tradition of CDC, but that's what its management decided to do.

I'm not sure what you mean by *Even ..model 99_ ..in microcode* but not all
C180 machines implement 128-bit floating point in microcode. In fact, the
smallest C180 machine, the 930, performs 128 bit add, subtract, and multiply
almost entirely in hardware (microcode is only used for 128 bit data transfers
using the 64 bit internal bus and has no effect on instruction performance
vs. doing the same thing entirely in hardware). 128 bit divide is done mostly
in hardware with some microcode for end case checking. Unless we are told
otherwise, 64 bit floating point operations are considered to be far more 
important than 128 bit because of the low frequency of use of 128 bit operationsso making the hardware/microcode tradeoff would seem reasonable to most hardware
designers. As the cost of a gate goes down hardware implementations
of 128 bit arithmetic are likely to become more common.

		- Ed Bender

Disclaimer : These opinions are my own, and do not necessarily reflect
	     those of my employer.