Re: Implementor support for the binary interchange formats
From: Steven Hobbs <Steven.Hobbs@xxxxxxxxxxxxxxxx>
To: Dan Zuras IEEE <forieee@xxxxxxxxxxxxxx>, Michel Hack <hack@xxxxxxxxxxxxxx>
CC: stds-754 <stds-754@xxxxxxxxxxxxxxxxx>
Date: Tue, 1 Mar 2011 17:12:03 -0500
Subject: RE: Implementor support for the binary interchange formats
It has been a long time but I remember that the HP 128-bit FP and the Sun 1=
28-bit FP were different. The HP 128-bit float representation had a 112 bi=
ts in the significand plus a "hidden" bit for 113 bits of precision. The S=
un 128-bit FP float representation also had 112 bits in the significand but=
had no hidden significand bit for 112 bits of precision. Note that the In=
tel 80-bit extended type is like the Sun 128-bit float in that neither have=
a hidden significand bit.
When the Alpha architecture was designed, it included both a 128-bit VAX FP=
data type and a 128-bit IEEE extended data type. There were no instructio=
ns for these two floating point types. Implementation was in software. Th=
e 128-bit VAX FP had been in the VAX hardware since 1980 and included 1 sig=
n bit, 15 exponent bits, 112 significand bits plus one additional hidden si=
gnificand bit for a total of 113 bits of precision. The Alpha 128-bit IEEE=
extended data type was identical to the HP 128-bit float type. Alpha chos=
e the HP 128-bit representation over the Sun 128-bit representation because=
it was more compatible with the VAX 128-bit representation which would mak=
e implementation easier (as well as making the architecture more regular.)
. . .
This is interesting.
I distinctly remember mentioning over dinner at an ARITH
conference that I had copied the quad parameters from
something I heard from Sun. Then my dinner companion
(from Sun) said, "That's funny. We got it from you guys
From what you say it is quite possible that I looked at
the VAX parameters & misunderstood about the lack of a
hidden bit turning ours into a 113 bit significand.
Then the Alpha guys might have used our parameters to
We have always said we copied from each other. It looks
like we may have done just that. They just did a better
job of it than I did. :-)
The things you learn decades after the fact...