IEEE double extended forma

[Turkowski, Ken]

P.754 specifies that there should be a single extended and a double extended,
whose primary purpose is to provide extra precision for intermediate
calculations.
 
What you have described could certainly be used for a double extended, but it
is overkill. It is really more of a quad format.
 
I have seen other propose this as a quad format, but I can't find references
for it. Do you have pointers to documentation?
 
Another proposal can be found in the draft proposal IEEE P.1596.5/D0.95 "Shared
Data Formats optimized for Scalable Coherent Interface Processors". In there,
there is a "floating hexlet":
 
  1 sign
 23 exponent
104 mantissa
---
128 total
  1 implicit bit, so 105 bits for the significand
 
exponent bias of +4194303
 
It also provides for denormalized numbers, NaN's, and infinities.
 
The difference between the two is that yours allocates bits proportionately in
the same way that IEEE does when it goes from single to double. The SCI format
just doubles both the exponent and mantissa:
 
        single  double CrayQuad SCIQuad
expon     8       11     15       23
mant     23       52    112      104
 
Any ideas on what name to call this in "C"?
 
----------------------------------------------------------------------------
Item    5801761                         29-Jan-93        17:55
 
From:   KNAAKaJUNIPER.CRAY.COMaINTERNET# 
 
Sub:    IEEE double extended format
 
From: knaakajuniper.cray.com (David C. Knaak)
X-Mailer: ELM [version 2.3 PL11b-CRI]
To: ncegacray.com
 
 
The IEEE standard 754 does not specify an exact format for double extended
but only gives some minimum requirements.  It appears to me that a de facto
standard has emerged:
 
    1 bit for the sign
   15 bits for the exponent
  112 bits for the fraction
  ---
  128 bits total
    1 implicit bit, so 113 bits for the significand
 
      exponent bias of +16383
 
My questions are:
 
1)  Do you think that this is the appropriate format for double extended?
 
2)  Do you know of any manufacturer that is going a different direction,
    and if so, why?
 
3)  Is "quad format" an appropriate term for this format?
 
Thanks for your thoughts.
 
David Knaak
knaakacray.com