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From: hes@ecsvax.UUCP (Henry Schaffer)
Newsgroups: net.arch
Subject: Re: Data General Eclipse Floating Point Format
Message-ID: <983@ecsvax.UUCP>
Date: Tue, 24-Dec-85 19:41:01 EST
Article-I.D.: ecsvax.983
Posted: Tue Dec 24 19:41:01 1985
Date-Received: Wed, 25-Dec-85 23:28:18 EST
References: <55@dg_rtp.UUCP>
Organization: NC State Univ.
Lines: 52

> 
>     The following describes the Data General Eclipse floating point format.
> Note, that some of the submissions were wrong, in specifying that the DG 
> format
> is exactly the same as the IBM format.  It is true we use the same number of
> bits, but the IBM format expresses the exponent as a power of 2, whereas in 
> the
> DG format, the exponent is a power of 16.

   In the IBM format, the exponent (also called characteristic) is the power
of 16 (not 2) by which the fraction (also called mantissa) must be multiplied.
With a base of 2, the range of floating point numbers would be *much* too small
unless the exponent were much longer.

>  ...
> 
>     The exponent is in excess-64 notation, representing the appropriate power
> of 16.  The mantissa is grouped into 6 (single) or 14 (double) hexidecimal
> digits.
> 
   This also describes the IBM floating point format.
> 
> 	Floating Point	Hexidecimal
> 
> 	         0:	00000000  [*]
> 	         1:	41100000  [*]
> 	         2:	41200000  [*]
> 	         3:	41300000  [*]
> 	           ...
> 	         9:	41900000  [*]
> 	        10:	41A00000  [*]
> 	        11:	41B00000  [*]
> 	           ...
> 	        15:	41F00000  [*]
> 	        16:	42100000  [*]
> 	           ...
> 	        32:	42200000  [*]
> 	           ... 
> 	       256:	43100000  [*]
> 	           ...
> 	       0.5:	40800000  [*]
> 	      0.25:	40400000  [*]
> 	     0.125:	40200000
> 	  6.25e-02:	40100000  [*]
> 	        -1:	C1100000  [*]
> 
  *  These values were in the IBM manual I looked in and agree exactly
with the DG values shown.  I don't see any difference between the IBM and
DG formats from what is given here.
> 	Michael Meissner

--henry schaffer