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From: meissner@dg_rtp.UUCP (Michael Meissner)
Newsgroups: net.arch
Subject: Data General Eclipse Floating Point Format
Message-ID: <55@dg_rtp.UUCP>
Date: Tue, 24-Dec-85 11:37:53 EST
Article-I.D.: dg_rtp.55
Posted: Tue Dec 24 11:37:53 1985
Date-Received: Wed, 25-Dec-85 23:28:07 EST
Organization: Data General (Languages @ Westborough, MA.)
Lines: 70
Keywords: floating point,Data General


    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.

		Single Precision

	+-+-------+------------------ ........ --------+
	|S|  Exp  |      Mantissa		       |
	+-+-------+------------------ ........ --------+
	 1  7 bits		24 bits
	bit

		Double Precision

	+-+-------+------------------ ........ --------+
	|S|  Exp  |      Mantissa		       |
	+-+-------+------------------ ........ --------+
	 1  7 bits		56 bits


    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.  For the 16-bit Eclipses, the mantissa does not have to be normalized.
For the 32-bit MV line, the POOPS manual specifies that the number must always
be normalized, and implementation dependent results occur if the number is not
normalized.  In practice, the MV/8000 will normalize the number before doing
any calculations, and the MV/10000 will generate a floating point fault if
given an unnormalized number.  One special guarantee is given in that loads and
stores affecting floating point registers do not change the format, so that
a floating load, followed by a floating store can be used to move 64 bits of
information.


	Floating Point	Hexidecimal

	         0:	00000000
	         1:	41100000
	         2:	41200000
	         3:	41300000
	         4:	41400000
	         5:	41500000
	         6:	41600000
	         7:	41700000
	         8:	41800000
	         9:	41900000
	        10:	41A00000
	        11:	41B00000
	        12:	41C00000
	        13:	41D00000
	        14:	41E00000
	        15:	41F00000
	        16:	42100000
	        17:	42110000
	        32:	42200000
	        64:	42400000
	       128:	42800000
	       256:	43100000
	       512:	43200000
	       0.5:	40800000
	      0.25:	40400000
	     0.125:	40200000
	  6.25e-02:	40100000
	        -1:	C1100000

	Michael Meissner
	Data General Corporation
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