Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!decwrl!mejac!orchard.la.locus.com!prodnet.la.locus.com!richg From: richg@locus.com (Rich Greenberg) Newsgroups: comp.arch Subject: Re: Floating-point formats Message-ID: <20075@oolong.la.locus.com> Date: 29 Nov 90 05:39:52 GMT References: Organization: Locus Computing Corp, Los Angeles Lines: 22 In article andyo@glasperl.masscomp (Andy Oram) writes: >I would like to find out what binary floating-point formats are currently in >use, besides the familiar IEEE 754 and DEC PDP/VAX. If you know of such >existing systems, please tell me: The format used by the IBM 360/370/390 series is as follows: (I am not familiar with IEEE 754 or DEC PDP/VAX so I don't know if they are like this one, but knowing IBM I doubt it.) The first byte: leftmost bit = sign of mantissa portion right 7 bits = power of 16 the mantissa is raised to plus 127 (or mabe 128, not sure and the book is at work). Rest of number is the binary mantissa, can be: 3 bytes (short float, about 7 decimal digits precision) 7 bytes (long float, about 15 decimal digits precision) 14 bytes (extended float, about 30 decimal digits precision) The extended format actually has a second exponent in there that wastes a byte. Total is 16 bytes. Layout: emmm mmmm emmm mmmm as 4 32 bit words. Note that because the mantissa is raised to a power of 16 (not 2), that a normalized mantissa can have up to 3 zero bits on the left end. Rich