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Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: John Savard <quadibloc@servername.invalid> Newsgroups: comp.arch Subject: Making Lemonade (Floating-point format changes) Date: Sat, 11 May 2024 21:44:45 -0600 Organization: A noiseless patient Spider Lines: 38 Message-ID: <abe04jhkngt2uun1e7ict8vmf1fq8p7rnm@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Sun, 12 May 2024 05:44:48 +0200 (CEST) Injection-Info: dont-email.me; posting-host="ad22be1223e1a3c6594dd94543987e7e"; logging-data="2658606"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18Y5sK2ex4tDByQn+w0JqbzVdbjRHFuElo=" Cancel-Lock: sha1:FJR5BNy3snKPX6zv4MVY1Cpezhw= X-Newsreader: Forte Free Agent 3.3/32.846 Bytes: 2509 I've made another long-overdue change in the Concertina II architecture on the page about 17-bit instructions. Since I describe the individual instructions there, with their opcodes and what they do, I've illustrated the floating-point formats of the architecture on that page. The good people in charge of the IEEE 754 standard had seen fit to define a standard 128-bit floating-point format which included a hidden first bit. This annoyed me greatly, because I was going to take the 8087's temporary real format, and extend the mantissa for my 128-bit format. I've decided that it's necessary to fully accept the 128-bit standard and support it in a consistent manner. Therefore, I have taken the following actions: I have dropped the option of supporting 80-bit temporary reals entirely, as they are now incompatible as an internal format. I have instead defined a 256-bit format for floats which does not have a hidden first bit, which looks like the old temporary reals, except that the exponent field is one bit wider. And in addition, just as the IBM 704 used two single-precision floats to make a double-precision float, and the IBM System/360 Model 85 started using two double-precision floats to make an extended precision float... I've defined how the 256-bit internal format floats can be doubled up to make a 512-bit float. I'm not really sure such floating-pont precision is useful, but I do remember some people telling me that higher float precision is indeed something to be desired. Well, the IEEE 754 standard has forced my hand. John Savard