Path: utzoo!attcan!uunet!bu.edu!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!ncar!gatech!udel!nigel.ee.udel.edu!mccalpin From: mccalpin@perelandra.cms.udel.edu (John D. McCalpin) Newsgroups: comp.arch Subject: Re: 1 double or 2 singles Message-ID: Date: 12 Nov 90 16:30:54 GMT References: <11054@pt.cs.cmu.edu> <2511@charon.cwi.nl> <11056@pt.cs.cmu.edu> <1990Nov12.160009.28675@ux1.cso.uiuc.edu> Sender: usenet@ee.udel.edu Organization: College of Marine Studies, U. Del. Lines: 24 Nntp-Posting-Host: perelandra.cms.udel.edu In-reply-to: msp33327@uxa.cso.uiuc.edu's message of 12 Nov 90 16:00:09 GMT >On 12 Nov 90 16:00:09 GMT, msp33327@uxa.cso.uiuc.edu (Michael Pereckas) said: -> I don't know how hard it would be to do, but I wonder how useful it -> would be. Does anyone know how useful it was on the Cyber 205? Because the memory bandwidth was available, this feature (double-speed half-precision arithmetic) was very useful. Long vector codes (N>>1000) really did get 2:1 speed-ups when run in 32-bit precision. Short vector codes got less speedup because the startup overhead was effectively twice as large (although it was the same number of cycles). A more significant problem was that the perverse floating-point arithmetic on the machine was surprisingly inaccurate and many calculations that were OK in 32-bits on IEEE machines would fail miserably on the 205/ETA-10. For more on both the speed and accuracy, see my discussion in the paper entitled "Some Notes on 32-bit Arithmetic on the Cyber 205 and ETA-10" in Supercomputer (the Dutch journal), volume 5, number 2 (issue #24), March, 1988. -- John D. McCalpin mccalpin@perelandra.cms.udel.edu Assistant Professor mccalpin@vax1.udel.edu College of Marine Studies, U. Del. J.MCCALPIN/OMNET