Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!hercules!caveh From: caveh@csl.sri.com (Caveh Jalali) Newsgroups: comp.arch Subject: Re: why is 33 MHz a popular number? Message-ID: <22595@hercules.csl.sri.com> Date: 29 Nov 90 04:57:50 GMT References: <1990Nov27.192504.11191@ux1.cso.uiuc.edu> <1990Nov28.010513.17760@news.nd.edu> <37889.2753fde0@ccavax.camb.com> Sender: usenet@csl.sri.com Organization: Computer Science Lab, SRI International, Menlo Park, CA Lines: 43 33MHz is usually 33.33333 MHz. if you take the inverse to find the cycle time, we can see that it does follow a pattern: MHz cycle time (ns) 10 100 12.5 80 16.67 60 20 50 25 40 33.3 30 40 25 50 20 66.67 15 the pattern sort of makes sense in the ns column. the point of all this is a that the speed rating is pretty much arbitrary from the point of the view of manufacturers such as motorola and intel. they have no idea what your requirements are, thus they can't fine tune their speed ratings to allow you the best performance. they basically have to pick arbitrary norms at which they test their components for pass/fail. if a chip passes, it means they will require certain timing constraints that must be met in order for that chip to operate correctly, and conversely, the chip will satisfy certain timing specifications. often, a 25MHz processor is a 33MHz processor which failed the 33MHz test, so they really do have the freedom to pick arbitrary speed ratings. if you're a sufficiently large enough customer, you will suddenly see things like 15MHz 68020's altho i don't think moto ever sold those over the counter. basically, they don't want to test these chips too many times, so they might test at 33MHz, if it fails test at 25MHz, if it fails test for 20MHz, if it fails ther, it's junk. they could test for 24, 23, 22.. MHz, but i guess that's not feasible. if you develop your own processor, you have the freedom to choose your test clock frequency. that's why MIPS and IBM get to use oddball frequencies. -- 00c -- caveh@csl.sri.com "X is not a letter, it's a sentence."