Path: utzoo!attcan!uunet!nih-csl!lhc!ncifcrf!haven!uflorida!travis!tom From: tom@ssd.csd.harris.com (Tom Horsley) Newsgroups: comp.sys.m88k Subject: Re: Emulating other computers on 88K's and Benchmarks Message-ID: Date: 8 Oct 90 10:51:44 GMT References: <1990Oct3.095041.9295@canterbury.ac.nz> Sender: news@travis.csd.harris.com Organization: Harris Computer Systems Division Lines: 29 In-reply-to: newton@smoggy.gg.caltech.edu's message of 8 Oct 90 04:41:18 GMT >>>>> Regarding Emulating other computers on 88K's and Benchmarks; newton@smoggy.gg.caltech.edu (Mike Newton) adds: newton> [2] The memory model, including wait states. The lower end newton> DG machines have a fair number of wait states -- a fact newton> that surprised me, considering their memory is custom. Before you complain about memory wait states you should figure out where they are all coming from. A vast number of cycles are consumed by overhead in the 88200 MMU chip - as a rough example, if a certain configuration of memory and MMUs take 16 cycles to fill a cache line, 3 of those are the time it takes to walk through the data unit pipeline, 2 or 3 of the remaining cycles are the time it takes to access memory, and the remainder are consumed by the MMU. Even doubling the speed of memory would only reduce the 16 cycles to 14 or 15. Please Note: The above figures are from my memory of one example we worked out in detail - there are A LOT of different types of memory cycles and paths through the MMU and this was one specific example we worked through (I seem to recall it was doing a load from a non-cached memory location). The specific figures quoted may be wrong, but the approximate percentage speed improvement from using faster memory chips is about right (in other words, barely significant :-). -- ====================================================================== domain: tahorsley@csd.harris.com USMail: Tom Horsley uucp: ...!uunet!hcx1!tahorsley 511 Kingbird Circle Delray Beach, FL 33444 +==== Censorship is the only form of Obscenity ======================+ | (Wait, I forgot government tobacco subsidies...) | +====================================================================+