Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site utastro.UUCP Path: utzoo!watmath!clyde!burl!ulysses!harpo!seismo!ut-sally!utastro!nather From: nather@utastro.UUCP (Ed Nather) Newsgroups: net.arch Subject: Re: RISC perspective Message-ID: <26@utastro.UUCP> Date: Fri, 9-Mar-84 10:54:44 EST Article-I.D.: utastro.26 Posted: Fri Mar 9 10:54:44 1984 Date-Received: Sat, 10-Mar-84 12:43:30 EST References: <27900009@ucbesvax.UUCP> Organization: UTexas Astronomy Dept., Austin, Texas Lines: 35 <> I have advocated "cache-advisor" instructions for RISC's--instructions that don't *do* anything, from the programmer's point of view, but which inform the cache-manager of highly-probable-to-certain upcoming memory events. This is another trade-off--a loss of code-density with a possible net gain in performance. That need not complicate the architecture. It could, in fact simplify it--if cache-block- prefetching were more effective, then instructions could be looser and "fatter" (more easily decoded and executed) with less worry over the fetching costs. Possible analogies: impurity doping of semiconductors to increase electron mobility, or adding minute amounts of certain minerals in steel-making to improve its tensile strength. It's worth some experimentation, I think. --- Michael Turner (ucbvax!ucbesvax.turner) This sounds like a splendid idea to me. If the programmer inserts a single instruction that serves as "advice" to the cache-manager, then the overhead involved is just 1 fetch-time (plus decoding time, but that's true for any instruction). It would act like a "pseudo-op" in an assembler program, to help manage things but generate no output. If such advice saves only 1 fetch it breaks even; everything after that is gravy. I found that the "sieve" benchmark could be speeded up a factor of 2 on our Vax by advising the "C" compiler about what to put into registers. The "C" compiler is the only one we have humble enough to accept advice. I find the thought of a much closer relationship between compiler and programmer, via direct "advice" where it can really matter, a marvelous one. Advising the computer directly could be even better! -- Ed Nather ihnp4!{ut-sally,kpno}!utastro!nather Astronomy Dept., U. of Texas, Austin