Path: utzoo!attcan!uunet!husc6!mailrus!tut.cis.ohio-state.edu!bloom-beacon!mit-vax!spectre From: spectre@mit-vax.LCS.MIT.EDU (Joseph D. Morrison) Newsgroups: comp.arch Subject: Re: VLIW (was please re-send mail) Message-ID: <5087@mit-vax.LCS.MIT.EDU> Date: 11 Nov 88 16:18:41 GMT References: <70@armada.UUCP> <28200228@urbsdc> Reply-To: spectre@mit-vax.UUCP (Joseph D. Morrison) Organization: MIT Laboratory for Computer Science, Cambridge Lines: 45 In article <28200228@urbsdc> aglew@urbsdc.Urbana.Gould.COM writes: >I have to be careful saying this, since I now work for Motorola, >but it should be obvious that scoreboarding cannot take you as far as >VLIW. Scoreboarding is an appropriate choice for the current level >of microprocessor technology, but any computer architect will tell >you that you eventually have to get past the one operation/cycle >dispatch limit ... (more info about VLIW and scoreboarding) It seems to me that the issue of VLIW versus scoreboarding is the wrong one to discuss. Scoreboarding is but one of several techniques for managing a pipeline. (Some alternative techniques are micro-dataflow, simple stalling, or letting the compiler stick no-ops in the right places. The simple schemes can also be combined with "register bypass" to improve pipeline performance.) So I think we were actually arguing about "which is better for getting parallelism; pipelining or VLIW?" Phrased that way, I think the answer is obviously "use both". If each of your functional units takes 4 cycles to perform its operation, and you have a VLIW machine with 8 functional units, your average throughput will be 2 instructions per cycle. The obvious thing to do is to use pipelined functional units, and get the 8 instructions per cycle you deserve :-) Naturally, as soon as you do this you will need some mechanism for handling the various conflicts that occur when two instructions in the pipeline want to use the same register. This is when you can use scoreboarding, or whatever you want. In fact, what better way to test pipeline strategies! With all those functional units, the pipeline management will be pretty hairy... Joe Morrison -- MIT Laboratory for Computer Science UUCP: ...!mit-eddie!vx!spectre 545 Technology Square, Room 425 ARPA: spectre@vx.lcs.mit.edu Cambridge, Massachusetts, 02139 (617) 253-5881 -- "Back off, man; I'm a scientist!"