Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!aglew From: aglew@oberon.crhc.uiuc.edu (Andy Glew) Newsgroups: comp.arch Subject: Electro-optic bus Message-ID: Date: 17 Jul 90 02:59:01 GMT Sender: usenet@ux1.cso.uiuc.edu (News) Distribution: comp Organization: University of Illinois, Computer Systems Group Lines: 49 Here's a throwaway - the electro-optic star bus: We all know that electrical interconnects such as busses have major performance problems - loading, skews, etc. [*]. The longer and the more things you talk to, the slower. We all know that optics has much more bandwidth than electrical busses - right? (Maybe I should put a smiley here?) We all know that the biggest problem with optics is that Si doesn't interface to it well. GaAs does, but GaAs doesn't achieve Si's levels of integration [**]. What is, perhaps, less well known is that Si can build fairly good *receivers* for optics; Si just cannot build good transmitters. Even MOSIS's CMOS processes can build fairly good receivers. Throw-away idea: take, say, 4 Si microprocessors. Give them each a point-to-point (easier to make fast) electrical interconnect (wires), from the Si chips, to a GaAs chip. Let the GaAs chip take these 4 sets of electrical signals, and compress them all onto a faster optical bus that is sent back to the 4 microprocessors. Ie. use the GaAs chip as the hub of a star, with incoming signals in electronics, and outgoing signals in optics. The Si chips all receive the optical signals with native Si receivers. Since all of the signals are broadcast on the optics, we effectively have a bus. If the Si receivers and logic can be made fast enough, then we could snoop and do all those sorts things that are nice to do on busses. Ie. bus bandwidth is no longer the constraint - the constraint is how fast the Si receivers and/or the Si cache snooping logic can be made. Waiting anxiously to be shot down... [*] this is not to say that there aren't people who believe that existing bus technology cannot be milked. Such as making existing 25 MB/s busses run at more than 4 MB/s under real usage patterns. But that's another story. [**] I am, of course, neglecting the problem of coupling optical fibers (if they were fibers) to the Si chips. -- Andy Glew, aglew@uiuc.edu