Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!wuarchive!udel!rochester!pt.cs.cmu.edu!gandalf.cs.cmu.edu!lindsay From: lindsay@gandalf.cs.cmu.edu (Donald Lindsay) Newsgroups: comp.arch Subject: Re: Second-generation RISC Message-ID: <12464@pt.cs.cmu.edu> Date: 23 Mar 91 23:09:46 GMT References: <3189@inews.intel.com> Organization: Carnegie Mellon Lines: 44 In article <3189@inews.intel.com> kds@mipos2.intel.com (Ken Shoemaker) writes: >Though you can get increased bandwidth by increasing the number of pins >you have a hard time pushing the speeds of the interchip interfaces. As clock frequencies go up, some funny things start happening. A row of bus drivers starts looking like a phased array - and I'm talking about 150 MHz, not 1 GHz. So, you double rail all the lines, and interleave with power and ground lines, and add P/G planes, and you may still have to perturb the board layout to avoid parallel traces. One view is that we should just accustom ourselves to inevitable nuisances and drawbacks. The alternate view is that there's physics out in them thar hills, and we should be looking for it. In particular, optical communication is looking better and better. Over long distances, it takes less energy to push photons than to push electrons. One Bell Labs estimate puts the crossover distance at 200 microns. That is not a typo. The crossover is not at 200 meters (LAN size) but nearer 200 microns (onchip size). But is it fast? Well, the speed record for transistors is in the low picoseconds. The record for short optical pulses is in the low femtoseconds. The record for optical pulse repitition rate is 600 fs - 1600 GHz - also not a typo. But can it be compact? Well, face-emitting lasers can be as little as a micron across. Yes, they have been successfully coupled to optical fibers. Cheap? A wafer of thousands of face-emitting lasers is a few thousand dollars. Yields are 85-90%. The only difficulty with individually modulating each of them at a gigahertz is the wiring and signal distribution headache. It doesn't really matter that practical difficulties remain. As clock rates rise, all possible approaches have practical difficulties. Why not go for a win instead of a draw? -- Don D.C.Lindsay .. temporarily at Carnegie Mellon Robotics