Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!mips!winchester!mash From: mash@mips.COM (John Mashey) Newsgroups: comp.arch Subject: Re: 64 bits--why stop there? Summary: more explanation Message-ID: <41013@mips.mips.COM> Date: 22 Aug 90 04:11:58 GMT References: <5539@darkstar.ucsc.edu> <13285@yunexus.YorkU.CA> <30728@super.ORG> <41004@mips.mips.COM> Sender: news@mips.COM Lines: 45 In article <41004@mips.mips.COM>, mash@mips.COM (John Mashey) writes: .... > 3) So, I'd be amazed if a new general-purpose architecture would likely > be viable at 48 bits. Of course, several people wrote to point out various 24/48 bit machines. So I must not have made the point strongly enough. 1) Computer design is driven by the tradeoffs a) At that time of design b) Expected over the life of the design, which of course, could easily be as long as 25-30 years (S/360, so far :-) c) With a proper balance between a) and b), i.e., if you are a startup, you'd better weight a) enough that the first product out the door makes sense. Bigger organizations might make some kinds of longer-term tradeoffs that are non-optimal in first round, but better over the life. Obviously the best is to have things that are terrific at each round :-) -As an example, suppose you wanted to build the best thing you could in a given silicon technology, in a given amount of space. You'd put things as close together as possible. If you did this, the chip may well not shrink easily into the next technology, especially because you don't get to shrink the wires as fast as the transistors. An alternative (which is what MIPS does), is to make each base design as fast as it can, subject to allowing for 1 or more straightforward shrinks, so the point of optimization is more like the 2nd or 3rd technology, not the first. 2) But the tradeoffs change over time, and they can change a lot. What was a good idea 10 or 20 years may be the wrong choice now. A point of the earlier posting was that the software-related tradeoffs have changed radically in the last 10 years, such that anyone doing a 8-bit-byte, 32-bit word (or 64, sometime), two's-complement, byte-addressed architecture, gets a "free ride" with a huge amount of fairly portable software, compared to someone who does something much different. (This is not necessarily good, but I claim that it is true, and not likely to change for at least 5-10 years, because somethign different has got to be compelling.) -- -john mashey DISCLAIMER: UUCP: mash@mips.com OR {ames,decwrl,prls,pyramid}!mips!mash DDD: 408-524-7015, 524-8253 or (main number) 408-720-1700 USPS: MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086