Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83 v7 ucbtopaz-1.8; site ucbtopaz.CC.Berkeley.ARPA Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!ucbvax!ucbtopaz!mwm From: mwm@ucbtopaz.CC.Berkeley.ARPA Newsgroups: net.micro Subject: Re: Need a microprogramming consultant! Message-ID: <802@ucbtopaz.CC.Berkeley.ARPA> Date: Thu, 7-Mar-85 22:51:46 EST Article-I.D.: ucbtopaz.802 Posted: Thu Mar 7 22:51:46 1985 Date-Received: Sun, 10-Mar-85 07:07:13 EST References: <504@ima.UUCP> Reply-To: mwm@ucbtopaz.UUCP (Praiser of Bob) Organization: Missionaria Phonibalonica Lines: 36 Summary: In article <504@ima.UUCP> johnl@ima.UUCP writes: >I wish we saw more stuff here like the note from ucbtopaz!mwm on >microprogramming. Clear and to the point. But I can't resist a few niggles. Thank you. >One thing to keep in mind is that microprogramming is a mixed blessing. There >is an increasingly popular school of thought that states that the slowdown >from interpreting microcode isn't worth it, and you'd be better off making a >very simple machine that executes instructions directly, and depending on your >compiler to generate code for it. The Berkeley RISC is one of the best known >of the efforts to do this. Note that (some of) the people selling RISC machines (pyramid) are *still* using microcoded machines. The simpler target instruction set means you need less rom for microcode, and less "magic" hardware - a compromise between pure RISC and CISC (complex ...). In exchange for the interpretation time, you get more flexibility in the machine with less trouble. For instance, pyramid changed from word-aligned word fetches to unaligned word fetches by tweaking the microcode. Like any complex decision, you have to make some trade-offs. >Finally, it turns out that microprogramming is far from a new idea. It was >originally proposed for an early British machine in 1952! Not surpising. Most of the good "new" ideas in CS are rather old. Could you supply a reference so I can read about it? BTW, redwood!rpw3 posted a very detailed description of one form of vertical microcode; as it appears in I/O devices, etc. He also mentions bit-sliced machines (the AMD 2900), which make for fast design of fast custom hardware (like Wirth's Lilith). The reference for such is "Bit-slice Microprocessor Design" by Mick and Brick (no, I'm not kidding), from McGraw-Hill.