Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!mailrus!wasatch!cs.utexas.edu!sm.unisys.com!ism780c!haddock!ima!johnl From: johnl@ima.ima.isc.com (John R. Levine) Newsgroups: comp.arch Subject: Re: Why is the RT slow? Message-ID: <2927@ima.ima.isc.com> Date: 17 Nov 88 17:24:38 GMT References: <5046@polya.Stanford.EDU> <691@quintus.UUCP> Reply-To: johnl@ima.UUCP (John R. Levine) Organization: Not much Lines: 41 In article <691@quintus.UUCP> ok@quintus.UUCP (Richard A. O'Keefe) writes: >In article jk3k+@andrew.cmu.edu (Joe Keane) writes: >>I get the impression that they were paranoid about code space, giving a >>too-complicated instruction set ... The ROMP chip was designed to run without a cache, so they were concerned about minimizing the bandwidth needed for instruction fetching. The instruction decode is fairly heavily pipelined, I doubt that's a major bottleneck. >>* Shift paired. Where else do they pair registers? In the IBM 360 series, to some extent. >Speaking of which, could someone explain to me what those instructions >are good for, and how best to do double-length shifts? When writing the AIX C compiler, I found them extremely handy for building the shift and add chains for multiplication by a constant. For double length shifts, something that we didn't need to do, I would shift each part separately since that's fast, and then use and's and or's to put the pieces together. The problem with the RT's instruction set is that it was exquisitely optimized for PL.8, their systems programming language, but the only real thing written in PL.8 that I'm aware of is the VRM, and even half of that ended up being rewritten in assembler. It's not a bad instruction set for C, but you need a far more sophisticated compiler than PCC to make good use of it. >... My guess is that it was a marketing decision: >IBM decided to scale the thing to what they thought people would buy >and guessed wrong. Of course -- they really didn't understand how much faster the workstation market moves than their more familiar mainframe and small business market, so by the time the RT came out it was obsolescent. Now they do understand, but who knows what they'll do about it. When the IBM came out, they published the "RT Personal Computer Technology" book which has a bunch of fairly interesting articles giving a somewhat sanitized version of how the RT came to be. It's order number SA23-1057. -- John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 492 3869 { bbn | spdcc | decvax | harvard | yale }!ima!johnl, Levine@YALE.something Kids spend more time with their parents than parents spend with their kids.