Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!psuvax1!wuarchive!uunet!sunquest!venus.sunquest.com!terry From: terry@venus.sunquest.com (Terry R. Friedrichsen) Newsgroups: comp.arch Subject: Re: bizarre instructions Summary: bizarre != stupid Keywords: instructions bizarre symmetry Message-ID: <14710@sunquest.UUCP> Date: 25 Feb 91 20:52:39 GMT Sender: news@sunquest.UUCP Followup-To: comp.arch Distribution: usa Organization: Sunquest Information Systems, Tucson Lines: 43 Donald Lindsay writes: >In fact, the old literature (and old machines!) are full of "bizarre" >instructions that proved ill-advised. A few examples: > > [ some good examples removed] > >- addressing modes which studies could not find a single use of. > (PDP-11 autodecrement deferred - omitted from the VAX.) This example differs somewhat from his others, which were basically deliberate design decisions that are not now generally seen as the wise way to go. But the autodecrement deferred addressing mode arose more out of symmetry (== simple instruction decoding) than somebody saying "gee, I'll bet that would be a useful addressing mode"). The pdp-ll had autoincrement/autodecrement, and it had deferred/non-deferred. The "ill-advised" combination of autodecrement deferred was just a result of symmetrical instruction decode logic. DEC's pdp-10 instruction set had the same oddities. The machine had a plethora of no-op equivalent instructions (some of which referenced memory and some of which did not), not because the designers thought it would be advisable to have lots of no-ops, but because the very simple, highly gate-efficient instruction decode logic took advantage of instruction-set symmetries. (I think I have notes around here that say the instruction decode for the original instruction set was done in something like 700 gates - but don't quote this; I may have the number wrong ...) The whole point is just that unused instructions or opcodes don't necessarily equate to stupidity or lack of foresight on the part of the designers. Sometimes they're really a result of cleverness :-). Terry R. Friedrichsen terry@venus.sunquest.com (Internet) uunet!sunquest!terry (Usenet) terry@sds.sdsc.edu (alternate address; I live in Tucson) Quote: "Do, or do not. There is no 'try'." - Yoda, The Empire Strikes Back Brought to you by Super Global Mega Corp .com