From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: tuhs-bounces@minnie.tuhs.org X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_NONE autolearn=ham autolearn_force=no version=3.4.1 Received: from minnie.tuhs.org (minnie.tuhs.org [45.79.103.53]) by inbox.vuxu.org (OpenSMTPD) with ESMTP id cd6ae286 for ; Sat, 16 Jun 2018 22:14:41 +0000 (UTC) Received: by minnie.tuhs.org (Postfix, from userid 112) id 9A502A18E0; Sun, 17 Jun 2018 08:14:40 +1000 (AEST) Received: from minnie.tuhs.org (localhost [127.0.0.1]) by minnie.tuhs.org (Postfix) with ESMTP id 430CB9EDEA; Sun, 17 Jun 2018 08:14:06 +1000 (AEST) Received: by minnie.tuhs.org (Postfix, from userid 112) id 0009B9EDEA; Sun, 17 Jun 2018 08:14:02 +1000 (AEST) Received: from Psilocybe.Update.UU.SE (Psilocybe.Update.UU.SE [130.238.19.25]) by minnie.tuhs.org (Postfix) with ESMTP id 55EAD9BC34 for ; Sun, 17 Jun 2018 08:14:02 +1000 (AEST) Received: from localhost (localhost [127.0.0.1]) by Psilocybe.Update.UU.SE (Postfix) with ESMTP id A47D7802E2 for ; Sun, 17 Jun 2018 00:14:01 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at Update.UU.SE Received: from Psilocybe.Update.UU.SE ([127.0.0.1]) by localhost (Psilocybe.Update.UU.SE [127.0.0.1]) (amavisd-new, port 10040) with ESMTP id UrOI7Zq5kNFJ for ; Sun, 17 Jun 2018 00:14:00 +0200 (CEST) Received: from Trollet.local (178-82-175-247.dynamic.hispeed.ch [178.82.175.247]) by Psilocybe.Update.UU.SE (Postfix) with ESMTPSA for ; Sun, 17 Jun 2018 00:14:00 +0200 (CEST) To: tuhs@minnie.tuhs.org References: From: Johnny Billquist Message-ID: <86dcd19d-f805-f338-f190-ab38d1ac82c1@update.uu.se> Date: Sun, 17 Jun 2018 00:14:00 +0200 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Subject: Re: [TUHS] core X-BeenThere: tuhs@minnie.tuhs.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: The Unix Heritage Society mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: tuhs-bounces@minnie.tuhs.org Sender: "TUHS" On 2018-06-16 21:00, Clem Cole wrote: > below... > On Sat, Jun 16, 2018 at 9:37 AM, Noel Chiappa wrote: >> >> Let's start with the UNIBUS. Why does it have only 18 address lines? (I >> have >> this vague memory of a quote from Gordon Bell admitting that was a mistake, >> but I don't recall exactly where I saw it.) > ​I think it was part of the same paper where he made the observation that > the greatest mistake an architecture can have is too few address bits.​ I think the paper you both are referring to is the "What have we learned from the PDP-11", by Gordon Bell and Bill Strecker in 1977. https://gordonbell.azurewebsites.net/Digital/Bell_Strecker_What_we%20_learned_fm_PDP-11c%207511.pdf There is some additional comments in https://gordonbell.azurewebsites.net/Digital/Bell_Retrospective_PDP11_paper_c1998.htm > My understanding is that the problem was that UNIBUS was perceived as an > I/O bus and as I was pointing out, the folks creating it/running the team > did not value it, so in the name of 'cost', more bits was not considered > important. Hmm. I'm not aware of anyone perceiving the Unibus as an I/O bus. It was very clearly designed a the system bus for all needs by DEC, and was used just like that until the 11/70, which introduced a separate memory bus. In all previous PDP-11s, both memory and peripherals were connected on the Unibus. Why it only have 18 bits, I don't know. It might have been a reflection back on that most things at DEC was either 12 or 18 bits at the time, and 12 was obviously not going to cut it. But that is pure speculation on my part. But, if you read that paper again (the one from Bell), you'll see that he was pretty much a source for the Unibus as well, and the whole idea of having it for both memory and peripherals. But that do not tell us anything about why it got 18 bits. It also, incidentally have 18 data bits, but that is mostly ignored by all systems. I believe the KS-10 made use of that, though. And maybe the PDP-15. And I suspect the same would be true for the address bits. But neither system was probably involved when the Unibus was created, but made fortuitous use of it when they were designed. > I used to know and work with the late Henk Schalke, who ran Unibus (HW) > engineering at DEC for many years. Henk was notoriously frugal (we might > even say 'cheap'), so I can imagine that he did not want to spend on > anything that he thought was wasteful. Just like I retold the > Amdahl/Brooks story of the 8-bit byte and Amdahl thinking Brooks was nuts; > I don't know for sure, but I can see that without someone really arguing > with Henk as to why 18 bits was not 'good enough.' I can imagine the > conversation going something like: Someone like me saying: *"Henk, 18 bits > is not going to cut it."* He might have replied something like: *"Bool > sheet *[a dutchman's way of cursing in English], *we already gave you two > more bit than you can address* (actually he'd then probably stop mid > sentence and translate in his head from Dutch to English - which was always > interesting when you argued with him). Quite possible. :-) > Note: I'm not blaming Henk, just stating that his thinking was very much > that way, and I suspect he was not not alone. Only someone like Gordon and > the time could have overruled it, and I don't think the problems were > foreseen as Noel notes. Bell in retrospect thinks that they should have realized this problem, but it would appear they really did not consider it at the time. Or maybe just didn't believe in what they predicted. Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt@softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol