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[96.241.37.234]) by smtp.gmail.com with ESMTPSA id f12sm9398377qkh.22.2021.11.30.00.07.16 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 30 Nov 2021 00:07:16 -0800 (PST) From: To: "'TUHS main list'" Date: Tue, 30 Nov 2021 03:07:15 -0500 Message-ID: <010901d7e5c1$4a0c7c20$de257460$@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit X-Mailer: Microsoft Outlook 16.0 Thread-Index: AdflwThp1c/XnIbPRSSrtfVO4pd7FQ== Content-Language: en-us Subject: [TUHS] Encoding an ISA: Random Logic vs. Control Stores X-BeenThere: tuhs@minnie.tuhs.org X-Mailman-Version: 2.1.26 Precedence: list List-Id: The Unix Heritage Society mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: 'Eugene Miya' Errors-To: tuhs-bounces@minnie.tuhs.org Sender: "TUHS" I believe that the PDP-11 ISA was defined at a time when DEC was still using random logic rather than a control store (which came pretty soon thereafter). Given a random logic design it's efficient to organize the ISA encoding to maximize its regularity. Probably also of some benefit to compilers in a memory-constrained environment? I'm not sure at what point in time we can say "lots of processors" had moved to a control store based implementation. Certainly the IBM System/360 was there in the mid-60's. HP was there by the late 60's. -----Original Message----- From: TUHS On Behalf Of Larry McVoy Sent: Monday, November 29, 2021 10:18 PM To: Clem Cole Cc: TUHS main list ; Eugene Miya Subject: Re: [TUHS] A New History of Modern Computing - my thoughts On Sun, Nov 28, 2021 at 05:12:44PM -0800, Larry McVoy wrote: > I remember Ken Witte (my TA for the PDP-11 class) trying to get me to > see how easy it was to read the octal. If I remember correctly (and I > probably don't, this was ~40 years ago), the instructions were divided > into fields, so instruction, operand, operand and it was all regular, > so you could see that this was some form of an add or whatever, it got > the values from these registers and put it in that register. I've looked it up and it is pretty much as Ken described. The weird thing is that there is no need to do it like the PDP-11 did it, you could use random numbers for each instruction and lots of processors did pretty much that. The PDP-11 didn't, it was very uniform to the point that Ken's ability to read octal made perfect sense. I was never that good but a little google and reading and I can see how he got there. ... --lm