Talk:PDP-11 Extended Instruction Set

XOR?

When talking about the PDP-11 EIS, it's a little complicated. You have the KE-11, which was the separate EIS option for the PDP-11/40, and you have all the other PDP-11s with EIS by default. The complication is that the instructions provided differ in that XOR was not included in the KE-11, but on all other models. So the question is, should XOR be mentioned on this page or not? --Bqt (talk) 18:36, 27 October 2022 (CEST)

Actually, XOR was included in the base KD11-A of the -11/40, which is why it's not in the KE11-E. And there's also SOB, SXT, MARK, and RTT in the base KD11-A, which are not in the -11/20, -11/05 and -11/04, but are in all the later machines, including the LSI-11. Those are covered here. So I'm not sure there is an error? Jnc (talk) 22:43, 27 October 2022 (CEST)

You are right that XOR was already in the base of the -11/40 instruction set. I should probably have thought that comment through a bit more. I usually make it simple for myself by just grouping XOR with the MUL/DIV/ASH/ASHC since it use the same register-destination format for the opcode. But technically, there are a few more instructions that exists on all models except for the 11/20. And I basically think of them all as being the EIS (or extended instruction set). So yes, also SOB/MARK/SXT/RTT. The 11/20 defined the basic instruction set. What was added in the 11/45 was then EIS. Just that the EIS option for the 11/40 is needed for some of those instructions the 11/45 added. But that options is called EIS. There is no clear definition of what EIS means from DEC, sadly. (Or none that I know of right now. I'll poke around some...) --Bqt (talk) 01:50, 28 October 2022 (CEST)

I think the reason that XOR (and SOB, etc) weren't included in the KE11-E, but the others (MUL, etc) were, was purely pragmatic engineering. The four instructions added in the KE11-E (MUL, DIV, ASH, ASHC) used additional data path elements (internal registers, etc) that were on the KE11-E board; the other instructions (XOR, SOB, etc) could be accomplished with the basic data path elements already in the main CPU. That's all there was to it!

The KE11-E (with the additional data path elements needed for those instructions) did increase the cost of the -11/40 by a non-trivial amount; almost certainly a significant factor for a machine which was intended to be a 'low-cost' entry in the product line. I have an -11/40 price list from January, 1973; the base -11/40 was US$13K, and the KE11-E was US$1200. I am quite sure that's why they didn't just include the KE11-E functionality in the 'standard' -11/40, but made it an option.

Should I add that to the article? Jnc (talk) 06:56, 28 October 2022 (CEST)

I would agree with that position. But it is a conclusion we are drawing based on reasoning, not by any specific document from DEC. But I would probably vote for saying that in the article. And the basic instruction set is also all there is in 11/05 and 11/10 as well as the 11/04 if I remember right (in addition to the 11/15 and 11/20). --Bqt (talk) 09:17, 28 October 2022 (CEST)