PDP-11 Extended Instruction Set

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The Extended Instruction Set is an extension to the basic PDP-11 instruction set; it includes hardware multiply and divide, and also multi-bit shifts. The exact instructions included in the EIS are:

  • MUL
  • DIV
  • ASH
  • ASHC

It was not available on the earliest PDP-11 models (the PDP-11/20 and the PDP-11/05), and the later PDP-11/04. It was available as an option for the PDP-11/40 and the PDP-11/03; it was standard on all other PDP-11 models.


The reason that of the instructions added to the PDP-11 architecture, quite a few (XOR, SOB, etc) weren't included in the KE11-E Extended Instruction Set, but the others (MUL, etc) were, was almost certainly purely pragmatic engineering. The four instructions added in the KE11-E 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 KD11-A CPU.

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. An -11/40 price list from January, 1973 shows that the base -11/40 was then US$13K, and the KE11-E was US$1200. This almost certainly why DEC didn't just include the KE11-E functionality in the 'standard' -11/40, but made it an option instead.

See also