Difference between revisions of "PDP-11 Extended Instruction Set"
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| − | The '''Extended Instruction Set''' is an extension to the basic [[PDP-11]] [[PDP-11 architecture|instruction set]]; it includes [[hardware]] multiply and divide, and also multi-bit shifts. The exact | + | The '''Extended Instruction Set''' is an extension to the basic [[PDP-11]] [[PDP-11 architecture|instruction set]]; it includes [[hardware]] multiply and divide, and also multi-bit shifts. The exact [[instruction]]s included in the EIS are: |
* MUL | * MUL | ||
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* ASHC | * 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 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. |
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| + | ==Rationale== | ||
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| + | The reason that of the [[PDP-11 architecture#Added later|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 [[register]]s, 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]]. | ||
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| + | 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 [[Digital Equipment Corporation|DEC]] didn't just include the KE11-E functionality in the 'standard' -11/40, but made it an option instead. | ||
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==See also== | ==See also== | ||
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* [[KEV11-B Extended Instruction Set]] | * [[KEV11-B Extended Instruction Set]] | ||
[[Category: PDP-11s]] | [[Category: PDP-11s]] | ||
Latest revision as of 16:20, 28 October 2022
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.
Rationale
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.
