Details about IBM's early 'scientific' computers

Paul Koning paulkoning at
Wed Nov 15 07:25:48 CST 2017

> On Nov 14, 2017, at 10:58 PM, Jon Elson via cctalk <cctalk at> wrote:
> On 11/14/2017 11:20 AM, Chuck Guzis via cctalk wrote:
>> It's always struck me how revolutionary (for IBM) the change in
>> architecture from the 700x to the S/360 was.  The 709x will probably
>> strike the average reader of today as being arcane, what with
>> sign-magnitude representation, subtractive index registers and so on.
>> The 7080, probably even more so.  But then, most of IBM's hardware
>> before S/360 had its quirky side; the only exception I can think of,
>> offhand, would be the 1130, which was introduced at about the same time
>> as the S/360.
> Pretty much all computers of that early-60's vintage, where a maze of logic was used to decode instructions, and everything was done with discrete transistors and diodes, had quirky arcane instruction sets.  Some of this was due to the prevailing thought on instruction sets, but part of it was done to save a few transistors here and there, and to heck with the side effects.  Most of these computers had very few registers, or put the "registers" in fixed core locations, due to the cost of a flip-flop.  The 709x series was certainly like that.  Hard to BELIEVE, with 55,000 transistors!

I can't remember how many transistors a CDC 6600 has.  A lot more than that, I'm pretty sure.

On "quirky arcane instruction sets" -- some yes, some no.  The CDC 6000 series can make a pretty good argument for being the first RISC machine.  Its instructions are certainly quite nicely constructed and the decoding involved is pretty compact.  While I don't think the term "orthogonal" had been applied yet to instruction set design -- I first saw that used for the VAX -- it fits the 6000 too.

Another example of an instruction set design that's pretty orthogonal is the Electrologica, especially in the X1 (from 1958).  It's a one address machine, not a register machine like the 6000 or traditional RISC, but in other ways it looks a lot like RISC.  Wide instructions with fixed fields allocated for fixed purposes (like register numbers, operation numbers, conditional execution modifiers, etc.).

The 360 was certainly significant in delivering many of these things in a very successful commercial package.  And I can believe it being revolutionary for IBM -- but not quite so much for the industry as a whole.


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