[Simh] Simh Digest, Vol 198, Issue 16

[Don North]

On 7/10/2020 7:20 AM, Johnny Billquist wrote:
> On 2020-07-10 14:19, Paul Koning wrote:
>>
>>
>>> On Jul 9, 2020, at 10:40 PM, Johnny Billquist <bqt at softjar.se> wrote:
>>>
>>> On 2020-07-10 04:37, Don North wrote:
>>>> On 7/9/2020 6:25 PM, Bob Supnik wrote:
>>>>> Yes, the PDP11 Architecture Handbook was a post-facto effort. The J11 was 
>>>>> finished; DEC did not intend to do another PDP11 processor. (I wrote a 
>>>>> spec for one, primarily as an exercise in trying to do a different 
>>>>> microcode structure than the PLA/ROM of the LSI11/F11/J11, but I lost it.) 
>>>>> The only formal part of the PDP11 architecture was the Commercial 
>>>>> Instruction Set extension, DEC STD 168, which was only implemented by the 
>>>>> F11 and the 11/44.
>>>> AND the PDP-11/74 CIS option, I might add. Fully implemented, never sold.
>>>
>>> Not to mention that the 11/74 in itself was fully implemented, but never 
>>> sold... Not even the 11/70MP...
>>
>> Were there actually two prototypes called 11/74?  I know the MP machine, 
>> which the RSX-11 development team owned.  And in Merrimack (home of RSTS and 
>> some of the compiler teams) there was an 11/74 with CIS, for COBOL.  But that 
>> one wasn't an MP machine.  Perhaps a coincidence, I don't have a real memory 
>> either way.
>
> Well, yes and no...
>
> The PDP-11/70MP is a modified KB11-C. In the end it's really just a modified 
> 11/70. There are changes to the microcode, and if I remember, a couple of 
> boards were changed. (See 
> http://www.bitsavers.org/pdf/dec/pdp11/1174/EK-70MP-TM_PRE_1170mp_Prelim_Technical_Manual_1977.pdf).
>
> The PDP-11/74 is the KB11-E. This was a more major redesign, which then 
> allowed for the addition of CIS. (See 
> http://www.bitsavers.org/pdf/dec/pdp11/1174/Prelim_KB11-E_Diffs_Aug78.pdf).
>
> But depending on what documentation you read, they might both be called 11/74.
>
>   Johnny
>
What Johnny said is basically correct. Way back then there were two separate 
programs going on:

1) a research program led by Verell Boaen (IIRC) on turning a standard 11/70 in 
an MP config. As indicated this required minor updates to the 11/70 microcode 
(to make the ASRB instruction act as a hardware interlock) and the IIST board 
for interprocessor communication. Multiport memory boxes also as I recall. This 
project was termed '11/70-MP'.

2) a hardware program led by Dick Brown (IIRC) in PDP-11 engineering to add CIS 
functionality to the 11/70 and update the CPU to what was called, then, the 
11/74. Some changes to CPU microcode to support CIS instruction dispatch and 
communication with the four board CIS processor. Changes to 11/70 backplane. 
Changes to a few CPU modules (not many and not complex).

At some point the CPU microcode update for ASRB was folded into the 11/74 CPU 
also, making it MP compatible.

I even have a plex of the updated 11/74 front panel I 'appropriated' when the 
11/74 was canceled, Here is a scan:

https://www.ak6dn.com/stuff/1174.jpg

Ultimately DEC marketing canceled the 11/74 CIS. There have been arguments and 
justifications over the years as to exactly why, but my communications with the 
marketing folks at the time was that when benchmarking Cobol performance the 
11/74 CIS significantly outperformed the 11/780 (which had just been released). 
So marketing had a positioning problem and the solution was to kill the 11/74 in 
favor of promoting the 11/780 family to eliminate the issue. Inarguably a wise 
decision at the time as the VAX was to be the future of DEC. At least until Alpha...

The 11/44 had a parallel implementation of the same CIS architecture (in fact I 
have an 11/44 with the hardware CIS option in my garage that is fully 
functional. Sadly no 11/74 ...) but it was much lower in performance (and cost) 
than the 11/74 CIS.

Don
DEC 11/74 microcode 1979-1980 or so

-- 
Don North
AK6DN