Original 11/74 front panel

Don North ak6dn at mindspring.com
Thu Feb 9 14:01:18 CST 2006

Johnny Billquist wrote:
> "Julian Wolfe (FireflyST)" <fireflyst at earthlink.net> wrote:
>> Okay, what I immediately noticed is that these machines say "MP" and 
>> Don's panel doesn't.
> More differences: Don's panel have the location for the key moved to 
> the other side of the parity lamps. Which is rather weird.
> Also, the screenprint of 11/74 looks shadowed, which is weird. And the 
> right hand side have a different color than the left hand side.
> And of course, the 11/74s from Tim don't have any CIS stuff on the 
> front panel.
Besides moving the keyswitch, the dial selector knobs on the righthand 
side were moved around as well to make space to label all the extra 
switch positions.

As I mentioned in another reply, the artifacts are due to scanning the 
panel on a 14" flatbed scanner in two pieces, and merging the scans. 
When I get around to it I'll take a high quality single-view photograph.
> (I'm still thinking that Don's panel have never had a matching piece 
> of real hardware, since I don't think that CPU was ever built.)
I can only say that I worked on real 11/74 hardware in 1978-9 writing 
microcode for the 11/74 CIS (commercial instruction set) accelerator 
(packed decimal, string instructions, etc). See 
for a memo written by Ray Boucher, one of the hardware designers (Al, 
how did you ever come across a memo such as this???). The other hardware 
designed was Mike Brown. The other microcoders were Dave Sager and Lewis 
Costas. The project manager was Dave Barry. How much more do you need to 

Not a lot of 11/74 CIS systems were built, probably no more than a dozen 
or so maximum. The 11/74 folded in the 11/70MP changes which had been 
done about a year earlier, so an 11/74 was really an 11/70MP base plus 
the CIS option. Later when the 11/74 CIS was killed as a product, the 
11/74 name lived on, but it was strictly a renamed version of the 11/70MP.
> Tim's picture however are of a real machine. Don't know which one, 
> though. If it's just a two CPU machine, it might have been POLLUX::, 
> since CASTOR:: was four CPUs. However, other machine have existed in 
> the past.
>> Were there non-multiprocessor 11/74s?  If so, what would have been 
>> the benefits over 11/70s?
> Yes there were. And the benifits were none. Actually, they were 
> slightly worse than normal 11/70s, but the difference were minimal.
> After DEC decided to not make the 11/74 into a commercial product, 
> they used 11/74 parts for 11/70 machines. Atleast inhouse. Not sure if 
> any of those parts found their way into customers machines.
> Used as such, the differences were basically related to cache: The MMU 
> have the cache bypass bit. The ASRB instruction always bypass the 
> cache, and you can also order the machine to explicitly bypass the 
> cache (unless my memory fails me).
> But nothing of this was used by any OS normally, so the only thing 
> noticeable would be the slower ASRB instruction.
>     Johnny
All true statements. ASRB to memory was modified to act as the semaphore 
instruction and thus always bypassed the cache AND did an atomic 
read/modify/write cycle to the memory. The valid semaphore memory values 
were 0 and 1; ASRB would then always set the memory value to 0 and 
return the old value in the C-bit from the right shift carry out.

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