CDC disk drives (was Re: what IBM system is this?)

[Paul Koning]

> On Jul 9, 2015, at 9:27 AM, P Gebhardt <p.gebhardt at ymail.com> wrote:
> 
> 
> 
>> I remember encountering them mostly as replacements for the terrible 821 
>> drives, which had the appearance of an 808, but substantially increased 
>> capacity.  I don't think any 821s were ever installed at a customer's 
>> site, however--I saw the lot of them that we had falling to the CE 
>> sledgehammer.
>> 
>> They were replaced by banks of 844s--lots of them.  I think one site had 
>> well over 100 installed, all hooked together with a MAC on at least 2 
>> Cyber 74s.
>> 
>> 
>> --Chuck
> 
>> 
> 
> 
> Wow, more than hundred of these CDC drives??? This must have been a massive 
> 
> installation! I didn't know so many disk drives could be combined with 
> 
> Cyber systems. Thanks for the insights!
> I was already amazed by pictures provided by the University of Auckland:
> 
> https://www.cs.auckland.ac.nz/historydisplays/FifthFloor/MagneticDataStorage/DataStorageImages/DiskPacks/CDCComputerRoom.jpg
> 
> It doesn't say, though, where this picture was taken.

Wow.  It isn’t clear how many systems are behind those drives.

And I thought CERL (University of Illinois PLATO) was a big setup.  When I was there (1975-1978) it had, I believe, about 20 844 drives.  Those were all connected to a single 7054 (via extenders of course, which aren’t well documented in the manuals I have seen) and from there to a dual mainframe setup (Cyber 73 and CDC 6500).  Both mainframes would do I/O, and on each there were two PPUs involved in the I/O to allow 1:1 interlace on 70-series channels.  This setup seriously exercised the drive reservation logic.

I ran into a controlware bug there, trying to use the Deadstart function (03uu).  It runs a sequence of conventional operations internally in the 7054, but as supplied it did not correctly handle a drive interlock status and would get things wedged up.  That wasn’t hard to fix; it gave me my one exposure to the controlware code, which is why I have a copy of it, though unfortunately not a copy of the manual.

In theory, a 7054 with extenders can support 64 drives, since the unit number is a 6-bit value.  And also, again in theory, you could hook up multiple 7054s if you need more drives than that.  So well over a 100, accessible to a single system, is at least theoretically possible.

Chuck, what (in this context) is a “MAC”?

	paul