On Tue, Jun 19, 2018 at 8:23 AM, Noel Chiappa <jnc@mercury.lcs.mit.edu> wrote:

> From: Doug McIlroy <doug@cs.dartmouth.edu>

> Yet late in his life Forrester told me that the Whirlwind-connected
> invention he was most proud of was marginal testing

Given the above, I'm totally gobsmacked to hear that. Margin testing was
important, yes, but not even remotely on the same quantum level as core.

Wow -- I had exactly the same reaction. To me, core was the second most important invention (semiconductors switching being he first) for making computing practical. I was thinking that systems must have been really bad (worse than I knew) from a reliability stand point if he put marginal testing up there as more important than core.

Like you, I thought core memory was pretty darned important. I never used a system that had Williams tubes, although we had one in storage so I knew what it looked like and knew how much more 'dense' core was compared to it. Which is pretty amazing still compare today. For the modern user, the IBM 360 a 1M core box (which we had 4) was made up of 4 19" relay racks, each was about 54" high and 24" deep. If you go to

CMU Computer Photos from Chris Hausler

and scroll down you can see some pictures of the old 360 (including a copy of me in them circa 75/76 in front of it) to gage the size).

FWIW:

I broke in with MECL which Motorola invented / developed for IBM for System 360 and it (and TTL) were the first logic families I learned with which to design. I remember the margin pots on the front of the 360 that we used when we were trying to find weak gates, which happened about ones every 10 days.

The interesting part to me is that I'm suspect the PDP-10's and the Univac 1108 broke as often as the 360 did, but I have fewer memories of chasing problems with them. Probably because it was a less of an issue that was causing so many people to be disrupted by the 'down' time.