The PDP-1 was, as far as we were concerned, the first effective, elegant mini-computer. It arrived at the Laboratory in 1961. We had expected delivery in 1960, but some unexpected purchasing politics in Washington caused the delay. The computer originally was intended to support certain input/output functions; the card-to-tape and the inverse, tape-to-printer and tape-to-film functions. Doing that would leave more time for the big, important user problems to run on all the worker computers.

A second, and far more interesting, purpose was its use as a place where things could be tested before attempting to use them on the big machines. We called the PDP-1 our "Romper Room" because all of us were very much learners and idea try-outers. Here, we could play around with all sorts of things that might be useful on the larger "Physics Computers," and not be badgered by others needing to use the computer. We could test ideas, both software and hardware and, most importantly, get immediate feedback.

There were three features that endeared the PDP-1 to us users. First, since one could sit at the console, it was very easy to write and correct programs. Second, it was very easy to attach all sorts of I/O devices and, third, is was easy to get time on this machine. We learned very quickly, that only the card-to-tape, and possibly the tape-to-film services had a beneficial effect on the production loads of the worker computers. The other services did not represent enough volume to make a difference, and using them took too much time--in the opinions of the users who demanded that such services, especially film recording, be available on the worker computers.

The Romper Room role, however, was an outstanding success. By the time the PDP-1 arrived, we had begun thinking about the grand interconnection of all our computers to facilitate resource sharing. This planning ultimately resulted in the development of the "Octopus" network. All sorts of ideas for Octopus hardware and software were first tried on the PDP-1. There were two main reasons why this worked so well: Building interfaces for this machine was very easy and, when using it, there was no shouting and screaming from users waiting for you to finish and get out of the way so they could run their (important) problems. On the big machines, every second was scheduled; people evolved very devious schemes to start their turn early and run a bit into the next person''s time slot. In contrast, work on the PDP-1 was peaceful. Also, by virtue of sitting there while the program was executing, one could get a feeling for how the program ran, as well as time to pause and think, and make changes. Very civilized.

The general problem with our Romper Room activities was that anything that took significant time to complete but didn''t have anything to do with nuclear research was "frowned upon," and any needed support was only slowly and grudgingly provided.

As the Octopus idea became more specific, the PDP-1 was used to study what kind of keyboard would be best for use with the terminals to be installed in the users'' offices. The two keyboards most heavily studied were the IBM Selectric I/O typewriter and the Teletype ASR 33. Based on initial costs and the expected costs for maintenance, the ASR 33 was chosen.

The PDP-1 had two registers, the AC or Accumulator and an IO register. The basic cycle time for instructions that referenced memory was 10 microseconds. Instructions that did not need a memory cycle took 5 microseconds, and certain combinations of such instructions could be combined into one operation. These included the shift/rotate group, the skip group, and the general-register-manipulation or operate group.

I can''t resist noting that the modern personal computer comes with over 200 million 8-bit bytes (my Macintosh has 1.2 billion bytes). Initially, there were 12K 6-bit bytes (4K 18-bit words) available on the PDP-1. Modern memories tend to be filled with very sloppily-cleaned up system programs. That was definitely not true for PDP-1 system programs; not a single memory word was ignored.

Table 1 shows a rough summary of the I/O devices installed on the PDP-1.