Computer History: The DEC PDP 11/70

Last week my friend Wesley Twittered about his first computer. It got me thinking about one of the first computers I used, so I replied to him about the DEC PDP 11/70. I also recalled a skill I developed at that time to convert octal (base 8) to binary in my head. More on that later. Here is what the control panel looked like for many PDP systems.

Photo credit: Wikipedia

The PDP family of computers were known as mini-computers to distinguish them from mainframes that ran in the data center. The workhorse PDPs ran in offices, on factory floors, and a privileged few were coddled in data centers too.

Getting this computer running was not as easy as pushing a button on a modern PC. You had to use the switches on the front panel to load an address, deposit a value, and then hit the start switch on the far right. Once it was booted, you could also tell what operating system was running by looking at the motion of the data lights when the CPU was idle. It it was running RSTS/E the lights made a marquee from left to right, and for RSX the lights started at both ends and met in the middle.

The address and deposited value were 7 digit values in octal (base 8) that you would find taped up on the front of the computer. The switches are in color coded groups of three, and you had to set the value using the 21 digit binary equivalent of the octal numbers. You can see the switch array at the bottom of the picture, numbered 0 to 20 from right to left. Programmers and hardware designers count starting a zero. Go figure.

Now I'm sure you all know how to convert octal to binary. You don't? Alright, here is a hint; each octal value can be represented by three binary digits. The binary digits have positional values of 4, 2, and 1. So 5 octal is 101 in binary. Here is a quick coversion table:

0 = 000
1 = 001
2 = 010
3 = 011
4 = 100
5 = 101
6 = 110
7 = 111

Now that you are equipped, what is 7742036 octal as a binary value? Quick now! Got it? The answer is 111 111 100 000 011 110.

Anyone who can convert octal to binary in your head raise your hand. Or post a comment since we are a diminishing breed and should probably meet.

If you'd like to see and experience some computer history for yourself, you should visit the Computer History Museum in Mountain View, CA. My personal experience of the museum was at its original location at Digital Equipment Corporation, where I worked at the time, and their facility on Museum Wharf in Boston. I'm looking forward to paying them a visit at their new West coast home. (Read more ...)