My first earliest experiences with "large" computers were an HP-2100 and a sigma-7, using a teletype and punch cards/line printer respectively.

I've stumbled upon an old image of a PDP-6 and PDP-6 here which had a large (16 inch) circular CRT monitor (and it seems a light pen as well).

Where these experimental devices or special graphics applications, or were these used to program and/or operate the computers routinely? If so, how?

below: from ultimate.com PDP-6 serials

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below x3: from PDP-1

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below x2: from archive.computerhistory.org PDP-1_Online

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    Spacewar!, of course. :-)
    – dirkt
    Dec 7, 2018 at 9:23
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    @dirkt This is clearly the correct answer!
    – uhoh
    Dec 7, 2018 at 9:26
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    A routine use of a light pen I've observed not on a PDP, though, but on a MIR-2 computer. For text editing with a CRT display and an electric typewriter, the light pen was used pretty much the same way the mouse is used today to indicate the text insertion point, or to copy-paste.
    – Leo B.
    Dec 7, 2018 at 23:47
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    By "main uses", are you asking for a list? There's Spacewar, already commented. There's project PEPR (Precision Encoding and Pattern Recognition), There's SHRDLU for the PDP-6. There are dozens of others. My effort, which I included as an answer is dwarfed by any of the above. Dec 9, 2018 at 12:39
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    @WalterMitty I think your answer is particularly interesting; (solving for, and then) envisioning molecular structure really leveraged every bit of technology available as it evolved, that must have been quite a lot of fun! So do you think it's safe to say that this is a specialized I/O device available to developers for a variety of applications, but for many uses or basic programming it was not required? Or was the CRT an essential part of computer operation? These are plenty of examples for me (others may enjoy seeing more) but it's the overall role I'm interested in. Thanks!
    – uhoh
    Dec 9, 2018 at 13:06

7 Answers 7


I got a job in the summer of 1965, programming a PDP-6 to put a model of a protein molecule on the display, as a stick figure. The molecule was 3D and the display was of course 2D, so I had to simulate depth. I used perspective, shading, and motion.

The idea of the display was to allow a human crystallographer to evaluate the actions of another computer program, to see if they seemed real. The other program, running on an IBM 7094, was bending and twisting the molecule so as to optimize something like bonding energy.

This was hard, and I only had a couple of computer courses and a little random hacking under my belt at the time. The hardest part was programming the modem that was networked to the IBM 7094. It took me all summer just to get something going.

Edit: There are a number of applications that used the CRT on either the PDP-1 or the PDP-6. These were all much more impressive than my efforts. A partial list follows:

SHRDLU. This is a program that could manipulate a virtual set of blocks using a virtual robotic arm. It acted on the blocks, based on natural language input from the user. The CRT was just to let the user see what SHRDLU was doing with the blocks. The main focus of SHRDLU wasn't the display. It was early exploration of natural language recognition.

Expensive Desk Calculator. This was a predecessor of VisiCalc, Lotus 1-2-3, Excel, or any of the electronic spreadsheets.

Expensive Typewriter. This was a predecessor of Word*Star, Word Perfect, Word, or any of the word processors.

Spacewar. My favorite, and already mentioned in another answer. Often used as a demo of the capabilities of the CRT. Predecessor of Space Invaders, Asteroids, or any of the video games.

Project PEPR. (Precision Encoding and Pattern Recognition) This was about automating the scanning and evaluation of millions of bubble chamber photographs. The CRT was used for, among other things, showing the user an image of the current photograph.

I don't know whether the Harmony Compiler belongs in this list or not. I'm not sure it used the CRT. But it sure used the speakers! This was a predecessor to the audio CD, MP3, and all that.

  • 1
    Were the display and light pen used by the OS at all, or only by applications? Dec 7, 2018 at 21:20
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    At the time, there wasn't really an OS for the 6. I was running on the raw iron. Some other people may have had an OS available to them, but I didn't know about it. The OS, if any, would have been an early version of ITS. Dec 7, 2018 at 21:23
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    Come to think of it, DEC must have had an early version of the Monitor out by 1965, but I never saw it that summer. Dec 7, 2018 at 21:41
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    What controlled the display timing? Could one generate a bunch of coordinates, send them to the display as a group, and then have the hardware visit each in sequence while code worked on the next batch, or did one have to send each coordinate to the display individually? If the latter, were there any timer-tick interrupts or other timer hardware resources to help the programmer output points at a uniform rate?
    – supercat
    Dec 11, 2018 at 21:39
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    I've forgotten the details of how the CRT controller worked. Hey, it's been over fifty years. I think it had vectors, and I think I programmed it in the simplest possible way. I was just a college kid at the time, and not one of the grand wizards. Dec 12, 2018 at 11:23

On the PDP1 at least they were used to play video games! Specifically SpaceWar, a predecessor to Asteroids (and every other top down game, really)



There was a system named Automatic Fingerprint Identification System that used PDP6, PDP8 and PDP11

  • 3
    Welcome to Retrocomputing! This answer is too short. Can you improve it by adding more detail about what this system was, and how it used the CRT and light pen? Links to or quotations from other sources would also help.
    – DrSheldon
    Jan 7, 2019 at 0:35

Short answer:

Where these experimental devices

No. They could be ordered as standard options.

or special graphics applications,

That was the intention. They where more like plotters or printers.

In fact, DEC made good sales of them as 'graphics terminals' on mainframes. The mainframe did whatever calculations and the results were either transmitted as a set of coordinates toward the PDP to display them directly (playing dumb terminal), or more often, the 'standard' (read: paper) output of the mainframe was redirected to the PDP, interpreted there by a special-to-type program to display a related graph. The latter was quite handy, as it required only small modifications to the mainframe applications and left all graphic interpretation (programming) to the 'intelligent' terminal.

or were these used to program and/or operate the computers routinely?

There have been applications to use them that way, but otherwise not. Programming was still a job for punch tape and TTYs.


PDP-5's as wells

enter image description here

I just ran across the YouTube video The Incredible Machine (1968) that shows the screen and light pen being used to do interactive circuit design and simulation.

GIF was made at about 02:44 in the video.

Once again, this is an example of a specific interface application, rather than to "program and/or operate the computers routinely."

From the video's caption:

The Bell Labs 'Graphic 1' computer system consisted of a Digital Equipment Corporation 'PDP-5' computer coupled with input devices such as the 'Type 370' light pen and Teletype Corporation 'Teletype Model 33' keyboard, married to a Digital Equipment Corporation 'Type 340' precision incremental display backed by 36-bit Ampex 'RVQ' buffer memory capable of storing 4096 'words'. The resolution on the monitor was 1024×1024.

This system was designed to transform the graphics-based input into output to be fed into a IBM '7094' (200 Kflop/s). The entire thing was attached to a microfilm-based recorder - the Stromberg Carlson 'SC 4020', which took hours to read and record the data.


You asked "Where these experimental devices or special graphics applications, or were these used to program and/or operate the computers routinely?" I'm not sure this was answered directly?

The PDP-6 display is a Type 340, and the PDP-1 display is the similar Type 30. The latter is "point plotting", i.e. there is a PDP-1 instruction to send the X, Y coordinates to draw a point. Whereas with the Type 340, the PDP-6 will make a display list in mememory, from which the display will fetch instructions to draw points, lines, and characters by itself.

The Type 30 and Type 340 were not experimental. They were in the stock DEC product list. However the primary device used to operate the machines were teletypes, not CRT displays. In that sense, the graphics applications were a bit special and not part of routine operation.


I used to work for DEC. One of my customers had a GT40 that in conjunction with an 11/70 to make plots for radiation treatments.

  • Welcome to Retrocomputing Stack Exchange. Please read the tour. It'd be good if you could elaborate on this answer; how did it use the light pen, and how did it use the CRT?
    – wizzwizz4
    Apr 14, 2019 at 13:47
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    I never saw it in action. The main reason is that when I was there, there system would be down for maintenance. I did play lunar lander on it a few times, though.
    – Tom Gallo
    Apr 16, 2019 at 22:17

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