From a mouse to a light pen

While I’m remembering things from 40 years ago I thought I’d write a little more about working on the PDP-1 computer. While debugging the mouse interface I wrote about previously, I added a small enhancement to the light pen. The original light pen simply set a hardware flag when the pen detected light as the point was being painted. The software had to continually poll the state of this hardware flag to check if the light pen had detected anything. This was inefficient. I simply added a few gates and some control circuitry to request an interrupt when the pen detected light. This took the polling instructions out of the display loop, and on a slow computer like the PDP-1 meant more points could be displayed before flicker set in.

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