There is a light pen!

[Bill E]

Well, more or less. I was looking at the display emulator code, p7sim. It emulates a light pen  via a mouse click. But, the pidp1 emulator doesn't ever check it. Hmm. Looks like it might be time for another distraction.

I'm missing some details about how the light pen worked, though. Here's what I know:

When a light pen hit occurs, the check-status bit 0 and program flag 1 are set.

The cks flag is cleared by a cks or the next dpy call.

What I'm missing:

There was one mention I saw that said the x, y coords of the hit are returned in the AC and IO registers when a dpy i is done, but I haven't found just what this means. What instruction returns it? The ioh? The next dpy? Any clues?

Does pf1 get cleared when the cks status is cleared?

Implementing this will require an emulator mod, but I've already made a number of those.

Bill

[Unibus]

Hi,

Don't know if any of this helps.

The Light Pen description includes:-

The Light Pen is designed to be used with the CRT Display Type 30. By "writing" on the face of the CRT, stored or displayed information can be expanded, deleted or modified. Specifically, each time a light-pulse strikes the pen, the Light Pen status bit is set to one (see definition of Check Status Instruction) and Program Flag 3 is set to one. At the time the status bit is set, the x-y coordinates of the -point just "seen" by the pen are in bits 0 through 9 of the AC and bits 0 through 9 of the 10, respectively (if the Display Instruction was given with the optional in-out wait). A program designed to accept Light Pen input would periodically check the Light Pen status bit and when found to be set, would store the C (AC) and C (10) thereby defining the point just "seen" by the pen. 

This should mean the X,Y position values from the previous dpy are still available in the AC/IO Registers. Light Pen status Flip-Flops are reset by the next dpy or the reset instruction for the Type 33 Symbol Generator. If not using the optional In-out completion wait then you are responsible for AC/IO Registers and the cks instruction.

There is a Light Pen Test Diagnostic Program. Catch is it is written for the PDP-4 and you would have to manually convert the code for the PDP-1. The diagnostic has a description on how to run but the code is undocumented.

http://www.bitsavers.org/pdf/dec/pdp7/DIGITAL-7-78-M_370LightPenDiag_Apr64.pdf

Architectural Evolution in DEC’s 18-Bit Computers, by Bob Supnik, February 2016 will provide an overview. Catch is he is showing the steps forward from PDP-1 to PDP-4 and you would have to reverse this conversion process.

https://simh.trailing-edge.com/docs/architecture18b.pdf

Regards,

Garry

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[Bill E]

Cool. I had seen that description, but it's vague on just how and when the AC and IO are valid. But, the program listing should help. There were functional differences between the way the -1 and -4 interacted with the display, but should give some clues.

Thanks, Bill