My Atari 2600 Pong Clock

August 6, 2014 | Atari 2600 | By: Mark VandeWettering

While looking for something completely different, I ran across the code and binary images for my old Atari 2600 “Pong Clock”. I realized that my previous post on the matter didn’t have pictures of my final version, so just for fun, here are a couple of Stella screengrabs (in NTSC “TV” mode, for enhanced realism).

I included a tiny intro screen. I showed this at a 2010 get together. The Conway glider at the top is animated:

simple.bin_6

It plays a game of pong against itself, with the score representing the current time. You can set the time using the left joystick. When the minutes tick change, the right player wins. When the hours change, the left player does.

simple.bin_7

It also works on black and white tvs. I never did the changes necessary to make it play in PAL mode, although it should be pretty straightforward. The Atari 2600 was practically built for implementing Pong.

Still, it’s got some finesse in it. I never could have done it without all the hints from the Atari Age forums and the Stella 2600 emulator. My source code references a nice little bit of code from an Atari Age tutorial series, which I shamelessly purloined. I left a comment saying:

;;; According to the documentation, A isn’t really the position (0-160),
;;; you have to add +7 to the position. But I find that the offset in
;;; stella is +5. I haven’t done the cycle counting to figure it out,
;;; but I’ve had good luck trusting stella, so that’s what I’m going
;;; with.

Perhaps I’ll revisit that sometime and figure out what was right.

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