foo, lat /load the 18-bit AC from the Test Word
add v /add the contents of memory location v to the AC
dac v /deposit the contents of the AC into v
rcl 9s /Rotate the combined AC-IO 9 bits to the left
xor v /exclusive-or the contents of v into the AC
dpy-i 300 /plot point at x = high ten bits of AC, y = high ten bits of IO
I have tried to reproduce this calculation accurately at the bit level, including the use of ones'-complement addition and "minus-zero gronking" (if a minus-zero results after an addition,it is replaced with plus-zero).
I said that the instruction used was "dpy" (pronounced "dippy"), but it was commoner to use "dpy-i 300."The dpy mnemonic included a 50 microsecond delay to allow the beam to settle, or something. If you knew that the loop was long enough to allow beam settling, you could eliminate the delay by subtracting the "i" bit, speeding up the loop. An intensity argument allowed setting the intensity to different levels, 300 being the highest available.
One problem with "dpy-i 300" was that if, by any chance, your algorithm hit a singularity in which it produced the same point over and over, the beam would be stationary, and a stationary point plotted with dpy-i 300 created a serious risk of creating a phosphor burn.
The use of point-plotting displays with long-persistence phosphors lent itself to a form of recreational programming that has no real equivalent in a world of raster displays. Perhaps the nearest thing I have seen to it recently is the creative experimentation with Mandelbrot sets in the nineties.
Indeed, it seems to me in retrospect that some of these display hacks would now be described as "chaotic" (in the sense of chaos theory).