One of the original “hacks” of the PDP-1 in 1964, with a downloadable reproduction.
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