dpy 720007 Display One Point Clears the light pen status bit and displays one point using bits 0 through 9 of of the AC to represent the (signed) X coordinate of the point and bits 0 through 9 of the IO as the (signed) Y coordinate. The third lowest nibble (bits 9-11) encodes the intensity (brightness) in 1's complement, where 3 is the brightest, 0 the default intensity, and 7 (-0) barely visible.
msq, lat ; Load AC from Test Word switches sas msv ; Skip next instruction if C(AC) = C(Y), msv = 777777 sza i ; Skip on ZERO (+0) AC law 2345 ; Load the AC with the number N add 7 ; Add C(Y) to C(AC) dac 7 ; Deposit C(AC) in Y rcl 9s ; Rotate combined AC & IO left 9s = 777 xor 7 ; Exclusive OR C(Y) with C(AC) dpy-i+300 ; Display One Point jmp msq ; Take next instruction from Y msv, 777777
HAKMEM Item 2:
HAKMEM Item 3 (Schroeppel):
DATAI 2 ADDB 1,2 ROTC 2,-22 XOR 1,2 JRST .-4