The answer is ... it isn't generating one
There seems to be a bug in the program in that a RANDOM command is entirely missing.
If you look at the flowchart of the program on the previous page there's a RANDOM after the LOW 4 which didn't make it into the program code on page 33.
Also note in the flowchart that it's RANDOM b1 ... This should be RANDOM w0 or something similar to generate a long sequence of random numbers.