The cycle length is 2^17-1 = 131071. The number of pixels is 320*200 = 64000. Therefore each pixel is hit at least once. The authors likely used 17 bits instea

The cycle length is 2^17-1 = 131071. The number of pixels is 320*200 = 64000. Therefore each pixel is hit at least once. The authors likely used 17 bits instead of 16 because then the x and y coordinates can be obtained via masking rather than modulo (8 bits -> 256 values < 320 pixels).