127/255, 63/255, 31/255, 15/255, 7/255, 3/255, 1/255, 0.
PRN range [0;254]
carry isn't set if a == b
if PRN == 127, cp 127 won't set the carry
So on second use, PRN must land in [0;126] so that cp 127 sets the carry
This gives 127 successful outputs out of 255