BOAC,
just to be picky and fit in with the rest of the crowd...
Eight bits give you 2^8=256 combinations. Mode C uses 12 bits (3 bits gives 2^3=8 combinations, thus 0-7, times four digits = 12 bits) giving 2^12=4096 combinations. Mode S uses 64 bits IIRC, giving 2^64=18446744073709551616 possible combinations. When we reach that number of A/C flying with mode S we will have to think of something else!
Cheers,
/ft, former computer geek who decided that Al, Ti and Jet-A was more fun than Si