How many bits would be needed for the byte?
In a processor architecture of 64 bits the smallest single 'word' is 64 bits. that is each cyclic transfer from buffer to buffer occurs on a 64 bit buss. to identify a single byte of this native word in fact would only take 3 bits, but the actual register might in fact be 8-16 bits... if it was designed that way. the compiler that was designed to handle this would take all this into account and when you incremented a pointer it would do the conversion for you. as well as if the 'byte ' was actually an Unicode 16 bit character'. you need to look at the cpu programmers guide or manual for actual specifics and also the compilers manual, specifically with regards to data types and to addressing them.