I've seen plenty of hardware registers that use one bit for some X, two bits for some Y, and 5 bits for some Z. (Yes, in embedded programming.) One way of dealing with that is with C bitfields.

Now, I never did it that way myself. I just and-ed it with a bitmask to get the part that I wanted. But it's a bit messier to do it that way, because if you want one of the fields as a number, and it's not from the least-significant bits, then you have to shift it as well as mask it, whereas with bitfields you can just read it as a number and get the right value.