How do you set only certain bits of a byte in C without affecting the rest?

Question

Say I have a byte like this 1010XXXX where the X values could be anything. I want to set the lower four bits to a specific pattern, say 1100, while leaving the upper four bits unaffected. How would I do this the fastest in C?

Answer 1

In general:

value = (value & ~mask) | (newvalue & mask);

mask is a value with all bits to be changed (and only them) set to 1 - it would be 0xf in your case. newvalue is a value that contains the new state of those bits - all other bits are essentially ignored.

This will work for all types for which bitwise operators are supported.