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

You can set all those bits to 0 by bitwise-anding with the 4 bits set to 0 and all other set to 1 (This is the complement of the 4 bits set to 1). You can then bitwise-or in the bits as you would normally.

ie

 val &= ~0xf; // Clear lower 4 bits. Note: ~0xf == 0xfffffff0
 val |= lower4Bits & 0xf; // Worth anding with the 4 bits set to 1 to make sure no
                          // other bits are set.