Mask (computing)

In computer science, a mask is data that is used for bitwise operations, particularly in a bit field.

Using a mask, multiple bits in a byte, nibble, word (etc.) can be set either on, off or inverted from on to off (or vice versa) in a single bitwise operation.

Common bitmask functions

Masking bits to 1

To turn certain bits on, the bitwise OR operation can be used, following the principle that Y OR 1 = 1 and Y OR 0 = Y. Therefore, to make sure a bit is on, OR can be used with a 1. To leave a bit unchanged, OR is used with a 0.

Example: Masking on the higher nibble (bits 4, 5, 6, 7) the lower nibble (bits 0, 1, 2, 3) unchanged.

Masking bits to 0

More often, in practice, bits that are to be ignored are "masked off" (or masked to 0) rather than "masked on" (or masked to 1). There is no way to change a bit from on to off using the OR operation. Instead, bitwise AND is used. When a value is ANDed with a 1, the result is simply the original value, as in: Y AND 1 = Y. However, ANDing a value with 0 is guaranteed to return a 0, so it is possible to turn a bit off by using AND with 0: Y AND 0 = 0. To leave the other bits alone, use AND with 1.