Bit mask

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.

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.

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.

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

It is possible to use bitmasks to easily check the state of individual bits regardless of the other bits. To do this, turning off all the other bits using the bitwise AND is done as discussed above and the value is compared with 0. If it is equal to 0, then the bit was off, but if the value is any other value, then the bit was on. What makes this convenient is that it is not necessary to figure out what the value actually is, just that it is not 0.

Example: Querying the status of the 4th bit

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