The Basics

Suppose we have a single bit x.

  1. To set it, let x = x | 1.
  2. To clear it, let x = x & 0
  3. To read it (i.e. not modifying any bit), read the result of x & 1
  4. To toggle it, let x = x ^ 1

Create the Mask

Suppose we have an unsigned int number of 10111011.
Suppose index start at 0. Suppose n = 3, m = 5

To set bits, we want our interest interval to be all 1’s.

  1. To set the nth bit: 
     mask = (1 << n)         # mask = 1000
 x = x | mask
  2. To set the last n bits: 
     mask = (1 << n) - 1   # mask = 1000 - 1 = 111
 x = x | mask
  3. To set from mth bits to nth bits, inclusive 
     mask = ((1 << m) - 1) - ((1 << n-1) - 1) 
 x = x | mask
  4. To clear the nth bit: 
     mask = (1 << n) - 1
 x = x & mask