Simple Bit Manipulation Macros

Stephen Friederichs March 12, 20134 comments Coded in C

Everyone working with low-level registers has needed to set, toggle and clear individual bits without disturbing the other bits.  This requires a simple but often difficult to remember series of ORs, ANDs and NOTs that are more often easier to just implement as a macro.  This code snippet implements macros to set a certain bit to '1', to clear a bit (set it to '0'), to toggle a bit (switch from '1' to '0' or '0' to '1') and to read the value of a bit.

/* Basic bit manipulation macros
   No one should ever have to rewrite these
*/

//Set bit y (0-indexed) of x to '1' by generating a a mask with a '1' in the proper bit location and ORing x with the mask.

#define SET(x,y) x |= (1 << y)

//Set bit y (0-indexed) of x to '0' by generating a mask with a '0' in the y position and 1's elsewhere then ANDing the mask with x.

#define CLEAR(x,y) x &= ~(1<< y)

//Return '1' if the bit value at position y within x is '1' and '0' if it's 0 by ANDing x with a bit mask where the bit in y's position is '1' and '0' elsewhere and comparing it to all 0's.  Returns '1' in least significant bit position if the value of the bit is '1', '0' if it was '0'.

#define READ(x,y) ((0u == (x & (1<<y)))?0u:1u)

//Toggle bit y (0-index) of x to the inverse: '0' becomes '1', '1' becomes '0' by XORing x with a bitmask where the bit in position y is '1' and all others are '0'.

#define TOGGLE(x,y) (x ^= (1<<y))