Bitwise Operations

Bit operations is very popular questions in any firmware interview, you might be asked about it many times in a interview (by different interviewers), and those questions are very similar!!! 

Bitwise Operators

&     bitwise AND

|     bitwise inclusive OR

^     bitwise exclusive OR

<<    left shift

>>   right shift

~     one's complement (unary)

Swap a and b around

a ^= b; b ^= a; a ^= b;

or equivalently,

a ^= b ^= a ^= b;

note: a^b obtains a mask which has 0 on bits that a and b are the same, and has 1 on bits that a and b are oppsite

Return a mask that set the first 1

c & (~c + 1) or c & -c

Return a mask that set the first 0

~c & (c + 1)

  

Count number of 1

int count_one1(int c)

{

int i;

for (i = 0; c; i++)

c ^= (c & -c);

return i;

}

int count_one2(int c)

{

int i;

for (i = 0; c; i++)

c &= (c - 1);

return i;

}

int count_one3(int c)

{

int n = 0, i;

for (i = 0; i < 32; i++) {

if (c & 1)

n++;

c >>= 1;

}

return n;

}

  

Left rotate a byte by certain bits

char rotate(char c, int i)

{

i %= 8;

return (c << i) | ((unsigned char)c >> (8 - i));

}