Ytukyg

I'm pretty new to C and I seem to be messing up my bitmasking. From what I understand, it's a way to grab or create something of a subset from a binary value.

Say I want to grab the last 8 bits of an unsigned int value, containing 00001111000011110000111100001111.

How would I use AND/OR to grab those last 8?

Here's a more general solution to generate the mask based on how many bits you're interested int.

unsigned int last_n_bits(unsigned int value, int n)

{

    unsigned int mask = -1;

    if (n < sizeof(unsigned) * CHAR_BIT)

        mask = ((1<<n)-1);

    return value & mask;

}

You can use the BINARY AND operator for do a binary mask:

unsigned char last_eight_bits = my_number & 0b11111111

Mask-out all bits > 0xff:

value & 0xffu

Say I want to grab the last 8 bits of an unsigned int value, containing
00001111000011110000111100001111.

How would I use AND/OR to grab those last 8?

In this case, you would use AND. The following code will grab the 8 least significant bits:

unsigned int number = 0x0F0F0F0Fu;

unsigned int mask = 0x000000FFu; // set all bits to "1" which you want to grab

unsigned int result = number & mask; // result will be 0x0000000F

The &-Operator is used for an AND-Operation with each bit:

    00001111000011110000111100001111

AND 00000000000000000000000011111111

------------------------------------

    00000000000000000000000000001111

Be aware that "0 AND X = 0" and that "1 AND X = X". You can do further investigation at: https://en.wikipedia.org/wiki/Boolean_algebra.