How do I extract bits from 32 bit number

Question

I have do not have much knowledge of C and im stuck with a problem since one of my colleague is on leave.

I have a 32 bit number and i have to extract bits from it.

Answer 1

You need masks to get the bits you want. Masks are numbers that you can use to sift through bits in the manner you want (keep bits, delete/clear bits, modify numbers etc). What you need to know are the AND, OR, XOR, NOT, and shifting operations. For what you need, you'll only need a couple.

You know shifting: x << y moves bits from x *y positions to the left*.

How to get x bits set to 1 in order: (1 << x) - 1

How to get x bits set to 1, in order, starting from y to y + x: ((1 << x) -1) << y

The above is your mask for the bits you need. So for example if you want 16 bits of 0xD7448EAB, from 10 to 25, you'll need the above, for x = 16 and y = 10.

And now to get the bits you want, just AND your number 0xD7448EAB with the mask above and you'll get the masked 0xD7448EAB with only the bits you want. Later, if you want to go through each one, you'll need to shift your result by 10 to the right and process each bit at a time (at position 0).

The answer may be a bit longer, but it's better design than just hard coding with 0xff or whatever.