Question

I am trying to understand how to use Bitwise AND to extract the values of individual bytes.

What I have is a 4-byte array and am casting the last 2 bytes into a single 2 byte value. Then I am trying to extract the original single byte values from that 2 byte value. See the attachment for a screen shot of my code and values.

The problem I am having is I am not able to get the value of the last byte in the 2 byte value.

How would I go about doing this with Bitwise AND?

Answer 1

The problem I am having is I am not able to get the value of the last byte in the 2 byte value.

Your 2byte integer is formed with the values 3 and 4 (since your pointer is to a[1]). As you have already seen in your tests, you can get the 3 by applying the mask 0xFF. Now, to get the 4 you need to remove the lower bits and shift the value. In your example, by using the mask 0xFF00 you effectively remove the 3 from the 16bit number, but you leave the 4 in the high byte of your 2byte number, which is the value 1024 == 2^10 — 11th bit set, which is the third bit in the second byte (counting from the least representative)

You can shift that result 8 bits to the right to get your 4, or else you can ignore the mask altogether, since by just shifting to the right the lowest bits will disappear:

4 == ( x>>8 )

More interesting results to test bitwise and can be obtained by working with a single number:

int x = 7;              // or char, for what matters:
(x & 0x1) == 1;
(x & (0x1<<1) ) == 2;   // (x & 0x2)
(x & ~(0x2)) == 5;