I have a question regarding some "borrowed" code:
How does the 0xff ensure that only 8 bits per byte are used? I'm not making the connection here. Can someone break this down in terms of "C for 5th graders".Code:#define ASIZE sizeof(int); unsigned int uival; unsigned char bytes[ASIZE]; int i; ... for (uival = 0, i = 0; i < ASIZE; i++) { uival = 256 * uival + (bytes[i] & 0xff); }
Thanks
Last edited by cdalten; 02-11-2006 at 12:56 PM.
0xFF is a value with the lower 8 bits set. ANDing with this value leaves only these lower 8 bits.
7. It is easier to write an incorrect program than understand a correct one.
40. There are two ways to write error-free programs; only the third one works.*
Okay, at the risk of being told I can google this, what the heck is bit sets and more to the point. What happens to the upper 8 bits?
Wait, I think I got it
0xff in binary is 11111111
now say I have ike the number 5.
then AND 0xff and 5 is
11111111
00000101
00000101
or 5
Set being 1, clear being 0.
FAQ > Explanations of... > Bit shifting and bitwise operations
FAQ > Prelude's Corner > Bit manipulation
7. It is easier to write an incorrect program than understand a correct one.
40. There are two ways to write error-free programs; only the third one works.*
I still don't get what happens to the upper 8 bits. Are they still there?
ANDing with zero clears a bit. So if bits 8 and above are zero and you and with anything, bits 8 and higher in the result will be cleared.
7. It is easier to write an incorrect program than understand a correct one.
40. There are two ways to write error-free programs; only the third one works.*
I'll start off with an example:
If any operand in an AND operation is 0 then the result is 0,Code:1001 1010 0111 1100 & 0000 0000 1111 1111 --------------------- 0000 0000 0111 1100
0 & 1 = 0
0 & 0 = 0
Which means that the zeroed part of the mask will be zeroed in the result too.
If any operand in an AND operation is 1 then the result is that of the other operand,
1 & 1 = 1
1 & 0 = 0 (one operand was 0 so result is 0 here)
So the part of the mask with ones will have the same value as the corresponding positions of the variable that was operated on.
And now I got it. Thanks.
You can check out this website as well
http://www.gamedev.net/reference/art....asp?the_id=69
