Bit manipulation

Hi, my question is to do with writing individual bits. I want to be able to write individual bits rather than a whole char or int. If I allocated myself a byte of memory how can I then write the individual bits myself, is this even possible? I want to be able to choose the 1/0 status myself and write this accordingly.

EG:
say I wanted to write 10010111 into a byte of memory how would I do this?

Thanks

>is this even possible?
Of course, C offers several bitwie operators that can be used in a cryptic and arcane manner to do exactly what you want. Not really, it just seems complicated at first.

>say I wanted to write 10010111 into a byte of memory how would I do this?
Here is one way:

Code:

#include <stdio.h>

int main ( void )
{
  int i = 0;
  int mask[] = { 1, 2, 4, 16, 128, -1 };
  unsigned char bits = 0;

  while ( mask[i] != -1 ) {
    bits |= mask[i]; /* Set the bit */
    ++i;
  }

  printf ( "%d\n", bits );

  return 0;
}

-Prelude

I wrote an FAQ entry about this, but L@D has yet to put it up...*grumbles* lemme try and find a transcript.

ahh...here we go!

Code:

Bit Manipulation/Bit Shifting - Basic overview
================================================

Bit Manipulation/Shifting is the process of taking data, and toying with
the binary of it (the basic 1s and 0s that it's comprised of).

Bit operations in C/C++ are carried out with the following operators:

       -----------------------------------------------------------
       |Operator          Definition                                               |
       -----------------------------------------------------------|
       |  &        |    Bitwise AND (Not to be confused with &&)|
       |  |         |   Bitwise OR (Not to be conused with ||)      |
       |  ^        |    XOR (Exclusive OR)                                     |
       |  <<     |    Shift left                                                      |
       |  >>     |    Shift right                                                    |
       |  ~        |    Unary (One's compliment)                          |
       ------------------------------------------------------------


OR is fairly simple: 
The OR operator (|) compares two bits and returns true (1) if either or both bits are set to true. 

code:--------------------------------------------------------------------------------
(1 | 2)

   00000001
|  00000010
-----------------
   00000011
--------------------------------------------------------------------------------


It basically just adds the binary of the numbers. It doesn't actually add the numbers though, remember that. 

AND is just as easy: 
The AND operator (&) compares two bits and returns true (1) if both bits are set to true. 

code:--------------------------------------------------------------------------------
(1 & 3)

   00000001
&  00000011
------------------
   00000001
--------------------------------------------------------------------------------


See what it did? everywhere there was a 1 in the first one and the second, there was a 1 in the resulting number. 

XOR is trickier: 
The XOR operator (^) compares two bits and returns true (1)  if only one of the bits are set to true. 

code:--------------------------------------------------------------------------------
(1 ^ 3)

   00000001
^  00000011
------------------
   00000010
--------------------------------------------------------------------------------



Anywhere there was a 1 in both numbers, it became a zero. 

UNARY is the easiest. It basically means binary opposite: 


code:--------------------------------------------------------------------------------
(~1)

~0000000001 = 1111111110
--------------------------------------------------------------------------------


<< and >> (shift left and right) are incredibly simple, and used the most: 

000000010 >> 1 = 000000001 
000000001 << 1 = 000000010 

It basicall goes like this: number >> Amount to shift by 
It moves the numbers down by the number you place to the right. 

>> Is actually the quickest way to divide by 2 
<< Is actually the quickest way to multiply by 2 

Another thing about shifts, is that the numbers don't carry! soo: 

000000001 >> 1 = 00000000, NOT 10000000 


And remember I wrote the numbers out in binary instead of the actual numbers you'd put in there. ( 000000011 is 3, 0000000010 is 2, and 000000001 is 1 binary) So remember, you don't write the numbers in binary, you write them in decimal to use the bit operators.

How about bitfields?

Code:

typedef unsigned char byte;
struct bitfield{
  byte bit0 : 1;
  byte bit1 : 1;
  byte bit2 : 1;
  byte bit3 : 1;
  byte bit4 : 1;
  byte bit5 : 1;
  byte bit6 : 1;
  byte bit7 : 1;
};

There are a few drawbacks to bitfields however it does make it possible for you to do this

Code:

struct bitfield bf;

bf.bit0 = 1;
bf.bit1 = 1;
bf.bit2 = 1;
bf.bit3 = 0;
bf.bit4 = 1;
bf.bit5 = 0;
bf.bit6 = 0;
bf.bit7 = 1;