OR Bit Operation

closed account (S6k9GNh0)
Could someone explain flags and their use of '|' or OR. Could you also note what it does internally and externally?
Disch (13742)
0 OR 0 = 0
0 OR 1 = 1
1 OR 0 = 1
1 OR 1 = 1

This is done with every bit in the variable.

Ex:

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unsigned char foo = 0x36;
foo |= 0x1A;

/*
  foo now = 0x3E because:

  36 =  0011 0110
  1A =  0001 1010
  -------------------
  3E =  0011 1110
*/


Basically, OR is used to make sure a specific bit is flipped on. For example:

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flags |= 0x10;
// no matter what 'flags' was to start, we now know for a fact
//  that the 0x10 bit is flipped on.
//  the status of all other bits remain unchanged.


================================================

AND is the opposite (almost)

0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1

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unsigned char foo = 0x36;
foo &= 0x1A;

/*
  foo now = 0x12 because:

  36 =  0011 0110
  1A =  0001 1010
  -------------------
  12 =  0001 0010
*/


AND is used to isolate a specific bit, or to turn specific bits off:

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if(flags & 0x10)
{
  // this code executes only if the 0x10 bit of flags was set
}


The complement operator (~) flips all bits of a byte:

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unsigned char a = 0x0F;
a = ~a;

 // a now = 0xF0 because all 8 bits have been flipped


This can be used with AND to turn off a specific bit:

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flags &= ~0x10;

/*
  no matter what flags was to start, we now know that the 0x10 bit is clear

  all other bits remain unchanged
*/
closed account (S6k9GNh0)
Thank you. This helped quite a bit.
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