---

Using the bit-wise operators in C

---

• Using the bitwise operators
  • The bit-wise operations are typically used do the following:

    Set a certain bit in a byte, short, int, or long variable Clear a certain bit in a byte, short, int, or long variable Flip a certain bit (0 ⇒ 1 or 1 ⇒ 0) in a byte, short, int, or long variable Test a certain bit in a byte, short, int, or long variable

    ---

  • Note:

    The bit-wise operations often use a certain bit pattern (or bit mask) The bit pattern can often be expression more easily using different number systems We will also look at how to express a value in different number system in C

  ---

  ---

  ---

  ---

• Setting a certain bit in a bit pattern
  • Set a certain bit in a bit pattern

    Suppose the variable a contains a bit pattern: a = 1010...x...1101 Problem: Set the bit x to the value 1 without changing all other bits in the pattern. We can set the bit x inside the bit pattern to 1 using the OR operation with the following specific pattern: a = 1010...x...1101 0000..010..0000 OR --------------- 1010...1...1101

    ---

  • Example: set the 2nd bit and the 4th bit in a bit pattern

    int main( int argc, char* argv[] ) { char a = 0; /* 0 = 00000000 */ /* Bit pos: 76543210 */ a = a | 4; /* 16 = 00000100 - set 2nd bit */ a = a | 16; /* 16 = 00010000 - set 4th bit */ printf( "a = %d\n", a ); }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc bit-op2.c To run:          ./a.out

  ---

  ---

  ---

  ---

• Observation: we need a better way to make bit patterns
  • Fact:

    The decimal number system is terrible for expression a bit pattern

    ---

  • Solution:

    Use different number systems !!!