C Programming Tutorial - Low Level Operations

If byte b is "binary anded" with a byte m where all the bits of m are 0 except for the bit in b that we are interested in, then we can isolate that bit. If the value of the expression is non zero then that bit is set, if it zero it isn't.

So

if (a & b)
    printf("a & b was non zero") ;
else
    printf("a & b was zero") ;

Beware - Trap Ahead!

You might think from this that there seems little difference between (a && b) and (a & b). Sometimes this is true, but not always. E.g. if a is 4 and b is 2, then (a && b) is true as both are non zero but (a & b) is false!

a     00000100₂
b     00000010₂
-------------------
a & b 00000000₂

Setting Bits

Use the "binary or" operation for this which sets any bits to 1 in the result if either (or both) of the two numbers has a 1 in its corresponding bits.

a     10010110₂
b     01111100₂
--------------------
a | b 11111110₂

So to set the bit in a (below) at position 2 to a 1 we or with b = 00000100

a    10010010₂
b     00000100₂
--------------------
a | b 10010110₂

Toggle that bit!

The 3rd binary operation is "exclusive or" or xor which in C uses the ^ operator. If two numbers a and b are xored together then each bit in the result is 0 If the corresponding bits are identical, 1 if they are different. If it helps, think of xor as a "Not the same" function.

To toggle any bit, simply "binary xor" the number with a 1 set in the appropriate bit position. If the corresponding bit is 1, the result bit is 0 etc.

a    10010010₂
b    00000100₂
--------------------
a ^ b 10010110₂

On the next page - An example of printing binary numbers.