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Macro definition to write a program to test a bit.

Posted on 2003-12-09
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Last Modified: 2010-04-15


how do you write a macro to test whether a bit in a byte is set or not set?

For example if we want to test the fourth bit of a byte, how do we do it using the macro.

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Question by:suresh204
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by:sunnycoder
ID: 9909926
Hi suresh204,

#define ISSET4(X) (X)&(0x8)

Cheers!
Sunny:o)
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by:sunnycoder
ID: 9909940
a small explanation is in order ...

given any byte say
                          1 1 0 1   1 0 1 0
to determine if nth bit is 1, bitwise AND it with a number which has all bits except nth bit as 0 ... here to determine, 4th bit we use
                          0 0 0 0   1 0 0 0

                          1 1 0 1   1 0 1 0
                     &   0 0 0 0   1 0 0 0
----------------------------------------------------
                          0 0 0 0    1 0 0 0
since answer is non zero, fourth bit is set

this is also referred to as masking .. since in the constant used, all other bits are set to 0 and 0 anded with anything gives a zero, the result depends on the only set bit in the constant (mask)
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ssnkumar earned 50 total points
ID: 9910136
You can use the following macro also:
#define IS_BIT_SET(X, pos) (0x01 << (pos - 1)) & (X)

"pos" is the bit which you want to test for number "X". Count starts from right. So rightmost bit is 1, next is 2, etc.
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