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Say I have a byte like this 1010XXXX where the X values could be anything. I want to set the lower four bits to a specific pattern, say 1100, while leaving the upper four bits unaffected. How would I do this the fastest in C?
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Setting a bitUse the bitwise OR operator ( | ) to set a bit. number |= 1UL << n; That will set the n th bit of number . n should be zero, if you want to set the 1 st bit and so on upto n-1 , if you want to set the n th bit.
The idea is to use bitwise <<, & and ~ operators. Using expression “~(1 << (k – 1))“, we get a number which has all bits set, except the k'th bit. If we do bitwise & of this expression with n, we get a number which has all bits same as n except the k'th bit which is 0.
Clearing bit using macro: We use the bitwise AND operator (&) to clear a bit. x &= ~(1UL << pos); it will clear nth bit. You must invert the bit string with the bitwise NOT operator (~), then AND it.
In general:
value = (value & ~mask) | (newvalue & mask); mask is a value with all bits to be changed (and only them) set to 1 - it would be 0xf in your case. newvalue is a value that contains the new state of those bits - all other bits are essentially ignored.
This will work for all types for which bitwise operators are supported.
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