How to create mask with least significat bits set to 1 in C

c bit-manipulation bitmask
sebi picture sebi · Sep 14, 2012 · Viewed 34.9k times · Source

Can someone please explain this function to me?

A mask with the least significant n bits set to 1.

Ex:

n = 6 --> 0x2F, n = 17 --> 0x1FFFF // I don't get these at all, especially how n = 6 --> 0x2F

Also, what is a mask?

Answer

Jerry Coffin picture Jerry Coffin · Sep 14, 2012

The usual way is to take a 1, and shift it left n bits. That will give you something like: 00100000. Then subtract one from that, which will clear the bit that's set, and set all the less significant bits, so in this case we'd get: 00011111.

A mask is normally used with bitwise operations, especially and. You'd use the mask above to get the 5 least significant bits by themselves, isolated from anything else that might be present. This is especially common when dealing with hardware that will often have a single hardware register containing bits representing a number of entirely separate, unrelated quantities and/or flags.

Related questions

What is Bit Masking?

I am fairly new to C programming, and I encountered bit masking. Can someone explain to me the general concept and function of bit masking? Examples are much appreciated.

Read More
What does (x ^ 0x1) != 0 mean?

I came across the following code snippet if( 0 != ( x ^ 0x1 ) ) encode( x, m ); What does x ^ 0x1 mean? Is this some standard technique?

Read More
I want to calculate the inverse mask for an unsigned char

I would like to calculate an inverse mask for an unsigned char.meaning if the original mask 0xc0 the the inverse mask should be 0x3f.that is to say all the bits should be flipped or inverted.I have …

Read More