How can I access a specific group of bits from a variable?

Question

I have a variable with "x" number of bits. How can I extract a specific group of bits and then work on them in C?

Answer 1

You would do this with a series of 2 bitwise logical operations.

[[Terminology MSB (msb) is the most-significant-bit; LSB (lsb) is the least-significant-bit. Assume bits are numbered from lsb==0 to some msb (e.g. 31 on a 32-bit machine). The value of the bit position i represents the coefficient of the 2^i component of the integer.]]

For example if you have int x, and you want to extract some range of bits x[msb..lsb] inclusive, for example a 4-bit field x[7..4] out of the x[31..0] bits, then:

1. By shifting x right by lsb bits, e.g. x >> lsb, you put the lsb bit of x in the 0th (least significant) bit of the expression, which is where it needs to be.

2. Now you have to mask off any remaining bits above those designated by msb. The number of such bits is msb-lsb + 1. We can form a bit mask string of '1' bits that long with the expression ~(~0 << (msb-lsb+1)). For example ~(~0 << (7-4+1)) == ~0b11111111111111111111111111110000 == 0b1111.

Putting it all together, you can extract the bit vector you want with into a new integer with this expression:

(x >> lsb) & ~(~0 << (msb-lsb+1))

For example,

int x = 0x89ABCDEF;
int msb = 7;
int lsb = 4;
int result = (x >> lsb) & ~(~0 << (msb-lsb+1));
//      ==   0x89ABCDE  & 0xF
//      ==   0xE (which is x[7..4])

Make sense?

Happy hacking!