C#, bits & bytes - How do I retrieve bit values from a byte?

Question

I'm reading some values from a single byte. I'm told in the user-manual that this one byte contains 3 different values. There's a table that looks like this:

I interpret that has meaning precision takes up 3 bits, scale takes up 2 and size takes up 3 for a total of 8 (1 byte).

What I'm not clear on is:

1 - Why is it labeled 7 through 0 instead of 0 through 7 (something to do with significance maybe?)

2 - How do I extract the individual values out of that one byte?

Answer 1

It is customary to number bits in a byte according to their significance: bit x represents 2^x. According to this numbering scheme, the least significant bit gets number zero, the next bit is number one, and so on.

Getting individual bits requires a shift and a masking operation:

var size = (v >> 0) & 7;
var scale = (v >> 3) & 3;
var precision = (v >> 5) & 7;

Shift by the number of bits to the right of the rightmost portion that you need to get (shifting by zero is ignored; I added it for illustration purposes).

Mask with the highest number that fits in the number of bits that you would like to get: 1 for one bit, 3 for two bits, 7 for three bits, 2^x-1 for x bits.

Answer 2

You can do shifts and masks, or you can use the BitArray class: http://msdn.microsoft.com/en-us/library/system.collections.bitarray.aspx

Example with BitVector32:

BitVector32 bv = new BitVector32(0);

var size = BitVector32.CreateSection(7);
var scale = BitVector32.CreateSection(3, size);
var precision = BitVector32.CreateSection(7, scale);

bv[size] = 5;
bv[scale] = 2;
bv[precision] = 4;

LINQPad output: