adding together the LSB and MSB to obtain a value

Question

I am reading data back from an imaging camera system, this camera detects age, gender etc, one of the values that comes back is the confidence value, this is 2 bytes, and is shown as the LSB and MSB, I have just tried converting these to integers and adding them together, but I don't get the value expected.

is this the correct way to get a value using the LSB and MSB, I have not used this before.

Thanks

just as an example, I have a value of 242 if I convert the LSB to an int, and 2 for the MSB, I am sure it is a simple equation, once its pointed out. — Racing57, Mar 03 '15 at 11:53
You can use bitwise operators: `(myByte & 1) + ((myByte & 128) >> 7)`, that's what they're for after all. You can also perform the whole calculation without addition if you'd like. — Benjamin Gruenbaum, Mar 03 '15 at 11:54

score 7 · Answer 1 · answered Mar 03 '15 at 11:55

7

Your value is going to be:

Value = LSB + (MSB << 8);

Explanation:

A byte can only store 0 - 255 different values, whereas an int (for this example) is 16 bits.

The MSB is the left hand^ side of the 16 bits, and as such needs to be shifted to the left side to change the bits used. You can then add the two values.

I would suggest looking up the shifting operators.

^ based on endienness (Intel/Motorola)

answered Mar 03 '15 at 11:55

Sarima

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Why is an int 16 bits in this example? The inferred type of MSB << 8 is Int32. – Ceisc Mar 03 '15 at 12:15
this seems to work a treat, the maximum confidence reported back I 1000, and I am getting up to that value.. I do need to read up a little on this I think, as I need to understand exactly what it is doing. but thanks !!! – Racing57 Mar 03 '15 at 12:28

score 1 · Answer 2 · answered Mar 03 '15 at 11:56

1

Assuming that MSB and LSB are most/least significant byte (rather than bit or any other expansion of that acronym), the value can be obtained by MSB * 256 + LSB.

answered Mar 03 '15 at 11:56

Ceisc

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Shifting is sometimes faster than multiplication: http://stackoverflow.com/questions/1168451/is-shifting-bits-faster-than-multiplying-and-dividing-in-java-net – Sarima Mar 03 '15 at 12:00
Yes, sometimes. It doesn't make the answer incorrect, however. – Ceisc Mar 03 '15 at 12:07
Yes, the jitter knows that as well. – Hans Passant Mar 03 '15 at 12:37
this also works, so which is the most efficient way of doing it, or are they both as good as each other ? – Racing57 Mar 03 '15 at 12:41

adding together the LSB and MSB to obtain a value

2 Answers2