Converting byte[] of binary fixed point to floating point value

Question

I'm reading some data over a socket. The integral data types are no trouble, the System.BitConverter methods are correctly handling the conversion. (So there are no Endian issues to worry about, I think?)

However, BitConverter.ToDouble isn't working for the floating point parts of the data...the source specification is a bit low level for me, but talks about a binary fixed point representation with a positive byte offset in the more significant direction and negative byte offset in the less significant direction.

Most of the research I've done has been aimed at C++ or a full fixed-point library handling sines and cosines, which sounds like overkill for this problem. Could someone please help me with a C# function to produce a float from 8 bytes of a byte array with, say, a -3 byte offset?

Further details of format as requested:

The signed numerical value of fixed point data shall be represented using binary, two's-complement notation.For fixed point data, the value of each data parameter shall be defined in relation to the reference byte. The reference byte defines an eight-bit field, with the unit of measure in the LSB position. The value of the LSB of the reference byte is ONE. Byte offset shall be defined by a signed integer indicating the position of the least significant byte of a data element relative to the reference byte. The MSB of the data element represents the sign bit. Bit positions between the MSB of the parameter absolute value and the MSB of the most significant byte shall be equal in value to the sign bit.

Floating point data shall be represented as a binary floating point number in conformance with the IEEE ANSI/IEEE Std 754-2008. (This sentence is from a different section which may be a red herring).

Answer 1

Ok, after asking some questions from a local expert on the source material, it turns out CodeInChaos was on the right track...if the value is 8 bytes with a -3 byte offset, then I can use BitConverter.ToInt64 / 256^3, if it is 4 bytes with a -1 byte offset then BitConverter.ToInt32 / 256 will produce the correct answer. I guess that means BitConverter.ToXXX where XXX is signed is smart enough to handle the twos-complement calculations!

Thanks to those who tried to help out, I thought it couldn't be too complicated but getting that 256 offset from the reference document wording was very confusing:-)

Answer 2

System.BitConverter works very slow, so if performance is significant to you, i'd recommend to convert bytes to int by yourself (via logical shifts). Also, please specify in what exact format floats are sent in your protocol.