Oh, the pain, the pain and the embarrassment…
I just came to realize that although a “long” in C# is 64 bits, in C++ it is still 32 bits. In order to get a 64 bit value in MSVC++ you must type either “long long” or “__int64″. I didn’t know that. 
This means that although the assembler function I just presented correctly calculates a 64 bit value, it will be truncated to 32 bits because the surrounding C++ function is declared as a long.
This in turn means that for bitmaps larger than 138 x 138 pixels – the correct result cannot be guaranteed. (With 64 bit values, the bitmap can instead be 9724315 x 9724315 pixels in size before an overflow can occur.)
Unfortunately, although I had unit test to verify the correctness of the function, I only tested with small bitmaps.
I have uploaded a new version. Ekeforshus
