8 bytes data type

Hi,

I need to be able to keep track of files of over 2 GB so I need more than 4 bytes variables to store the byte length in.

I also need to be able to add or substract e.g. Len = Len1 + Len2 ;

The variable (may / has to be) unsigned.

I tried double (is double the only 8 bytes variable ??) but this one is of the type float ...

operators such as % don't work.
The byte mapping in the variable is also not what I expected.
E.g. I tried to memcpy the HiWord and LoWord out of doubles but this doesn't work.
When I looked at how the data is stored in the 8 bytes I couldn't make sense out of it (not what you e.g. can see in a DWORD)

Advice appreciated !
Sorry 40 points is all I have left