Add/subtract 2 quad integers as if they were unsigned?

[Michael Mattias]

The goal of Rosetta Code is to compare the same program in different languages, so I used the same function and variables names of the original one.
Of course it's possible to write a completely different algorithm using meaningful names and comments, perhaps better, but this is not the purpose in this case.

I hope you are not trying to compare the runtime performances of C++ code written for a 64-bit CPU to PowerBASIC code written for a 32-bit CPU. That's not even comparing apples with oranges, it's comparing apples with elephants.