Thanx Kunal
Done it as 24 bits before i posted the question, I will look into the carry flag idea and see how it goes.
looks like it would save lots of time because when the 17th bit is 0 everything is 16 bits so i expect the timing would be improved by at least 25%.
Maybe one easy solution would be if bit 17 = 0 do math 16 else do math 24.

Regards
Mahmood