64bit integer conversion

hi!
i was trying to create a processor that could run at 32 or 64 bits
for the 32 bits part, no problems
however, while synthetising for the 64 one, it fails with

ERROR:Xst:808 - "rom.vhd" line 81: Argument of 'to_integer' function must have size less than 32.

from what i understand, the to_integer function from numeric_std works with 32 bits vectors max!
is there a way to make it accept 64 bits vectors?
or is there some other function i should use?

thanks a lot!
Koda

tanukichu