why a opcode can't using in CCS

   I am  coding  using  Linear Assemble in CCS3.3 ,target: a C64X chip,

   the address i access,need increasing by degrees.for example:  ptrout +=ncols;

so :I using the opcode:addu ,

the following is the compile information:

       "vert.sa", ERROR!   at line 20: Illegal operand combination
             addu prout,piany_col,prout

PS: i first using the opcode :add,

the piany_col is 384,i print the number of prout ,following,

      -2147383740  -2147382204  -2147380668

,I Know,that is error

So I choose the add instruction with unsigned.but ADDU can't play a role.NEED help!