PROBLEM IN MODULE POOL PROGRAM

HI TO ALL

I AM CREATING A MOD POOL PROGRAM IN WHICH A SCREEN IS CREATED .

screen fields are

billid,order_id, qty , unit_price .

tolal is the field in the table.

plz tell me what calculation i should use to calculate the total as I enter the data and it should be updated in table.

plz tell me the solution.

Thanks & Regards

Anubhav