Programming in PIC16F877a

I wrote an assm. program for pic16f84a which controls two stepper motor functions. Now I want to use this program with pic16f877a. I declared all nesesarry ports. when I compile this program with mplab it gives no errors, but when I use this program in isis it gives an error. Tell me What to do?

Could be all sorts of things:

-Did you turn the '877 ADC off?
-do you run your '877 at the same speed as the 'f84
-Did you check all the files you use are in the same banks and/or available on the other µC...