An integrated circuit for executing external program codes comprises a processor, a read only memory for storing program codes of a first routine and a second routine, and a random access memory comprising a first memory block and a second memory block. The processor executes the first routine and uses a plurality of first memory units in the first memory block for accessing data. The processor executes the second routine and uses a plurality of second memory units in the first memory block for accessing data. The first and second memory units comprise one or more common memory units. The processor executes a third routine stored in an external read only memory and accesses the data of the third routine in the second memory block.