“Subroutines allow sequences of instructions to be used over and over from many places within the program. In effect, the subroutine-calling convention allows the programmer to define new instructions by using sequences of other instructions. The program accesses a subroutine by using a Jump instruction to load the program counter with the address of the subroutine; but before doing so, the computer saves the previous contents of the program counter in a special memory location. At the end of the subroutine, another instruction reads this return address and jumps back to the location from which the subroutine was called.”