Jump Instruction does not work for Subroutine Call

Especially useful subroutines are put into subroutine libraries and can be called from many locations in many programs, and so, of course, they must return to many different locations.

There is a problem with using j for the call of and return from a subroutine.

The problem is illustrated at right. The main routine is written to call a useful subroutine sub at several locations in the code. But, as it is written, sub always returns control to the same location. Usually this will not work.

In the past, before the concept was completely understood, hardware support for subroutines was missing. Various nasty software tricks were used to implement the idea.

What is needed is a way to send a return address to the subroutine when it is called. Now when the subroutine finishes, it passes control to that return address.

Of course, "passing control to a return address" means that the subroutine loads the PC (program counter) with the return address. The next instruction fetch of the machine cycle will get the instruction from that address.