Consider a machine language instruction CALL X, which stands for call procedure at location X. If the register apprach is used, CALL X causes the following actions:

RN         PC    +   IL
PC         X

where RN is a register that is always used for this purpose, PC is the program counter and IL is the instruction length. The called procedure can now save the contents of RN to be used for the later return.

A second possibilities is to store the return address at the start of the procedure. In this case, CALL X causes

X             PC   +   IL
PC           X    +   1

Both of these approaches have been used. The only limitation of these approaches is that they prevent the use of reentrant procedures. A reentrant procedure is one in which it is possible to have several calls open to it at the same time.