Subroutine, Subroutine nesting and Stack memory

1. Subroutine –
A set of Instructions which are used repeatedly in a program can be referred to as Subroutine. Only one copy of this Instruction is stored in the memory. When a Subroutine is required it can be called many times during the Execution of a Particular program. A call Subroutine Instruction calls the Subroutine. Care Should be taken while returning a Subroutine as Subroutine can be called from a different place from the memory.

The content of the PC must be Saved by the call Subroutine Instruction to make a correct return to the calling program.

Figure – Process of subroutine in a program

Subroutine linkage method is a way in which computer call and return the Subroutine. The simplest way of Subroutine linkage is saving the return address in a specific location, such as register which can be called as link register call Subroutine.

2. Subroutine Nesting –
Subroutine nesting is a common Programming practice In which one Subroutine call another Subroutine.

Figure – Subroutine calling another subroutine

From the above figure, assume that when Subroutine 1 calls Subroutine 2 the return address of Subroutine 2 should be saved somewhere. So if link register stores return address of Subroutine 1 this will be (destroyed/overwritten) by return address of Subroutine 2. As the last Subroutine called is the first one to be returned ( Last in first out format). So stack data structure is the most efficient way to store the return addresses of the Subroutines.

Figure – Return address of subroutine is stored in stack memory

3. Stack memory –
Stack is a basic data structure which can be implemented anywhere in the memory. It can be used to store variables which may be required afterwards in the program Execution. In a stack, the first data put will be last to get out of a stack. So the last data added will be the First one to come out of the stack (last in first out).

Figure – Stack memory having data A, B & C

So from the diagram above first A is added then B & C. While removing first C is Removed then B & A.

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