SUBROUTINE &STACK

In a given program ,it is often necessary to perform a particular task many times on

different data values .When a program branches to a subroutine, we say that it is calling the

subroutine .The instruction that performs the branch operation is called a call_subroutine

instruction.After a subroutine has been executed, subroutine is said to return to the program

that called it by executing a Return instruction .The calling program must resume

execution , continuing immediately after the instruction that called the subroutine.

Call subroutine is a special branch instruction that performs the following operations

•store the contents of the PC in the link register (LR)

•branch to the target address specified by the instruction

The return from a subroutine branches to the address contained in the link register .

When one subroutine calls another,The return address of the second call is also stored in

the link register .It is essential to save the content of the link register in another location

before issuing a new call ,otherwise the return address of the first subroutine will be

lost . A stack data structure can be used to store the return addresses associated with

subroutine calls .Call-subroutine pushes the content of the PC onto the stack and loads

the subroutine address into the PC .The return instruction pops the return address from

the stack into the PC .

The exchange of information between a calling program and a subroutine is referred to as

parameter passing.

When calling a subroutine, a program may provide to the subroutine the parameters, ( the

operands or their addresses) to be used in the computation.

Later, the subroutine returns other parameters, ( the results of the computation. ) to the

calling program .

If many parameters are involved, there may not be enough general-purpose registers

available for passing them to the subroutine.

Alternatively, parameters can be placed on the stack ( stack can handle a large number of

parameters)

In the below example subroutine uses three registers , the contents of these registers are

saved by pushing them on the stack using the move_multiple instruction .Before return,

they are restored .

Tutorial By Namitha