Subroutine

In computer science, a subroutine (function, procedure, or subprogram) is a sequence of code which performs a specific task, as part of a larger program, and is grouped as one or more statement blocks; such code is sometimes collected into software libraries. Subroutines can be "called", thus allowing programs to access the subroutine repeatedly without the subroutine's code having been written more than once. In many programming languages the word function is reserved for subroutines that return a value; however, the C programming language and its programmers view subroutines simply as functions that do not return a value.

Low-level implementation

This section deals with the modern implementation of having subroutine data stored on one or more stacks.

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Due to usage of a stack, a subroutine can call itself (see recursion) or other subroutines (nested calls), and of course it can call the same subroutine from several distinct places. Assembly languages generally do not provide programmers with such conveniences as local variables or subroutine parameters. They get to be implemented by passing values in registers or pushing them onto the stack (or another stack, if there is more than one).

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When there is just one stack, the return addresses must be placed in the same space as the parameters and local variables. Hence, a typical stack may look like this (for a case where function1 calls function2):

Related Topics:
Parameters - Local variable

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• previous stack data
• function1 local variables
• parameters for function2
• function1 return address (of the instruction which called function2)
• function2 local variables.

This is with a forwards-growing stack — on many architectures the stack grows backwards in memory. Forward and backwards-growing stacks are useful because it is quite practical to have two stacks growing towards each other in a common scratch space, using one mainly for control information like return addresses and loop counters and the other for data. (This is what Forth does.)

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The parts of the program which are responsible for the entry into and exit out of the subroutine (and hence, the setting up and removal of each stack frame) are called the function prologue and epilogue.

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If the procedure or function itself uses stack handling commands, outside of the prologue and epilogue, e.g. to store intermediate calculation values, the programmer needs to keep track of the number of 'push' and 'pop' instructions so as not to corrupt the original return address.

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