Call Stack - Functions of The Call Stack

Functions of The Call Stack

As noted above, the primary purpose of a call stack is:

Storing the return address

When a subroutine is called, the location (address) of the instruction at which it can later resume needs to be saved somewhere. Using a stack to save the return address has important advantages over alternatives. One is that each task has its own stack, and thus the subroutine can be reentrant, that is, can be active simultaneously for different tasks doing different things. Another benefit is that recursion is automatically supported. When a function calls itself recursively, a return address needs to be stored for each activation of the function so that it can later be used to return from the function activation. This capability is automatic with a stack.

A call stack may serve additional functions, depending on the language, operating system, and machine environment. Among them can be:

Local data storage

A subroutine frequently needs memory space for storing the values of local variables, the variables that are known only within the active subroutine and do not retain values after it returns. It is often convenient to allocate space for this use by simply moving the top of the stack by enough to provide the space. This is very fast compared to heap allocation. Note that each separate activation of a subroutine gets its own separate space in the stack for locals.

Parameter passing

Subroutines often require that values for parameters be supplied to them by the code which calls them, and it is not uncommon that space for these parameters may be laid out in the call stack. Generally if there are only a few small parameters, processor registers will be used to pass the values, but if there are more parameters than can be handled this way, memory space will be needed. The call stack works well as a place for these parameters, especially since each call to a subroutine, which will have differing values for parameters, will be given separate space on the call stack for those values.

Evaluation stack

Operands for arithmetic or logical operations are most often placed into registers and operated on there. However, in some situations the operands may be stacked up to an arbitrary depth, which means something more than registers must be used (this is the case of register spilling). The stack of such operands, rather like that in an RPN calculator, is called an evaluation stack, and may occupy space in the call stack.

Pointer to current instance

Some object-oriented languages (e.g., C++), store the this pointer along with function arguments in the call stack when invoking methods. The this pointer points to the object instance associated with the method to be invoked.

Enclosing subroutine context

Some programming languages (e.g., Pascal and Ada) support nested subroutines, allowing an inner routine to access the context of its outer enclosing routine, i.e., the parameters and local variables within the scope of the outer routine. Such static nesting can repeat - a function declared within a function declared within a function... The implementation must provide a means by which a called function at any given static nesting level can reference the enclosing frame at each enclosing nesting level. Commonly this reference is implemented by a pointer to the encompassing frame, called a "downstack link" or "static link", to distinguish it from the "dynamic link" that refers to the immediate caller (which need not be the static parent function). For example, languages often allow inner routines to call themselves recursively, resulting in multiple call frames for the inner routine's invocations, all of whose static links point to the same outer routine context. Instead of a static link, the references to the enclosing static frames may be collected into an array of pointers known as a display which is indexed to locate a desired frame. The Burroughs B6500 had such a display in hardware that supported up to 32 levels of static nesting.

Other return state

Besides the return address, in some environments there may be other machine or software states that need to be restored when a subroutine returns. This might include things like privilege level, exception handling information, arithmetic modes, and so on. If needed, this may be stored in the call stack just as the return address is.

The typical call stack is used for the return address, locals, and parameters (known as a call frame). In some environments there may be more or fewer functions assigned to the call stack. In the Forth programming language, for example, ordinarily only the return address, counted loop parameters and indexes, and possibly local variables are stored on the call stack (which in that environment is named the return stack), although any data can be temporarily placed there using special return stack handling code so long as the needs of calls and returns are respected; parameters are ordinarily stored on a separate data stack or parameter stack, typically called the stack in Forth terminology even though there is a call stack since it is usually accessed more explicitly. Some Forths also have a third stack for floating point parameters.