Gsm call flow diagram pdf

Please forward this error screen to sharedip-1601531662. Unsourced material may gsm call flow diagram pdf challenged and removed. A call stack is used for several related purposes, but the main reason for having one is to keep track of the point to which each active subroutine should return control when it finishes executing. An active subroutine is one that has been called but is yet to complete execution after which control should be handed back to the point of call.

If a called subroutine calls on yet another subroutine, it will push another return address onto the call stack, and so on, with the information stacking up and unstacking as the program dictates. They are given access only to a set of functions, and not the memory on the stack itself. 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. Stack structures provide this capability automatically. It is often convenient to allocate space for this use by simply moving the top of the stack by enough to provide the space. Note that each separate activation of a subroutine gets its own separate space in the stack for locals. 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.

Operands for arithmetic or logical operations are most often placed into registers and operated on there. 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. The depth of a routine’s lexical nesting is a known constant, so the size of a routine’s display is fixed. Also, the number of containing scopes to traverse is known, the index into the display is also fixed. 32 levels of static nesting.

The display entries denoting containing scopes are obtained from the appropriate prefix of the caller’s display. An inner routine which recurses creates separate call frames for each invocation. In this case, all of the inner routine’s static links point to the same outer routine context. Beside 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.

In some environments there may be more or fewer functions assigned to the call stack. Forth terminology even though there is a call stack since it is usually accessed more explicitly. Each stack frame corresponds to a call to a subroutine which has not yet terminated with a return. A diagram like this can be drawn in either direction as long as the placement of the top, and so direction of stack growth, is understood.

Furthermore, independently of this, architectures differ as to whether call stacks grow towards higher addresses or towards lower addresses. The logic of the diagram is independent of the addressing choice. The stack frame at the top of the stack is for the currently executing routine. Each stack frame contains a stack pointer to the top of the frame immediately below. The stack pointer is a mutable register shared between all invocations.

A frame pointer of a given invocation of a function is a copy of the stack pointer as it was before the function was invoked. The locations of all other fields in the frame can be defined relative either to the top of the frame, as negative offsets of the stack pointer, or relative to the top of the frame below, as positive offsets of the frame pointer. The location of the frame pointer itself must inherently be defined as a negative offset of the stack pointer. In most systems a stack frame has a field to contain the previous value of the frame pointer register, the value it had while the caller was executing. The value is saved upon entry to the subroutine and restored upon return. For some purposes, the stack frame of a subroutine and that of its caller can be considered to overlap, the overlap consisting of the area where the parameters are passed from the caller to the callee.

In some environments, the caller pushes each argument onto the stack, thus extending its stack frame, then invokes the callee. In other environments, the caller has a preallocated area at the top of its stack frame to hold the arguments it supplies to other subroutines it calls. Under this approach, the size of the area is calculated by the compiler to be the largest needed by any called subroutine. The actual call instruction, such as “branch and link”, is then typically executed to transfer control to the code of the target subroutine. The prologue will commonly save the return address left in a register by the call instruction by pushing the value onto the call stack.

Alternatively, some instruction set architectures automatically provide comparable functionality as part of the action of the call instruction itself, and in such an environment the prologue does not need to do this. If frame pointers are being used, the prologue will typically set the new value of the frame pointer register from the stack pointer. Space on the stack for local variables can then be allocated by incrementally changing the stack pointer. When a subroutine is ready to return, it executes an epilogue that undoes the steps of the prologue. Under many calling conventions the items popped off the stack by the epilogue include the original argument values, in which case there usually are no further stack manipulations that need to be done by the caller.

With some calling conventions, however, it is the caller’s responsibility to remove the arguments from the stack after the return. Returning from the called function will pop the top frame off of the stack, perhaps leaving a return value. In this case, the stack frame of a function contains one or more entries specifying exception handlers. Some languages have other control structures that require general unwinding.

This operation requires the stack to be unwound, removing as many stack frames as necessary to restore the proper context to transfer control to the target statement within the enclosing outer function. The call stack can sometimes be inspected as the program is running. Depending on how the program is written and compiled, the information on the stack can be used to determine intermediate values and function call traces. Ruby and Smalltalk, to implement first-class continuations. Taking regular-time samples of the call stack can be useful in profiling the performance of programs, because if a subroutine’s pointer appears on the call stack sampling data many times, it is likely a code bottleneck and should be inspected for performance problems.