GENERIC STACK-USAGE METHOD

Most microprocessors have an internal pointer (the stack pointer) which 
references memory so that the micro can keep track of the point of execution
as it varies because of interrupts, function calls, and so forth.  The stack 
(memory to which it points) is also used by many systems (sometimes 
unfortunately) as a storage place for local values, saved registers, and so 
forth.

Just prior to a call, the stack pointer, which is much like any pointer one 
defines, is pointing to some place in memory (designated by the programmer 
or the operating system) for its use.  When you call a function, it works 
something like this (its usage varies somewhat from language to language -- 
even within one language).

stack pointer -->| orig position | In most systems, the stack pointer moves toward
                 |               | lower addresses as you use it.
                 ~               ~

At call:         | orig position |
                 |   arguments   |
stack pointer -->| last argument |
                 |               |
                 |               |
                 |               |
                 |               |
                 ~               ~

There may be zero or more arguments.  They are pushed onto the stack in a 
predetermined order.  For C/C++, it is right-to-left.  The stack pointer moves 
with each push.

After call:      | orig position |
                 | argument here |
                 | (maybe more)
stack pointer -->| ret addr here |
                 |               |
                 |               |
                 |               |
                 ~               ~

Into procedure   | orig position | 
Arguments avail  | arguments...  | When you modify the argument(s), you modify
for use          | ret addr here | the value(s) stored here.  If an argument
                 |  saved regs,  | is a reference or pointer you may use it to
                 | locals, etc.  | modify the value pointed to elsewhere
                 | in this area  | (in the calling procedure, say).  If you write
stack pointer -->|               | more data to one of the local variables than it
                 ~               ~ can store, guess where the excess winds up.

The function does its work and unwinds the stack (locals, etc.)

Before return    | orig position | Immediately before the return, after storage
                 | arguments...  | for saved registers, locals, etc. has already
stack pointer -->| ret addr here | been recovered (and disappeared). The arguments
                 |               | are still on the stack.
                 |               |
                 |               |
                 ~               ~

After return:    | orig position | Immediately after the return.  The very first
stack pointer -->| arguments...  | thing the machine is going to do next is destroy
                 |               | the arguments, whether you've modified them or not.
                 |               | Sometimes the arguments are removed by the called
                 |               | function and the return address position adjusted
                 |               | appropriately.
                 ~               ~

stack pointer -->| orig position | And it's done; you are right back where you started,
                 |               | bookkeeping wise, when you made the call.  Any 
                 |               | changes you made to the arguments are history, for
                 ~               ~ all practical purposes (they may persist until the
                                   next stack operation).

If you passed an argument as a reference, any modifications you made to the
 value it referred to are, of course, in force.  If you modified the reference, 
itself, to point to something else, you could modify that something else, also 
(for example, subsequent bytes pointed to by a char *).  The reference itself 
disappears.  If you pass an argument by value, that value is perfectly usable 
to the called procedure; it could specify a length to use for some operation, 
for example.  If you modify the value, such modifications disappear when the 
arguments disappear, immediately after the called procedure returns to the 
caller.  If you want lasting changes in the caller, you need to make them by 
reference or RETURN a value from the called procedure.