Sams Teach Yourself C++ in 21 Days

Organizing into Functions 131

5

When data is pushedonto the stack, the stack grows; as data is poppedoff the stack, the

stack shrinks. It isn’t possible to pop a dish off the stack without first popping off all the

dishes placed on after that dish.

A stack of dishes is the common analogy. It is fine as far as it goes, but it is wrong in a

fundamental way. A more accurate mental picture is of a series of cubbyholes aligned top

to bottom. The top of the stack is whatever cubby the stack pointer (which is another reg-

ister) happens to be pointing to.

Each of the cubbies has a sequential address, and one of those addresses is kept in the

stack pointer register. Everything below that magic address, known as the top of the

stack, is considered to be on the stack. Everything above the top of the stack is consid-

ered to be off the stack and invalid. Figure 5.8 illustrates this idea.

FIGURE5.7
A stack.

FIGURE5.8
The stack pointer.

Stack

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YourAge 37

MyAge 50

on the stack

off the stack

stack pointer

theVariable^102