One of the principal hurdles for new programmers is grappling with the many layers of intellectual abstraction. Computers, of course, are just electronic machines. They don’t know about windows and menus, they don’t know about programs or instructions, and they don’t even know about 1s and 0s. All that is really going on is that voltage is being measured at various places on an integrated circuit. Even this is an abstraction: electricity itself is just an intellectual concept, representing the behavior of subatomic particles.

Few programmers bother much with any level of detail below the idea of values in RAM. After all, you don’t need to understand particle physics to drive a car, make toast, or hit a baseball, and you don’t need to understand the electronics of a computer to program one.

You do need to understand how memory is organized, however. Without a reasonably strong mental picture of where your variables are when they are created, and how values are passed among functions, it will all remain an unmanageable mystery.

When you begin your program, your operating system (such as DOS or Microsoft Windows) sets up various areas of memory based on the requirements of your compiler. As a C++ programmer, you’ll often be concerned with the global name space, the free store, the registers, the code space, and the stack.

Global variables are in global name space. We’ll talk more about global name space and the free store in coming chapters, but for now we’ll focus on the registers, code space, and stack.

Registers are a special area of memory built right into the Central Processing Unit (or CPU). They take care of internal housekeeping. A lot of what goes on in the registers is beyond the scope of this book, but what we are concerned about is the set of registers responsible for pointing, at any given moment, to the next line of code. We’ll call these registers, together, the instruction pointer. It is the job of the instruction pointer to keep track of which line of code is to be executed next.

The code itself is in code space, which is that part of memory set aside to hold the binary form of the instructions you created in your program. Each line of source code is translated into a series of instructions, and each of these instructions is at a particular address in memory. The instruction pointer has the address of the next instruction to execute. Figure 5.4 illustrates this idea.

Figure 5.4.The instruction pointer.

The stack is a special area of memory allocated for your program to hold the data required by each of the functions in your program. It is called a stack because it is a last-in, first-out queue, much like a stack of dishes at a cafeteria, as shown in Figure 5.5.

Last-in, first-out means that whatever is added to the stack last will be the first thing taken off. Most queues are like a line at a theater: the first one on line is the first one off. A stack is more like a stack of coins: if you stack 10 pennies on a tabletop and then take some back, the last three you put on will be the first three you take off.

When data is "pushed" onto the stack, the stack grows; as data is "popped" off 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.

Figure 5.5. A stack.

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 register) 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 considered to be off the stack and invalid. Figure 5.6 illustrates this idea.

Figure 5.6.The stack pointer.

When data is put on the stack, it is placed into a cubby above the stack pointer, and then the stack pointer is moved to the new data. When data is popped off the stack, all that really happens is that the address of the stack pointer is changed by moving it down the stack. Figure 5.7 makes this rule clear.

Figure 5.7. Moving the stack pointer.