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Random access memory is definitely a good thing, particularly for microprocessors, but sometimes it's advantageous to treat memory a little differently. Here's a form of storage that's neither random nor sequential: Suppose you work in an office where people come to your desk to give you jobs to do. Each job involves a file folder of some sort. Often when you're working on one job, you find that before you can continue you must do a related job using another file folder. So you leave the first folder on your desk and put the second one on top of it to work on that. Now someone comes to your desk to give you yet another job that has higher priority than the earlier one. You're handed a new file folder and you work with that one on top of the other two. That job requires yet another file folder, and soon you have a pile of four file folders on your desk.

Notice that this pile is actually a very orderly way to store and keep track of all the jobs you're doing. The topmost file folder always has the highest-priority job. After you get rid of that one, the next one on the pile must be attended to, and so on. When you finally get rid of the last file folder on your desk (the first one you started with), you can go home.

The technical term for this form of storage is a stack. You're stacking things from the bottom up and removing them from the top down. It's also called last-in-first-out storage, or LIFO. The last thing put on the stack is the first thing taken off the stack. The first thing put on the stack is the last thing taken off the stack.

Computers also can use a stack, not for storing jobs but for storing numbers, and it's something that turns out to be quite convenient. Putting something on the stack is called a push, and taking something off the stack is called a pop.

Suppose you were writing an assembly-language program that used registers A, B, and C. But you notice that you've reached a point where the program needs to do something else—another little calculation that also needs to use registers A, B, and C. You eventually want to come back to what you were doing before, however, and continue using A, B, and C with the values they previously had.

What you could do, of course, is simply store registers A, B, and C in various locations in memory and later load these locations back into the registers. But that requires keeping track of where you stored them. A much cleaner way to do it is to push the registers on the stack:

PUSH A

PUSH B

PUSH C

I'll explain what these instructions actually do in a moment. For now, all we need to know is that they somehow save the contents of the registers in last-in-first-out memory. Once these statements are executed, your program can use these registers for other purposes without worry. To get the earlier values back, you simply pop them from the stack in the reverse order, as shown at the top of the following page.

POP C

POP B

POP A

Remember: Last in, first out. Accidentally switching around these POP statements would constitute a bug.

What's particularly nice about the stack mechanism is that lots of different sections of a program can use the stack without causing problems. For example, after the program pushes A, B, and C on the stack, another section of the program could decide it needs to do the same thing with registers C, D, and E:

PUSH C

PUSH D

PUSH E

Then all that's necessary is for that section of the program to restore the registers this way:

POP E

POP D

POP C

before the first section popped C, B, and A.

How is the stack implemented? The stack is, first of all, just a section of normal RAM that isn't being used for anything else. The 8080 microprocessor contains a special 16-bit register that addresses this section of memory. That 16-bit register is called the Stack Pointer.

My examples of pushing and popping individual registers weren't quite accurate for the 8080. The 8080 PUSH instruction actually stores 16-bit values on the stack, and the POP instruction retrieves them. So instead of instructions like PUSH C and