Chapter 7: Following Your Instructions Meeting Machine Instructions up Close and Personal

Overview

The most visible part of any assembly language program is its machine instructions, those atoms of action that are the steps a program must take to get its work done. The collection of instructions supported by a given CPU is that CPU's instruction set. For example, the 8086 and 8088 CPUs share the same instruction set, which is why most people consider them the same CPU.

This cannot be said for the later CPUs in the family, all of which offer additional instructions not found in the original 8086/8088. I can't cover all the x86 machine instructions in this book, even the original set introduced with the 8086. Those that I will describe are the most common and the most useful, and the easiest for newcomers to understand. It's not just a space issue, either. Some of the instructions (and for the most recent CPUs, such as the Pentium, a good many of them) are dedicated to way-down-deep functions that support the workings of protected mode operating systems and virtual memory. I could spend a whole book the size of this one just explaining the concepts that go into such operating systems and would have to before I could explain the instructions from which one builds them.

Nor will I abandon the discussion of memory addressing begun in the last chapter. As I've said before, understanding how the CPU and its instructions address memory is more difficult but probably more important than understanding the instructions themselves. In and around the descriptions of the machine instructions I'll present from this point on there will be discussions and elaboration on memory addressing. Pay attention! If you don't learn that, memorizing the entire instruction set will do you no good at all.