Designing Embedded Hardware, 2nd Edition

Chapter 2. Assembly Language

For the things we have to learn before we can do them, we learn by doing them.

—Aristotle, Nichomachean Ethics

This chapter is about writing assembly-language software. This is a difficult subject to present, as it is a very diverse topic. There are many processors covered in this book. The assembly language of one processor bears no relation to that of another. To delve into the assembly language of each would require an entire book in its own right (or several books). Hence, I'm not even going to attempt to cover each one's instruction set. Instead, I'm going to concentrate on just two processors, the 68HC11 and the PIC, and use these as vehicles to show you some basic assembly- language techniques. I'm not going to dissect the instruction sets in detail. The User's Manual/datasheets for the processors give full descriptions of the instructions and their operation.

Assembly-language programming works down at the machine level, and as such, you really get a feel for what the processor is doing and how your computer actually works. While assembly-language programming can be fun, it can also be a daunting task to code major applications in it. To this end, assembly is usually used in only two instances. The first is for the development of simple test software during the early prototype development of a system. Such software may simply initialize the machine to a known state and perform some simple task such as flashing a LED or outputting the venerable "hello ...