Page 25 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

2 Chapter 1 Basic Computer Structure and the 6812

discussion, and we want to answer that objection a priori, so that those readers will not
miss the point. We will introduce a seemingly large number of terms. Don't miss the
objective: We are really introducing concepts. The reader should think about the concepts
rather than memorize definitions. Like your physics text, this text has to use terms in a
fairly precise way to avoid ambiguity. Your physics text, you may recall, used the word
"work" in a very precise way, as the product of force times distance, which is a bit
different from the conversational use of the word "work" as used in the expression, "He's
doing a lot of work." We will use terms such as "read" and "fetch" in a similar way.
When defined, they will be written in italics and will be listed in the index. We ask you
to learn the term and its meaning even though you do not have to memorize the wording
of the definition. But take heart, because although we do have a few concepts that have to
be learned, and we have to learn the terms for those concepts, we do not have many
formulas or equations to deal with. Accept our challenge to understand these terms; then
you will enjoy the latter discussions even more than if you muddle through this section
without thinking about the terminology.
You probably know what microcomputers and computers are, to some degree, but
let us discuss the term "computer" so that if you get into an argument about whether a
hand calculator is a computer, you can respond knowledgeably.
A microcomputer is a kind of computer or, more accurately, a kind of von Neumann
computer, named after the scientific giant of our century who invented it. All von
Neumann computers have four components: memory, controller, data operator
(sometimes called an arithmetic-logic unit), and input-output (I/O), which are
connected by an address and data bus. A simplified diagram of a computer is shown in
Figure 1.1. Briefly, the memory stores both the data and the program, and the input-
output provides the communication with the outside world. In a conventional personal
computer system input-output is done through peripherals such as CRTs, keyboards,
scanners, printers, modems, and so on. In typical microcontroller applications the input-
output system provides the necessary connections, or interfacing, to the device, of which
the microcontroller is a component, such as an automobile, kitchen appliance, toy, or
laboratory instrument. The data operator performs arithmetic and logical operations on
data, such as addition, ANDing, and so on. The controller controls the flow of
information between the components by means of control lines (which are not shown in
Figure 1.1), directing what computation is to be done. The input/output, controller, and
data operator may themselves contain a small amount of memory in registers.
A microcomputer is a computer that is implemented using low-cost integrated
circuits (ICs) arid is therefore cheap enough to be used in an incredible range of
applications where a large computer would be infeasible. For the purposes of this book,
if the data operator and the controller are together on a single 1C, but other ICs are needed
to complete the computer, that 1C is called a microprocessor, the computer that uses a

Figure 1.1. Simplified Computer Structure