2 Computer Organization: Basic Processor Structure
computation. The student also learns that this description can be run on a
computer to perform the computation. Many of us then start wondering what
kind of magical machine is capable of performing our computation, given only
a simple description.
The truth of the matter is that computers are indeed magical, in the sense
that they can do amazing things. However, they are less than magical, al-
though still amazing, when we examine how they work. Two areas that ex-
amine the operation of the computer are called computer organization and
computer architecture. The distinction between computer organization and
computer architecture is probably best described in terms of two other con-
cepts: implementation and interface. Computer organization is the study of
the implementation of a computer. That is to say, the hardware and circuitry
out of which the computing system is built. Computer architecture studies the
structure presented to a program, that can be used to perform a computation.
Computers are made up of many separate devices. To understand how a
computer performs a computation, probably the most interesting device to
examine is the central processing unit (CPU), which is often simply called
the processor. This is the device that actually executes a program, and is the
focus of this book. In this section we directly examine the question of how a
program performs a computation, by giving an overview of the structure of
the computer system. In this discussion the structure is presented as a set of
layers, one built on top of another. We begin our discussion at the top level,
the high-level language program which has been written, and work our way
down to the bottom level, which consists of digital circuitry.
1.1 HIGH-LEVEL, ASSEMBLY, AND MACHINE LANGUAGES
So, a user writes a program in a high-level language, with the intent of running
the program on a computer. Let us analyze that statement. We begin by
discussing what is meant by a high-level language.
1.1.1 High-Level Languages
Programming languages are used to describe a computation that is to be
executed on a computer. Languages are classified by level. The idea is that
low-level languages are closer to the hardware of the computer, and high-level
languages are further from the hardware, and more abstract. That is to say,
a low-level description of a computation would describe the computation in
terms of the various hardware devices that constitute the computer system.
A high-level description might describe the computation as the manipulation
of some abstract data structure, like an array, which is a data structure that
would normally not be implemented directly in hardware.
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