Hardware Reference
In-Depth Information
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A digital computer is a machine that can do work for people by carrying out
instructions given to it. A sequence of instructions describing how to perform a
certain task is called a
program
. The electronic circuits of each computer can rec-
ognize and directly execute a limited set of simple instructions into which all its
programs must be converted before they can be executed. These basic instructions
are rarely much more complicated than
Add two numbers.
Check a number to see if it is zero.
Copy a piece of data from one part of the computer's memory to another.
Together, a computer's primitive instructions form a language in which people
can communicate with the computer. Such a language is called a
machine lan-
guage
. The people designing a new computer must decide what instructions to in-
clude in its machine language. Usually, they try to make the primitive instructions
as simple as possible consistent with the computer's intended use and performance
requirements, in order to reduce the complexity and cost of the electronics needed.
Because most machine languages are so simple, it is difficult and tedious for peo-
ple to use them.
This simple observation has, over the course of time, led to a way of structur-
ing computers as a sequence of abstractions, each abstraction building on the one
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