Hardware Reference
In-Depth Information
3
THE DIGITAL LOGIC LEVEL
At the bottom of the hierarchy of Fig. 1-2 we find the digital logic level, the
computer's real hardware. In this chapter, we will examine many aspects of digital
logic, as a building block for the study of higher levels in subsequent chapters.
This subject is on the boundary of computer science and electrical engineering, but
the material is self-contained, so no previous hardware or engineering experience
is needed to follow it.
The basic elements from which all digital computers are constructed are amaz-
ingly simple. We will begin our study by looking at these basic elements and also
at the special two-valued algebra (Boolean algebra) used to analyze them. Next we
will examine some fundamental circuits that can be built using gates in simple
combinations, including circuits for doing arithmetic. The following topic is how
gates can be combined to store information, that is, how memories are organized.
After that, we come to the subject of CPUs and especially how single-chip CPUs
interface with memory and peripheral devices. Numerous examples from industry
will be discussed later in this chapter.
3.1 GATES AND BOOLEAN ALGEBRA
Digital circuits can be constructed from a small number of primitive elements
by combining them in innumerable ways. In the following sections we will de-
scribe these primitive elements, show how they can be combined, and introduce a
powerful mathematical technique that can be used to analyze their behavior.
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