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66 CHAPTER 3 / A TOP-LEVEL VIEW OF COMPUTER FUNCTION

KEY POINTS

◆ An instruction cycle consists of an instruction fetch, followed by zero or
more operand fetches, followed by zero or more operand stores, followed
by an interrupt check (if interrupts are enabled).
◆ The major computer system components (processor, main memory, I/O
modules) need to be interconnected in order to exchange data and control
signals. The most popular means of interconnection is the use of a shared
system bus consisting of multiple lines. In contemporary systems, there typ-
ically is a hierarchy of buses to improve performance.
◆ Key design elements for buses include arbitration (whether permission to
send signals on bus lines is controlled centrally or in a distributed fashion);
timing (whether signals on the bus are synchronized to a central clock or
are sent asynchronously based on the most recent transmission); and width
(number of address lines and number of data lines).

At a top level, a computer consists of CPU (central processing unit), memory, and I/O
components, with one or more modules of each type.These components are intercon-
nected in some fashion to achieve the basic function of the computer, which is to exe-
cute programs.Thus,at a top level,we can describe a computer system by (1) describing
the external behavior of each component—that is, the data and control signals that it
exchanges with other components; and (2) describing the interconnection structure
and the controls required to manage the use of the interconnection structure.
This top-level view of structure and function is important because of its explana-
tory power in understanding the nature of a computer. Equally important is its use to
understand the increasingly complex issues of performance evaluation.A grasp of the
top-level structure and function offers insight into system bottlenecks, alternate path-
ways, the magnitude of system failures if a component fails, and the ease of adding per-
formance enhancements. In many cases, requirements for greater system power and
fail-safe capabilities are being met by changing the design rather than merely increas-
ing the speed and reliability of individual components.
This chapter focuses on the basic structures used for computer component in-
terconnection. As background, the chapter begins with a brief examination of the
basic components and their interface requirements. Then a functional overview is
provided. We are then prepared to examine the use of buses to interconnect system
components.

3.1 COMPUTER COMPONENTS

As discussed in Chapter 2, virtually all contemporary computer designs are based
on concepts developed by John von Neumann at the Institute for Advanced Studies,
Princeton. Such a design is referred to as the von Neumann architecture and is based
on three key concepts: