Information Technology Reference
In-Depth Information
& CHAPTER 5
Processing Unit Design
In previous chapters, we studied the history of computer systems and the fundamen-
tal issues related to memory locations, addressing modes, assembly language, and
computer arithmetic. In this chapter, we focus our attention on the main component
of any computer system, the central processing unit (CPU). The primary function of
the CPU is to execute a set of instructions stored in the computer's memory. A
simple CPU consists of a set of registers, an arithmetic logic unit (ALU), and a con-
trol unit (CU). In what follows, the reader will be introduced to the organization and
main operations of the CPU.
5.1. CPU BASICS
A typical CPU has three major components: (1) register set, (2) arithmetic logic
unit (ALU), and (3) control unit (CU). The register set differs from one computer
architecture to another. It is usually a combination of general-purpose and special-
purpose registers. General-purpose registers are used for any purpose, hence the
name general purpose. Special-purpose registers have specific functions within
the CPU. For example, the program counter (PC) is a special-purpose register
that is used to hold the address of the instruction to be executed next. Another
example of special-purpose registers is the instruction register (IR), which is
used to hold the instruction that is currently executed. The ALU provides the cir-
cuitry needed to perform the arithmetic, logical and shift operations demanded of
the instruction set. In Chapter 4, we have covered a number of arithmetic oper-
ations and circuits used to support computation in an ALU. The control unit is
the entity responsible for fetching the instruction to be executed from the main
memory and decoding and then executing it. Figure 5.1 shows the main com-
ponents of the CPU and its interactions with the memory system and the input
/
output devices.
The CPU fetches instructions from memory, reads and writes data from and to
memory, and transfers data from and to input
output devices. A typical and
/
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