Chapter 3
The System Unit:
Processing and Memory
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Learning Objectives

Understand how data and programs are represented
to a computer and be able to identify a few of the
coding systems used to accomplish this.

Explain the functions of the hardware components
commonly found inside the system unit, such as the
CPU, memory, buses, and expansion cards.

Describe how new peripheral devices or other
hardware can be added to a PC.
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Learning Objectives, Cont’d.

Understand how the computer system’s CPU and
memory components process program instructions
and data.

Name and evaluate several strategies that can be
used today for speeding up the operations of
computers.

List some technologies that may be used in the future
PCs.
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Overview

This chapter covers:

How computers represent data and programs

How the CPU, memory, and other components are
arranged inside the system unit

How the CPU works

Strategies to speed up a computer and create
faster computers
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Data and Program Representation

Computers today are mostly digital computers—
devices that can only understand two states.

The two states of a digital computer are typically
represented by 0s and 1s; that is, in binary form.

Computers do all processing and communications in
binary form, so natural-language input and output are
translated to and from binary by the computer.
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The Binary Numbering System

We normally use the decimal numbering system,
which uses 10 symbols (0, 1, 2, 3, 4, 5, 6, 7, 8, and
9).

Computers use the binary numbering system,
which represents all numbers using just two symbols
(0 and 1).
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Coding Systems for Text-Based Data

ASCII and EBCDIC
 Fixed-length codes that can represent any single
character of data as a string of eight bits.

Unicode
 A longer (32 bits per character is common) code
that can be used to represent text-based data in
virtually any written language.
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Coding Systems for Other Types of
Data



Graphics data—often stored as a bitmap which the
color to be displayed at each pixel stored in binary
form.
Audio data—waveform audio is common; MP3
compression makes audio files much smaller.
Video data—requires a great deal of storage
space, but can be compressed.
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Machine Language

Machine language is the binary-based code used to
represent program instructions.

The basic set of machine language instructions that a
CPU can understand is that CPU’s instruction set.

Most programmers rely on language translators to
translate their programs into machine language for
them.
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Inside the System Unit

The system unit of a PC is the case that houses
processing hardware and other hardware.

All of the hardware contained within the system unit
is connected to the system board or motherboard.
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CPU

The CPU (central processing unit) does the vast
majority of processing for a computer.

CPUs contains a variety of circuitry and components and
are connected to the motherboard.

Also called the processor or microprocessor.
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CPU, Cont’d.

Processing speed (clock speed) is measured in
megahertz (MHz) or gigahertz (GHz) and measures
the number of clock ticks per second.

A computer word is a group of bits or bytes that a
CPU can manipulate at one time.

Other factors that affect the speed of the computer
include the amount of RAM and cache memory, bus
width, and bus speed.
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Memory: RAM

RAM (random access memory) is the computer’s main
memory and is used to temporarily storage programs and
data with which it is working.

RAM is volatile (erased when the power to the PC goes
off).

RAM comes in a variety of types, speeds, and size. Types
of RAM include:
DRAM
SRAM
SDRAM
DDR SDRAM
RDRAM
DDR-II SDRAM
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Memory: Other Types

Cache memory—fast memory chips located on or close
to the CPU chip (L1, L2, and L3).

Registers—high speed memory built into the CPU.

ROM (read-only memory)—non-volatile chips inside
which data or programs are stored.

Flash memory—non-volatile memory that can be erased
and reused. Used both within the PC and for portable
storage media (e.g digital cameras).
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Buses

Buses are electronic paths that data travels around
on a computer system.

Internal buses move data around within the CPU.

Expansion buses establish links with peripheral
devices.
 ISA, PCI, AGP, USB, FireWire.
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Expansion Slots and Cards

Expansion for desktop PCs.
 Expansion cards can be inserted into expansion
slots on the motherboard.
 Some external devices can be plugged into existing
USB or FireWire ports without adding another
expansion card.

PC cards—used with notebook PCs.

Expansion for handhelds and mobile devices—can have
proprietary, PC card, or SDIO slot.
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Ports

Connectors to which devices can be attached.

Common ports:

Serial

Keyboard

Parallel

Mouse

SCSI

Monitor

USB

Modem

FireWire

MIDI

Network

IrDA

Game
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How the CPU Works

The CPU is a collection of electronic circuits and
components.

Impulses from an input device passes through RAM and
enters the CPU via a system bus.

Within the CPU the impulses move through the circuits
and components to create new impulses.

Eventually, a set of electronic impulses leaves the CPU for
an output device.
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Typical CPU Components



Arithmetic/logic unit
(ALU)

Prefetch unit

Decode unit

Internal cache

Bus interface unit
Control unit
Registers
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The System Clock and the Machine
Cycle


The system clock synchronizes the computer’s
operations.
The machine cycle is the series of operations
involved in the execution of a single, machine-level
instruction.
 Fetch
 Decode
 Execute
 Store
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Making Computers Faster Now and
in the Future

Speeding up your system today.

Add more memory.

Perform system maintenance.

Buy a larger or second hard drive.

Upgrade your Internet connection.

Upgrade your video card.

Upgrade your CPU.
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Making Computers Faster Now and
in the Future Cont’d.

Strategies for making faster computers.

Moving circuits closer together.

Faster and wider buses; faster memory.

Improved materials (copper, SOI, new materials).

Pipelining.
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Making Computers Faster Now and
in the Future, Cont’d.

Strategies for making faster computers, cont’d

Multiprocessing (multiple CPUs, each working on a
different job) and parallel processing (multiple CPUs
working together to make one job finish sooner).

Hyperthreading.

Improved instruction set design.
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Future Trends

Nanotechnology—computer chips thousands of
times smaller than today’s components; built at the
atomic and molecular levels.

Organic computers—computers
containing organic matter
(biotechnology).


Quantum computing—uses
atoms or nuclei working
together as quantum bits
(qubits).
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Future Trends, cont’d.

Optical chips—use light waves to transmit data.

3-D chips—use multiple layers of circuitry or the
circuitry stands vertically.
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Summary

Data and program representation

Inside the system unit

How the CPU works

Making computers faster now and in the future
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