Chapter 3
The System Unit:
Processing and Memory
Data and Program Representation
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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.
The Binary Numbering System
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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).
Coding Systems for Text-Based Data
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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.
Coding Systems for Other Types of Data
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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.
Machine Language
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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.
Inside the System Unit
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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.
CPU
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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.
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.
Memory: RAM
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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:
Memory: Other Types
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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).
Buses
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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.
Expansion Slots and Cards
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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.
Ports
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Connectors to which devices can be attached.
Common ports:
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Keyboard
Mouse
Monitor
Modem
MIDI
IrDA
Game
How the CPU Works
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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.
Typical CPU Components
The System Clock and the Machine Cycle
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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
Making Computers Faster Now and in the Future
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Speeding up your system today.
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Add more memory.
Perform system maintenance.
Buy a larger or second hard drive.
Upgrade your Internet connection.
Upgrade your video card.
Upgrade your CPU.
Making Computers Faster Now and in the Future Cont’d.
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Strategies for making faster computers.
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Moving circuits closer together.
Faster and wider buses; faster memory.
Improved materials (copper, SOI, new materials).
Pipelining.
Making Computers Faster Now and in the Future, Cont’d.
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Strategies for making faster computers, cont’d
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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.
Future Trends
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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).
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Quantum computing—uses atoms or nuclei working together as quantum bits
(qubits).
Optical chips—use light waves to transmit data.
3-D chips—use multiple layers of circuitry or the circuitry stands vertically.
KEY TERMS
ALU
binary system
bit
bitmap
bus
byte
cache
CPU
DB-25
decimal system
DVD
EBCDIC
expansion slots
Extended Capabilities Port
gigabyte
IEEE 1394 port
IrDA port
legacy port
megabyte
MIDI port
mips
monitor port
MP3
network port
parallel port
parallel processing
pipelining
pixel
port
PS/2
RAM
RJ-45
ROM
SCSI port
serial port
Unicode
USB
USB port
vector graphic
word size