COMPONENTS IN THE SYSTEM UNIT
System Unit
The system unit is a box-like case (usually rectangular in shape) that houses the
electronic components of the computer.
For a desktop computer, the electronic components and most storage
devices such as floppy drive, hard disk, and CD-ROM drive, reside inside
the system unit. Other devices, known as the peripherals, such as keyboard,
mouse, monitor, and printer, are connected externally to the system unit.
For a laptop computer, the system unit houses almost all of its components
including the keyboard, pointing device, and monitor.
The Motherboard
Many of the electronic components in the system unit reside on a circuit board
called the motherboard or system board.
Several different types of chips can be found on the motherboard.
A chip is a small piece of semi-conducting material, on which one or more
integrated circuits are etched.
An integrated circuit (IC) is a microscopic pathway capable of
carrying electrical current.
Each integrated circuit can contain millions of transistors, which
act as electronic switches, or gates, that open or close the circuit
for electronic signals.
The central processing unit is one of the more important chips on the
motherboard.
Central Processing Unit
The central processing unit (CPU) or processor interprets and carries out the
instructions that operate a computer.
On larger computers, the various functions performed by the CPU are
spread across many separate chips and sometimes multiple circuit boards.
On a personal computer, the CPU usually is contained on a single chip and
is often called a microprocessor.
A microprocessor usually contains the control unit, the arithmetic and logic
unit, registers, and system clock.
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The control unit directs and coordinates most of the operations in the
computer.
For every instruction, the control unit repeats a set of four basic
operations called the machine cycle or instruction cycle.
Fetching is the process of obtaining a program instruction or data
item from memory.
The time taken to fetch is called instruction time, or I-time.
Decoding is the process of translating the instruction into
commands that the computer understands.
Executing is the process of carrying out the commands.
The time taken to decode and execute is called execution
time, or E-time.
Storing is the process of writing the result to memory.
Most processors used by personal computers today support pipelining,
which means that the CPU begins executing a second instruction
before the first instruction is completed. The result is faster processing.
Most newer processors can pipeline up to four instructions.
Superscalar CPUs have two or more pipelines that can process
instructions simultaneously.
A computer’s speed is very often measured according to the number of
instructions it can process in one seconds.
This speed is measured in MIPS, which stands for million
instructions per second.
MIPS refers only to the CPU speed, whereas applications
generally are limited by other factors such as input and
output speed.
The arithmetic and logic unit (ALU) performs the execution part of a
machine cycle.
Specifically, the ALU performs the arithmetic, comparison, and
logical operations.
Arithmetic
operations
include
addition,
subtraction,
multiplication, and division.
Comparison operations involve comparing one data item to
another to determine if the first item is greater than, equal to, or
less than the other item.
Logical operations work with conditions and logical operators
such as AND, OR, and NOT.
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Registers are temporary storage locations used to hold data and
instructions.
A microprocessor contains many different types of registers, and each
performs a specific function.
Storing the location from where an instruction was fetched.
Storing an instruction while it is being decoded.
Storing data while the ALU processes it, and
Storing the results of a calculation.
The system clock is a small chip that synchronizes or controls the timing of
all computer operations.
The system clock generates regular electronic pulses, or ticks, that set
the operating pace of components in the system unit.
Each tick is called a clock cycle and a CPU requires a fixed
number or clock cycles to execute each instruction.
The faster the clock, the more instructions the CPU can execute in a
second.
Most of today’s processors are superscalar, which means that they
can execute more than one instruction per clock cycle.
The speed at which a processor executes instructions is called clock
speed, or clock rate.
Clock speed is measured in megahertz (MHz), which equates to
one million ticks of the system clock, or in gigahertz (GHz),
which equates to one billion ticks of the system clock.
A higher clock speed means that the CPU can process more
instructions per second than the same CPU with a lower clock
speed.
Common Prefixes and their Meanings
Prefixes for small amounts
Meaning
Decimal Notation
Milli
One thousandth of
0.001
Micro
One millionth of
0.000001
Nano
One billionth of
0.000000001
Pico
One trillionth of
0.000000000001
Prefixes for large amounts
Meaning
Decimal Notation
Kilo
One thousand
1,000
Mega
One million
1,000,000
Giga
One billion
1,000,000,000
Tera
One trillion
1,000,000,000,000
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The clock speed affects only the CPU; it has no effect on
peripherals such as a printer or disk drive.
Clock speed is an important consideration when buying a
computer.
Recently, copper has replaced aluminum to create the electronic circuitry of
CPUs because
Copper is cheaper and also a better conductor of electricity.
Processor chips that use copper run faster and cost less than chips that
use aluminum.
Copper requires less electricity.
More suitable for use in portable computers.
Integrated CPU is a new type of microprocessor that combines functions of a
CPU, memory, and a graphics card on a single chip.
These chips are designed for lower-cost personal computers and
smaller-sized computers.
Most of today’s processors are equipped with MMXTM technology, in which a
set of instructions is built into the processor so that it can manipulate and process
multimedia data more efficiently.
Most of today’s computers are equipped with a zero-insertion force (ZIG)
socket, which has a small lever or screw designed to facilitate the installation
and removal of processor chips.
Newer processor chips generate a lot of heat and a heat sink is a small ceramic
or metal component with fins on its surface that is designed to absorb and
ventilate heat produced by electrical components.
A smaller device called a heat pipe is used to cool laptop computers.
A coprocessor is a special processor chip or circuit board designed to assist the
processor in performing specific tasks.
Users running engineering, scientific, or graphics applications will notice a
dramatic increase in speed with a floating-point coprocessor.
A floating-point coprocessor is also called a math or numeric
coprocessor.
Some computers support parallel processing, which can speed up processing
time by using multiple processors simultaneously to execute a program.
Parallel processing requires special software designed to recognize how to
divide up the problem and then bring the results back together again.
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Data Representation
Most computers are digital, meaning that they understand only two discrete
states: on and off.
Computers are electronic devices powered by electricity, which has only
two states: on, or off.
0 is used to represent off, and 1 is used to represent on.
In contrast, human speech is analog, meaning that it uses continuous signals
to represent data and information.
Computers use the binary system that has just two digits, 0 and 1.
Each of these digital values is called a bit, the abbreviation form for binary
digit.
A bit is the smallest unit of data the computer can handle.
Practically, computers process data in terms of bytes.
A bit is not very informative because it can only represent a value of either
0 or 1.
A byte is eight bits grouped together as a unit, and it provides enough
different combinations of 0s and 1s to represent 28 = 256 individual
characters that include numbers, letters, and symbols.
A byte is actually a series of on/off electrical states.
A coding scheme is the patterns of 0s and 1s combinations used to represent
characters.
The three most popular coding schemes used to represent data are:
ASCII (American Standard Code for Information Interchange,
pronounced as ASK-ee).
ASCII is the most widely used coding system to represent data.
ASCII is used on many personal computers and minicomputers.
EBCDIC (Extended Binary Coded Decimal Interchange Code,
pronounced as EB-see-dic).
EBCDIC is used primarily on mainframe computers.
Unicode
Unicode is a coding scheme capable of representing all the
world’s current languages.
ASCII and EBCDIC codes are sufficient for English and
Western European languages, but are not large enough for
Asian and other languages that use different alphabets.
Unicode is a 16-bit code that has the capacity of representing
more than 65500 characters.
The first 256 codes of Unicode are the same as ASCII.
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Memory
The memory in the system unit is used to store data, instructions, and
information.
Memory stores three basic items that include:
The operating system and other system software that control the usage
of the computer equipment.
Application programs designed to carry out a specific task such as
word processing.
The data being processed by the application program.
The role of memory to store both data and program is known as the stored
program concept.
A byte is the basic storage unit in memory.
Each byte is placed in a precise location in memory, called an address.
An address is a unique number identifying the location of a byte in
memory.
Any address in memory can either
Be empty, or
Holds a single byte of data at a time.
The size of memory is measured by the number of bytes available for use.
A Kilobyte (KB) of memory is equal to exactly 210 = 1024 bytes.
A Megabyte (KB) of memory is equal to exactly 210 = 1024 KB, or 210 x
210 = 1,048,576 bytes.
Memory can be volatile and nonvolatile.
The contents of volatile memory are lost when the computer power is
turned off.
RAM is an example of volatile memory.
The contents of nonvolatile memory remain even when the computer
power is turned off.
ROM, flash memory, and CMOS are examples of nonvolatile
memory.
Memory and Storage Sizes
Unit
Abbreviation
Exact amount (Bytes)
Approximate number of
pages of text
Kilobyte
KB or K
1,024
1/2
Megabyte
MB
1,048,576
500
Gigabyte
GB
1,073,741,824
500,000
Terabyte
TB
1,099,511,627,776
500,000,000
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Volatile Memory
RAM (Random Access Memory) is the memory chips in the system unit.
RAM is an example of volatile memory.
Items in RAM that are needed for future use must be saved to a storage
device before the computer is turned off.
There are two basic types of RAM, dynamic RAM and static RAM.
DRAM (Dynamic RAM) is a type of memory that must be re-energized
constantly or its contents will be lost.
SDRAM (Synchronous DRAM) is a faster version of DRAM because
it is synchronized to the system clock.
RDRAM (Rambus DRAM) is a newer type of DRAM that is even
faster than SDRAM.
Most computers today use some type of SDRAM or RDRAM.
SRAM (Static RAM) is faster and more reliable than any form of DRAM,
but are more expensive.
SRAM does not have to be re-energized as often as DRAM.
RAM chips are usually packaged on a small circuit board that is inserted into the
motherboard.
SIMM (Single Inline Memory Module) refers to the type that pins on
opposite sides of the circuit board are connected together to form a single
set of contacts.
DIMM (Dual Inline Memory Module) refers to the type that pins on
opposite sides of the circuit board are not connected and thus form two sets
of contacts.
RAM chips commonly used in SIMMs and DIMMs are SDRAM chips.
RDRAM chips are packaged on a circuit board called RIMM (Rambus
Inline Memory Module).
The necessary amount of RAM varies according to the types of work to do and
the types of applications to be used on the computer.
Most of the computers today use memory cache. Memory cache speeds the
processes of computing by storing frequently used instructions and data.
There are two types of memory cache:
L1 (Level 1) cache is also called the primary cache or internal
cache.
L1 cache is built directly into the processor chip (usually ranges
from 8 KB to 64 KB).
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L2 (Level 2) cache, or external cache, is not part of the processor
chip.
L2 cache is slightly slower than L1 cache, but has a larger
capacity (usually ranges from 64 KB to 2 MB).
A computer system with L2 cache usually performs at speeds that
are 10 to 40 percent faster than those without any cache.
A computer system should have from 256 K to 512 K of L2 cache
to realize the largest increase in performance.
The increase in performance is not significant above 512 K.
When the processor needs an instruction or data, it searches memory in the
following order:
1. L1 cache
2. L2 cache
3. RAM
4. Storage device (e.g., hard disk, floppy disk, CD-ROM)
Nonvolatile Memory
ROM (Read Only Memory) is the name given to memory chips storing data
that can only be read.
ROM is an example of nonvolatile memory.
ROM contains data, instructions, or information that is recorded
permanently when the chip is manufactured.
ROM chips that contain permanently written data, instructions, or
information are called firmware.
A programmable read-only memory (PROM) is a type of ROM chip on which
permanently items can be placed.
The instructions used to program a PROM chip are called microcode.
Once a PROM chip is written, it functions like a regular ROM chip and
cannot be erased or changed.
Flash memory or Flash ROM can be erased electronically and reprogrammed.
Flash memory is used to store programs and data on personal computers,
cellular phones, printers, digital cameras, pagers, and personal digital
assistants (PDA).
CMOS (Complementary metal-oxide Semiconductor) is used to store
configuration information about the computer.
CMOS chips use battery power to retain information even when the
computer is turned off.
Unlike ROM, information stored in CMOS can be changed.
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Memory Access Time
Memory access time is the speed at which the processor can access data from
memory directly.
For memory, access time is measured in nanosecond (ns), which is one
billionth of a second.
DRAM chips normally have access times ranging from 50 to 70 ns.
SRAM chips normally have access times ranging from 7 to 20 ns.
ROM chips normally have access times ranging from 55 to 250 ns
For disk storage, access time is measured in millisecond (ms), which is one
thousandth of a second.
A fast hard disk may have access time ranging from 8 to 15 ms.
(i.e., Accessing data in memory with a 70 ns access time is over
200,000 times faster than accessing data of a hard disk with a 15 ms
access time.)
Expansion Slots and Expansion Cards
An expansion slot is a socket where a circuit board, or expansion card, can be
inserted into the motherboard to add new devices, or capabilities, to the
computer.
Three popular types of expansion cards are
Video card, also called video adapter or graphics card, converts
computer output into a video signal that is sent through a cable to the
monitor.
Sound card that allows sound to be input through a microphone and output
through speakers.
Internal modem that enables computers to communicate via telephone
lines.
Many of today’s computers support Plug and Play, which means that a
computer can automatically configure expansion cards and other devices as they
are installed.
A PC Card is a thin credit card-sized device that is used to add capabilities to a
laptop computer.
All PC Cards conforms to the PCMCIA (Personal Computer Memory
Card International Association) standards, and they were originally called
PCMCIA cards.
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The height or thickness of PC Card various among three types
The thinnest Type I cards are used to add memory capabilities to the
computer.
Type II cards contain communications devices such as modems.
The thickest Type III cards are used to house devices such as hard
disks.
The ability to add or remove devices while a computer is running is called hot
plugging or hot swapping.
Ports
External devices are often attached by a cable to the system unit.
A port is the interface, or point of attachment, to the system unit.
Ports have different types of connectors.
A connector is used to join a cable to a device.
Male connectors have one or more exposed pins.
Female connectors have matching holes to accept the pins on a male
connector.
A gender changer is a device used to join two connectors that are either
both female or both male.
A serial port transmits only one bit of data at a time.
Serial ports are usually used to connect devices that do not require fast data
transmission rates, such as a mouse, keyboard, or modem.
A modem uses a serial port because the telephone line expects data to
be transmitted in a serial form.
Serial ports conform either to the RS-232 or RS-422 standard, which
specifies the number of pins used on the port’s connector.
Two commonly used connectors for serial ports are a male 25-pin
connector or a male 9-pin connector.
A parallel port is capable of transferring more than one bit of data at a time.
Parallel ports are usually used to connect printers and scanners.
Many printers connect to the system unit using a parallel port that can
transfer eight bits of data simultaneously through eight separate lines in
a single cable with a 25-pin female connector.
A parallel port is sometimes called a Centronics interface, after the
company first defined the standard for communication.
EPP (Enhanced Parallel Port) and ECP (Extended Capabilities Port)
use the same connectors as the Centronics port, but are more than ten times
faster.
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A USB (Universal Serial Bus) port can connect up to 127 different peripheral
devices with a single connector.
USB also supports hot plugging as well as Plug and Play.
Devices connect to the system unit using USB ports include keyboard,
mouse, scanner, disk drives, digital camera, …
A MIDI (musical instrument digital interface) port is designed to connect the
system unit to a musical instrument, such as an electronic keyboard.
MIDI is a standard of the electronic music industry.
A system unit with a MIDI port has the capability of recording sounds that
have been created by a synthesizer and then processing the sounds to create
new sounds.
A synthesizer is a peripheral or a chip that creates sound from digital
instructions.
A SCSI (small computer system interface) port is a special high-speed parallel
port used to attach peripheral devices such as printers and disk drives to the
system unit.
A SCSI port can transmit up to 32 bits of data at a time.
A total of seven SCSI devices can be daisy chained together.
The 1394 port, also called FireWire, can connect multiple types of devices
requiring faster data transmission speeds.
Devices that normally require 1394 port for connection include digital video
camcorders, digital VCRs, color printers, scanners, digital cameras, …
The 1394 port also supports Plug and Play.
An IrDA (Infrared Data Association) port allows wireless devices to transmit
data via infrared light waves.
The IrDA port on the computer and the IrDA port on the peripheral device
must be aligned so that nothing obstructs the path of the infrared light wave.
Devices that use IrDA ports include keyboards, mouse, printer, …
Buses
A bus is an electrical channel that allows the various devices inside and attached
to the system unit to communicate with each other.
All buses consist of two parts: a data bus and an address bus.
The data bus transfers actual data, and
The address bus transfers information about where the data should go in
memory.
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The bus width, or size of the bus determines the number of bits that can be
transmitted at one time.
The larger the number of bits handled by the bus, the faster the computer
transfers data.
Word size is the number of bits the CPU can process at a given time.
In most computers, the word size is the same as the bus width.
Every bus also has a clock speed.
The clock speed for a bus is measured in megahertz (MHz), which is equal
to one million ticks per second.
The higher the bus clock speed, the faster the transmission of data.
Most of today’s processors have a bus speed of either 100 MHz or 133
MHz.
The two basic types of buses found in a computer are system bus and expansion
bus.
A system bus connects the CUP to main memory.
An expansion bus allows the CPU to communicate with peripheral devices.
The types of expansion buses on the motherboard determine the types
of expansion cards a user can add.
Common types of expansion buses include
The ISA (Industry Standard Architecture) bus is the most
common and slowest expansion bus.
Devices that are normally connected through ISA buses
include mouse, modem card, sound cad, low-speed network
card, …
A local bus is a high-speed expansion bus used to connect higher
speed devices such as hard disks.
VESA local bus was the first standard local bus that was
used primarily for video cards.
PCI (Peripheral Component Interconnect) bus is the
current local bus standard because it is more versatile than
the VESA local bus.
Types of cards that inserted into a PCI slot include video
cards, SCSI cards, high-speed network card, …
The AGP (Accelerated Graphics Port) is a bus designed by Intel
to improve the speed with which 3-D graphics and video are
transmitted.
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The USB bus and 1394 bus eliminate the need to install
expansion cards into expansion slots.
USB devices are connected to each other outside the system
unit. The USB port then connects to the USB, which
connects to the PCI bus on the motherboard.
The 1394 bus works in a similar way as the USB bus.
The PC Card bus is the expansion bus for a PC Card.
Data travels on the PC Card bus to the PCI bus.
Bays
A bay is an open area inside the system unit used to install additional equipment.
A bay is different from a slot, which is used for the installation of expansion
cards.
Bays are most often used for disk drives.
Drive bays are bays used for disk drives.
An external drive bay or exposed drive bay allows access to the
drive from outside the system unit.
Floppy disk drives, CD-ROM drives, and tape drives are
examples of devices installed in external drive bays.
An internal drive bay or hidden drive bay is concealed entirely
within the system unit.
Hard disk drives are installed in internal bays.
Power Supply
The power supply is the component in the system unit that converts the wall
outlet AC power into DC power.
Some external peripheral devices also have an AC adapter, which is an external
power supply.
Laptop Computers
A docking station is a device into which a laptop computer is placed.
A docking station contains connections to peripherals such as keyboard,
monitor, printer, and other devices.
Some laptop computers have a port replicator, which is a device that allows
other peripheral devices such as printer, modem, and mouse to connect into it.
The port replicator is then connected to the laptop computer.
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