The Components of the
System Unit
Chapter 4
Lisa Schulke
What exactly is the System
Unit?
• A case that contains electronic components
used to process data
• Sometimes called the chassis which protects
the internal electronic components
– Includes the Motherboard which is the
main circuit board of the system unit
More about the Motherboard
• Many electronic components attach to the
board
• Those components include: adapter cards,
the processor, and memory chips
– A computer chip is a small piece of semi
conducting material which have integrated
circuits
• Other components are built into the board
The Processor
A component
• central processing
unit
• Interprets & carries
out basic
instructions that
operate a computer
• Manages the
computer’s
operations
• Contain 2 smaller
components
– Control unit
The Control Unit
• Directs &
coordinates
operation
• Interprets each
instruction issued by
a program & initiates
the appropriate
action to carry out
the instruction
The Arithmetic
Logic Unit
• Arithmetic operations
– Basic calculations
• Comparison operations
– Comparing 1 data to
another
• Logical operations
– And, or, and Not
The Machine Cycle:
4 basic operations of a processor
1. Fetching- obtaining a program instruction or
data from the memory
2. Decoding- translating instructions into
signals
3. Execute- carries out the commands
4. Storing- writing result to memory
Plenty more about
processors
• Contains:
– A register which is storage location that
temporarily holds data and instructions
– A system clock that control the timing of
computer operations
• Clock speed
Names of Processors
• Pentium- used by high performance PCs
• Celeron- used by less expensive basic PCs
• Xeon and Itanium- ideal for workstations and
low-end servers
Processor Installation and
Upgrades
• 3 types of Upgrades
– Chip-for-chip
– Piggyback
– Daughterboard
New chips cause heat
• A heat sink is a small ceramic component that
absorbs and ventilates heat produced by
electrical components
• A heat pipe cools processors in notebook
computers
Coprocessors and Parallel
Processing
• A coprocessor is a
special chip or circuit
board that assists the
processor in performing
tasks
– Floating point has
numeric capabilities
• Parallel processing is a
method that uses
multiple processors to
execute a program.
Data Representation
• Computers communicate digitally and
recognize only 2 discrete states: on and off
– recognizes the 2 states by the binary
system which only has 2 unique digits( 0 &
1), called bits
• 8 bits form a byte
Coding scheme
• The combinations of 0s and 1s that represent
characters are defined by patterns called the
coding scheme
– Example: #3 is represented by 00110101
• The 2 popular coding schemes:
– ASCII and EBCDIC
Memory
• Consists of electronic
components that store
instructions waiting to
be executed by the
processor, data, and the
results of processed
data (info)
What memory stores?
• 3 basic categories of items:
– The operating system and other system
software that control or maintain the
computer and its devices
– The application programs that carry out a
specific task, such as word processing
– The data being processed by the
application programs and resulting
information
More about Memory
• A byte resides temporarily in a location called
the address
– An address is a unique number that
identifies the location of the byte
• To access data in memory, the computer
references the addresses that contain bytes
of data
Sizes of Memory
•
•
•
A kilobyte is equal to exactly 1,024 bytes
A megabyte is equal to almost 1 million
bytes
A gigabyte equals almost 1 billion bytes
2 types of Memory
1. Volatile- when the computer’s power is
turned off it loses its contents
2. Nonvolatile- does not lose its contents
Ram
• Stands for Random access memory or main
memory
• Consists of memory chips that can be read
from and written to by the processor
• It can hold multiple programs simultaneously
• Most Ram is volatile
2 basic types of ram chips
• Dynamic Ram
– Must be reenergized
constantly or
they lose their
contents
• Static Ram
– Do not have to
be reenergized as
often
– More
expensive
– Faster and
more reliable
Where do RAM chips live?
• Usually reside on a memory module which is
a small circuit board
– Memory slots on the motherboard hold
memory modules
• 3 types of modules:
– SIMMS (single inline memory module)
– DIMMS (dual inline memory module)
– RIMMS
Ram Configurations
• The amount of RAM necessary in a computer
often depends on the types of software you
plan to use
• The more RAM a computer has, the faster it
• A software package will indicate the minimum
amount of RAM it requires
Cache: speeds up process
• Memory cache helps speed the processes of
the computer because it stores used
instructions and data
– L1 cache has the smallest capacity
– L2 cache has a larger capacity than L1
– L3 cache exists only with L2 advanced
transfer cache
• Disk cache
Read-only memory
• ROM refers to memory chips storing
permanent data
• Data can’t be modified
• ROM is nonvolatile
• Rom chips are called firmware which contain
permanently written data/info
• A PROM (programmable) chip is a blank
ROM which a programmer can write
Flash Memory
• A type of nonvolatile memory
• Can be erased electronically and
reprogrammed
• Useful for updates
• Flash memory cards store flash memory on a
removable device instead of a chip
An aide to chips
• Complementary metal-oxide
semiconductor (CMOS)
– Technology that provides high speeds
– Consumes little power
– Uses battery power to retain info
Memory Access Times
• Access time is the time it takes the
processor to read data/info
– Directly effects how fast the computer runs
– Nanoseconds and MHz are used to state
access times
– Accessing data from memory is faster than
from hard disk
Expansion slots and adapter
cards
• The slot is a socket that holds adapter card
• An adapter card is a circuit board (expansion
card)
– Enhances functions
– Provides connections to peripherals
• A peripheral connects to the system unit
and is controlled by the processor
–Examples: modems, printers,
scanners
Neat cards
• A sound card enhances sound by allowing
sound to be input thru a microphone and
output thru speakers
• A video card converts output info into a signal
which displays an image on the screen
• A modem card allows computer to
communicate via telephone lines, cable, etc..
• A network card allows computer to access a
network
Pc cards and Flash memory
cards
• A PC Card slot is an expansion slot that
holds PC cards
– A PC card adds memory, storage, sound,
fax/modem & communications
– PC cards are 1 type of flash memory card
• A flash memory card allows users to
transfer data from mobile devices to
desktop computers
Ports and Connectors
• The point where the
peripheral connects to
the system unit
• Send data
• The term jack is used to
identify audio/video
ports
• Personal computers
have ports in front
• Others have in back
• Joins a cable to a
peripheral
• 2 genders of cables:
– Male- 1+ exposed
pins
– Female-have
matching holes to
accept the pins
• Same gender can’t
connect
• Gender changer- join
same gender
connectors
Serial Ports
• Interface that connects a device to the system
by transferring bit by bit
• Connect devices that don’t require fast data
• Examples: mouse, keyboard
• The COM port is a type of serial port
Parallel Ports
• Interface that connects devices by
transferring more than 1 bit at a time
• Printers are connected by this port
• Also called a Centronics interface
USB Ports
•
•
•
•
Short for Universal serial bus port
Can connect up to 127 different peripherals
Uses only 1 single connector type
To attach multiple peripherals use Daisy
chain
– The 1st USB device to the USB port on the
computer, then the 2nd connects to 1st and
so on
• USB hub- plugs into USB port and contains
multiple USB ports
Special-Purpose Ports
• Firewire- connect devices that require faster
speeds
• Midi- connects the system unit to a musical
instrument
• SCSI (small computer system interface)connects disk drives and printers
• IrDa- transmits data via infrared light waves
• Bluetooth- transmits data using radio waves
Buses
• Electrical channels that bits transfer on
• Allows inside and outside devices to
communicate with each other
• Consists of 2 parts:
– A data bus transfers actual data
– An address bus transfers info where the
data is
• Bus width- the # of bits the computer can
transmit at 1 time
Word Size and buses
• The number of bits the processor can
interpret and execute at a give time
• Every bus has a clock speed (hertz)
• 2 basic types of buses:
– A system bus connects to main memory
– An expansion bus connects to peripherals
Expansion Bus
• Communicates with peripherals attached to
adapter card
• Types:
– ISA (Industrial Standard Architecture)most common
– Local- a high speed expansion that
connects higher speed devices
– AGP (Accelerated Graphics Port)designed to improve the speed with 3-D
graphics
– FireWire- eliminate the need to install
cards
Power Supply
• Converts the wall outlet AC power into DC
power
• Near the supply is a fan to keep unit cool
• Some external peripherals have an AC
adapter, which is an external power supply
Putting it all together
• Many components of the system unit
influence the speed and power of the
computer
• These include:
– Type of processor
– Clock speed of the processor
– The amount of RAM
– Clock speed of the bus
Chapter Summary
• Components of the system unit
• Described how memory stores data/info
• Discussed sequence of operations that occur
when a computer executes an instruction