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Chapter 2
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SYSTEM UNIT
• A system unit is the part of a computer that houses
the primary devices that perform operations and
produce results for complex calculations.
• System unit includes: the motherboard, CPU, RAM
and other components.
• System unit does not includes: the keyboard or
monitor or any peripheral devices.
• A system unit is sometimes called a box or main
unit.
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• A system unit includes the following parts:
- Motherboard
- Processing Devices
- Memory Chips
- System Clock
- Buses
-Ports
-Expansion Slots and Cards
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MOTHER BOARD
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Mother Board
• It is the Main circuit board in system unit
• Every component of the system unit connects to the
mother board.
• Contains chips, integrated circuits, and transistors
• The CPU, memory, hard drives, optical drives, video
card, sound card and other ports and expansion
cards all connect to the motherboard directly or via
cables.
• Also called system board or main board
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Processing Devices
central processing unit (CPU) :
• Interprets and carries out basic instructions that
operate a computer
• Also called the processor
• contained on a single chip called the “Microprocessor”.
The microprocessor has two basic components:
• Arithmetic/Logic Unit (ALU)
• Control Unit
# The speed of CPU measured by MHz or GHz
• Arithmetic Logical Unit (ALU) :
Consist of electronic circuitry to perform :
-Arithmetic operations (addition(+) , subtraction(-),
multiplication(*), division(/)..etc
-logical operations(and , or, not,..)
-comparisons operations (less-than<=) , (equal=)
• Control Unit (CU) :
It directs the movement of signals between the memory and
the Arithmetic-Logic Unit and between the CPU and
input/output devices.
MEMORY
• Temporary storage area for operating system,
application programs, and data
• Consists of one or more chips on motherboard
• Each byte stored in unique address
There are basically 2 types of memory:• Primary
• Secondary
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MEMORY
Memory
Primary
RAM
Secondary
ROM
CD
Floppy
Primary Memory
• Primary storage (or main memory or internal
memory), often referred to simply as memory, is
the only one directly accessible to the CPU.
There are 3 types of Primary memory :• RAM (Random Access Memory)
• ROM (Read Only Memory)
• CMOS.
RAM
1) This is the part of the computer that stores operating
system software , software applications and other
information for the CPU to have fast and direct access
when needed to perform tasks.
2) The data in the RAM can be read or modified. Hence
it is called read/write memory.
3) The data in most types of RAM is lost after the
program is terminated or the computer is switched off.
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RAM
- Main memory is available in to types:
1-Dynamic Random Access Memory (DRAM)
2-Static Random Access Memory (SRAM)
ROM
The two main characteristics of the ROM are:
1) Instructions in the ROM can be executed but cannot
be changed, hence the name Read Only Memory.
2) These instructions are not erased when the
computer is switched off . Its contents cannot be
changed by users.
- Example of ROM :
PROM ,EPROM,EEPROM.
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MEMORY cont…
PROM
ROM
EPROM
RAM
EEPROM
DRAM
SRAM
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MEMORY cont…
COMPARE
Primary Memory
•Fast
•Expensive
•Low capacity
•Connects directly to
the processor
Secondary Memory
•Slow
•Cheap
•Large capacity
•Not connected directly to
the processor
EXPANSION CARDS AND SLOTS
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The expansion card : An expansion card is an
electronic circuit board that adds more functionality
to a desktop computer.
Expansion card types
Video cards
Sound cards
Network cards
TV tuner cards
Modem cards
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Expansion slots are located on the motherboard,
and openings on the back of the computer allow the
ports on the cards that go in the slots to be accessed
Expansion slot Types
PCI Express
AGP
PCI
ISA
MCA
VLB
Express Card
Peripheral Component
Interconnect Express(PCIE)
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Industry Standard
Architecture (ISA)
Accelerated Graphics Port
(AGP)
PCI Local
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Ports
Connects external devices to system unit
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keyboard port
USB port
serial port
monitor port
game port
network port
mouse port
parallel port
speaker port
microphone port
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BUS
• Channel that allows devices inside computer to
communicate with each other
• On the motherboard
• System bus connects processor and RAM
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POWER SUPPLY AND CABLE
• A power supply unit (PSU): is the component that
supplies power to the other components in a computer.
- Desktop computers have a power supply unit located
within the system unit .
- Notebook computers use AC adapters and batteries that
are typically located outside the system unit.
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POWER SUPPLY AND CABLE
cables: are used to connect exterior to the
system unit via the ports