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System Unit

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Definition

The system unit/system cabinet is a container that

houses most of the electronic components that make

up the computer system.

Types of the system units vary depending on the type

of the microcomputer e.g. Desktop, Notebook, and

Tablet system units.

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System unit components

Motherboard/Mainboard CPU Memory Bus lines Ports and Cables Power Supply System Clock Expansion Slots

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Motherboard Board

Also known as main board or system board Allows I/O devices to communicate with the

system unit. It provides connections between processors,

memory units, and sockets or expansion slots.

A flat circuit board with sockets and chips

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Expansion Slots

Expansion slots are where additional circuit boards can be plugged in.

Each expansion slot has 1 external port where a peripheral device can be connected.

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Microprocessor

CPU control unit arithmetic and logic unitRegisters –

Instruction registerData register

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Control Unit

Tells system how to carry out program instructions

Directs flow between memory and the arithmetic and logic unit

Directs signal flow between the CPU and input and output devices

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Arithmetic Logical Unit (ALU)

performs arithmetic and logical operations on data

adds, subtracts, multiplies, compares

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Registers

Special-purpose High-speed Temporary storage Located inside CPU

Instruction register

Holds instruction currently being executed

Data register

Holds data waiting to be processed

Holds results from processing

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Microprocessor Chips

Word size Number of processed bits at a time Expresses chip capacity Size of word determines power

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Memory

Computer memory is a collection of chips on the motherboard where all computer processing and program instructions are stored while in use.

It enables the CPU to retrieve data quickly for processing.

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Memory Capacity

Kilobyte (KB) 1,000 bytes

Gigabyte (GB) 1 billion bytes

UUnniitt

CCaappaacciittyy

Megabyte (MB) 1 million bytes

Terabyte (TB) 1 trillion bytes

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Types of Memory

RAM-Random access Memory- also known as main memory or primary

memory

ROM – read only memory

CMOS-Complementary Metal-oxide semiconductor.

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holds data and programs being processed by CPU

Volatile - when power shuts off, contents of RAM are emptied

Exception - flash RAM can retain data when power disrupted, used in high end portable computers

RAM

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ROM

Read-only memory Also called Firmware

Cannot be changed by the user

Used for start-up instructions, keyboard control capabilities

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CMOS

Complementary metal-oxide semiconductor:

Flexible and expandable memory Holds startup information on system

components Non-volatile Its contents may be updated

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Bus Lines

Data pathways that connect system components

Data roadway for traveling bits

More lanes, faster traffic; 64 bit bus faster than a 32 bit

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Ports and Cables

Ports are connecting sockets

Cables connect input and output devices to ports

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System Clock Is a circuit that emits a continuous stream of

high and low pulses that are all exactly the same length

Controls speed and synchronizes operations

Expressed in megahertz

Determines the speed of the computer

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System board Holds the various other system components

Microprocessor Contains the CPU on a chip

Memory Holds programs and instructions

System clock Controls speed of computer operations

Expansion Connect to network and otherslots/boards system capabilities

Bus lines Connect various internal system components

Ports Connect outside devices to system unit

Component Function

SUMMARY OF THE SYSTEM UNIT COMPONENTS

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How does the computer process data?

A computer is an information-processing machine that works by converting all kinds of data/information into binary numbers (ones and zeros) and then using simple mathematics to make decisions about, or to rearrange, those numbers

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How does the computer process data? Data and instructions are represented in a

binary form

In binary form, ones, can be stored as magnetized spots while the zeroes can be stored as unmagnetized spots on a hard disc.

Movement and transformation of electrical pulses in electrical circuits.

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Data Representation

Binary.i.e Os and 1s Bit: BInary digiT

Smallest unit of information/storage - sufficient to hold one bit

Can take one of two values (true and false or 0 and 1, or yes or no)

Byte: Smallest addressable unit of storage Usually 8 bits Typically holds one character

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Word

– Fundamental unit of storage in a computer– Word size is one of chief distinguishing

characteristics of a computer– Typical size in modern computers: 4 bytes or

8 bytes– An instruction is usually one or more words

long– A word can be used to hold a whole number

of characters

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Data Data:

Numbers, characters, images, or other method of recording

Represented in binary. Has no meaning on its own. When interpreted by data processing system it takes

on meaning and becomes information. Data storage:

– Setting of individual bits to specific values, destroying its previous contents

Data retrieval:– Copying the contents of a particular memory cell to

another storage area.– Original data remains unchanged

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Binary Coding Schemes

ASCII – used for microcomputers

EBCDIC – used for large computers

Unicode – 16 bit code

Used to support International Languages

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0100 0001

How the letter is represented in ASCII code.How the letter is represented in ASCII code.

Binary Coding Schemes

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ASCII code

Common decimalrepresentation

Binary representation

3 + 5 0011 0011

0010 1011

0011 0101

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Machine Cycle

A machine cycle is a series of steps a CPU takes to execute an instruction Instruction Cycle

Fetching- for command or data Decoding – break down the command to

resemble those in the CPU’s instruction set. Execution Cycle

required data located instruction executed results stored

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How fast is a computer?

Cycle Time Measured in MIPS – millions of instructions

per second (also known as Megahertz) slow (milliseconds) medium (nanoseconds) fast (picoseconds)

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Microprocessor Technology and Speed

Affected by size of word (size of registers) how many bits the processor can read

Affected by width of bus How many bits can travel along the bus

Affected by Clock Speed Reduced Instruction Set Computing (RISC)

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