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The system unit is a box-like case that stores the electronic components of the computer used to process information.
Electronics components and other storage devices are located inside the system unit.
Components inside the system unit include:
 Processor
 Memory
 Module
 Cards
 Ports
 Connectors

 The motherboard or the system board, is the main circuit board in the system unit.
 Mother board includes different types of chips.
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A chip is a small piece of semi conducting material, on which integrated circuits are etched.
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There are various chip packages such as;
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Dual inline package
Pin grid array package
Flip chip package
Single edge contact cartridge

 The central processing unit , or the processor, interprets and carries out the basic instructions that operate a computer.
Control Unit/
Arithmetic/
Logic Unit
Input Devices Memory Output Devices
Storage Devices

 The CPU contains the control unit and the arithmetic/logic unit .
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The control unit directs and coordinates most of the operations in the computer.
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For every instruction, the control unit repeats a set of four basic operations called machine cycle:
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Fetch - obtain program instruction or data item from memory
Decode - translate instruction into commands
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Execute - carry out command
Store - write result to memory
The arithmetic/logic unit performs arithmetic, comparison, and logical operations.

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Pipelining
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With pipelining the CPU begins executing a second instruction before it completes the first instruction.
Registers
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The registers temporarily hold data and instructions.
o Stores location from where instruction was fetched o Stores instruction while it is being decoded o Stores data while ALU processes it
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The system Clock
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The system clock control the timing of all computer operations.
The clock speed is the speed at which a processor executes instructions.

Comparison of Personal Computer Processors
The most popular personal computer processor are:
Pentium
Xeon
Athlon
Celeron
Itanium
Duron
Processor Installation and Upgrades
Instead of buying a new computer it might be better to upgrade your processor to increase the computer’s performance.
Processor upgrades are identified as:
A chip for chip upgrade
A piggyback upgrade
A daughterboard upgrade

Most computers are digital which means they only recognize only two discrete states: on and off.
1 ON
0 OFF

Computers uses a binary system to recognize the two states.
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The binary system is a number that has just two unique digits, 0, 1, called bits.
Bit is the smallest unit of data the computer can represent.
When eight bits are grouped together as a unit, they form a byte, which provides enough different combinations of 0’s and 1’s to represent 256 individual characters.
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These characters include numbers, uppercase and lowercase letters of the alphabet, and punctuation marks.
Combinations of 0’s and 1’s are defined by patterns called coding scheme.
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Two popular coding schemes are:
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The American Standard Code for Information Exchange
The Extended Binary Coded Decimal Interchange Code

 What is memory?
 Temporary storage area for data, instructions, and information.
 It consists on one or more chips on motherboard
 How is memory measured?
 By the number of bytes available
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Kilobyte (KB)
Megabyte (MB)
Gigabyte (GB)

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There are two types of system unit memory.
Volatile memory, which loses its contents when the computers power is turned off.
Nonvolatile memory, which does not loses its content when the computers power is turned off.
 Memory in a computer refers to RAM (random access memory)
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Consists of memory chips that can be read and written to by the processor and other devices.
Also called main memory or primary storage.
Most RAM is volatile.
The more RAM a computer has the faster it responds.

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How Application Programs Transfer In and Out of
RAM?
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Step 1. When the computer is running, certain operating system files are in RAM. The operating system displays on the screen.
Step 2. When you start a program, the program loads into RAM from the hard disk. As you are creating the document the information is on RAM and it shows in the screen.
Step 3. When you quit the program, RAM may be used to store another program or data. The original program is removed from the screen, and the operating system’s user interface re appears.
Step 4. When you quit a program, its program instructions are removed from RAM. The Web browser no longer is displayed on the screen.

 What are two basic types of RAM chips?
Static RAM
(SRAM)
Dynamic RAM
(DRAM)
Faster and more reliable than DRAM chips
Used for special application such as cache
Most Common
Type
Faster variations of DRAM are SDRAM and RDRAM

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ROM (read-only memory) refers to memory chips that only can be read and used, which means they cannot be modified.
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ROM is nonvolatile memory (NVM), meaning that its contents are not lost when the computer’s power is turned off.
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A variation of the ROM chip, called programmable read-only memory (PROM), is a blank chip on which you can place items permanently.

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 also known as flash ROM or flash RAM, is nonvolatile memory that can be erased electronically and reprogrammed.
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 Another type of memory chip, it stores configuration information about the computer and uses battery power to retain information when the power to the computer is turned off.

 An expansion slot is an opening, or socket, where you can insert a circuit board into the motherboard.
 These circuit boards are called; card, expansion board, expansion board, adapter card, adapter, interface card, addin, and add-on.
 There are four types of expansion cards found in most of today’s computers;
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Video card
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Sound card
Network interface card
Modem card

 A cable often attaches external devices to the system unit. A port is the point of attachment to the system unit.
 Ports have different types of connectors which are used to connect a cable with a device.
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Male connectors have one or more exposed pins.
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Female connectors have matching holes to accept the pins.

 Most computers have three types of ports;
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Serial port
Parallel port
Universal serial bus

 How buses contribute to a computers processing speed?
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Bits are transfer internally within the circuitry of the computer along electrical channels.
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These channels are called a bus. They allow the various devices inside and attached to the system unit to communicate with each other.
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The bus width determines the number of bits that can be transferred at one time.
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In most computer word size is the number of bits the processor can interpret and execute at a given time.
A computer has two basic types of buses
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System bus – connects the CPU to main memory.
Expansion bus
– allows the CPU to communicate with peripheral devices.

 A bay is an open area inside the system unit in which you can install additional equipment.
 Drive bays
 External drive bay – allows access from outside the system unit.
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Floppy A disk
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DVD
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Zip drives
Internal drive bay – concealed entirely within the system unit.
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Hard disk drive

 Power supply is the component in the system unit that converts the wall outlet AC power into DC power.
 AC adapter – an external power supply.

 What is a mobile computer?
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Looks like a notebook
Weights between 2.5 and 8 pounds
 Mobile computers system unit contain;
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Motherboard
Processor
Keyboard
Speakers
Display