Chapter 4

System Unit Components

System unit

• The system i\unit is a case that contains electronic computer used to process data
• The inside of the system unit on a desktop personal computer includes:

1. Drive bay(s)
2. power supply
3. Sound card
4. Video card
5. Processor
6. Memory

• The motherboard is the main circuit board of the system unit    -A computer chip contains integrated circuits 

Processor

• The processor,also called the central processing unit(CPU),interprets and carries out the basic instructions that operate a computer                                                                        -contain a control unit and an arithmetic logic unit (ALU)

1. Multi-core processor
2. Dual-core processor
3. Quad-coreprocessor

•  The control unit is the component of the processor that directs and coordinates most of the operations in the computer
• The arithmetic logic unit (ALU) performs arithmetic,comparison,and other operations
• For every instruction,a processor repeats a set of four basic operations,which comprise a machine cycle  
•  Most current personal computers support pipelining                   -Processor begins fetching a second instruction before it completes the machine cycle for the first instruction

The processor contains registers,that temporarily hold data and instructions
The system clock controls the timing of all computer operations

• The pace of the system clock is called the clock speed,and is measured in gigahertz(GHz)
• The leading manufacturers of personal computer processor chips are Intel and AMD
• Determine how you plan to use a new computer before slecting a aprocessor
• 
• A processor chip generates heat that could cause the chip to burn up
• Require additional cooling                                                              -Heat sinks                                                                                    -Liquid cooling technology
• Parallel processing uses multiple processors simultaneously to execute a single program or task                                                   -Massively parallel processing involves hundreds or thousands of processor                                      

Data representation

1.  Analog signals are continuous and vary in strength and quality
2. Digital signals are in one of two states:on or off

• Most computers are digital
• The binary system uses two unique digits(0 and 1)
• Bits and bytes

A computer circuit represents the 0 or the 1 electronically by the presence or absence of an electrical charge
             
Eight bits grouped together as a unit are called a byte.A byte represents a single character in the computer 

• ASCII (American Standard Code for Information Interchange) is the most widely used coding scheme to represent data

Memory

• Memory consists of electronic components that store instructions waiting to be executed by the processor,data needed by those instructions, and the result of processing the data 
• Stores three basic categories of item:

1. The operating system and other system software
2. Application programs
3. Data being processed and the resulting information

• Each location in memory has an address
• Memory size is measured in kilobytes (KB or K),megabytes (MB),gigabytes (GB),or terabytes (TB)
• The system unit contains two types of memory:

1. Volatile memory-Loses its contents when power is turned off                                -Example includes RAM
2. Nonvolatile memory-Does not lose contents when power is                                          removed                                                                                      -Examples include ROM,flash                                                        memory,and CMOS

• Three basic types of RAM chip exist:

1. Dynamic RAM (DRAM)
2. Static RAM (SRAM)
3. magnetoresistive RAM(MRAM)

• RAM chips usually reside on a memory module and are inserted into memory slots
• The amount of RAM necessary in a computer often depends on the types of software you plan to use
• Memory cache speeds the processes of the computer because it stores frequently used instructions and data
• Firmware-Read-only memory (ROM)refers to memory chips storing permanent data and instructions
• EEPROM can be erased- A PROM (programmable read-only memory)chip is a blank ROM chip that can written to permanently
• Flash memory can be erased electronically and rewritten           -CMOS technology provides high speeds and consumes little power
• Access time is the amount of time it takes the processor to read from the memory 

Expansion slots and Adapter Cards

•  An expansion slot is a socket on the motherboard that can hold an adapter card
• An adapter card enhances functions of a component of the system unit and/or provides connections to peripherals

• With Plug and Play, the computer automatically can configure adapter cards and other peripherals as you install them
• Removable flash memory -Memory cards,UUSB flash drives, and PCcards 

Ports and connectors

-A port is the point at which a peripheral attaches to or communicates with a system unit

-A connector joins a cable to aport

Buses

• Expansion slots connect to expansion buses
• Common types of expansion buses include

      -PCIbus

      -PCI Express bus 

      -Acclerated Graphics Port

      -USB and FireWirebus

      -PC Card bus

Bays

• A bay is an opening inside the system unit in which you can install additional equipment 

Power Supply

-the power supply conversts the wall outlet AC power into DC power 

-Some external peripherals have an Ac adapter, which is an external power supply