1. Chapter 6
The System Unit

2. Competencies (1 of 2) Describe the four basic types of system units
Discuss how a computer uses binary codes to represent data in electronic form
Describe each of the major system unit components
Discuss microprocessors, including specialty processors
Describe the different types of memory
Page 154

3. Competencies (2 of 2) Discuss expansion slots and boards
Describe the five principal types of expansion buses
Discuss the four standard ports
Page 154

4. System Unit
Also known as the system cabinet is a container that houses most of the electrical components that make up a computer system.
Page 155

5. Electronic Data and Instructions
Data and instructions are represented electronically
Two-state system or Binary System
Off/on electrical states
Characters represented by 0s (off) and 1s (on)
Bits
Bytes
Our voices recognize analog (Key Term) signals
Continuous signals
Computers only recognize digital (Key Term) signals
Electrical
On/off state
The binary system consists of two digits – 0 and 1
Each 0 or 1 is called a bit – short for binary digit
Bits are combined into groups of eight bits called bytes
Smallest character = bit; eight bits = 1 byte or one character
Page 157

6. Binary Coding Schemes Three types of binary coding schemes
ASCII - American Standard Code for Information Exchange
EBCDIC - Extended Binary Coded Decimal Interchange Code
Unicode – handles languages with large numbers of characters
Knowing this information helps understand what takes place when data entered into computer – why it takes “so long” for the data to get transformed
Three basic schemes of coding
ASCII – primarily desktop
EBCDIC – primarily mainframe and super
Unicode – to handle languages with large numbers of characters
Page 158

7. System Board Connects all components
Allows communication between devices
Main board or motherboard
Circuit board electronic components
Sockets
Slots
Bus lines
Connects all system components and allows input and output devices to communicate with system unit
Other names: main board; motherboard
Located in the system unit, consists of a large flat circuit board with sockets and chips (Key Term)
Chips contain numerous circuits etched on a small wafer of layers of silicon and other materials
Chips are mounted on carrier packages (Key Term)
Chips are also referred to as a silicon chip (Key Term), semiconductor (Key Term), or integrated circuit (Key Term)
Page 159

8. Microprocessor Central Processing Unit (CPU) Two Basic Components
Control unit
Arithmetic-logic unit (ALU)
Located on the microprocessor chip and has two components - the control unit and the arithmetic-logic unit
Control Unit
Program instructions
Directs flow between memory and Arithmetic-Logic Unit
Directs flow between CPU and I/O devices
Arithmetic-Logic Unit - ALU
Two types of operations
Arithmetic operation (Key Term)
Logical operation (Key Term)
Microprocessor Chips
Word size
CISC
RISC
Smart Card (Key Term)
Page 161

9. Microprocessor Chips Chip capacities are expressed in word sizes
Two Recent Significant Developments
64-bit processors
Becoming more commonplace
Windows XP Professional X64 Edition
Dual-Core Chips
Can provide two separate and independent CPUs
Parallel processing
A word (Key Term) is the number of bits that can be accessed at one time by the CPU
Older computers process data and instructions in millionths of a second – microseconds
Newer computers
Nanoseconds– microcomputers
Picoseconds– supercomputers
Page 162

10. Memory Holding area for data, instructions, and information
Memory is contained on chips connected to the system board
Types of memory chips
RAM
ROM
CMOS
RAM (Random-access memory) (Key Term)
Frequently accessed
Acts as high-speed, temporary holding area
Faster processing results
Temporarily holds data and programs being processed by the 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
Cache (pronounced: “cash”)
Area in RAM set aside to store information frequently accessed
Acts as a high-speed, temporary holding area for parts of programs and data
Results in faster processing results for the user
ROM (Read-only memory) (Key Term)
Nonvolatile; CPU retrieves programs written on the ROM chip
Typical user does not go in and change information on ROM
Also called firmware
CMOS (complementary metal-oxide semiconductor) (Key Term)
Provides flexibility and expandability for system
Page 163

11. RAM Random Access Memory (RAM) chips hold the program and data
Cache memory or RAM cache
Flash RAM or flash memory
Other types of RAM
DRAM
SDRAM
DDR
Direct RDRAM
Cache memory – improves processing by acting as a temporary high-speed holding area between the memory and CPU
Flash memory – can retain data even if the power is disrupted
Most expensive type of RAM
Used for digital telephones, digital video cameras, and portable computers
Virtual Memory (Key Term) – large programs are divided into parts and the parts are stored on a secondary device, usually a hard disk. Each part is then read into RAM only when needed
RAM is also referred to as temporary or volatile storage
It is a good idea to save your work in progress to a second a secondary storage device. which is permanent or nonvolatile storage
Return
Page 163

12. ROM
Read-only memory (ROM) chips are not volatile and cannot be changed by the user
CPU can read, or retrieve data and programs but the computer cannot write
Contain special instructions
Needed to start a computer
Give keyboard keys their special capabilities
Put characters on screen
ROMs are also called firmware
Return
Page 164

13. CMOS
Complementary metal-oxides semiconductor (CMOS) chips provide flexibility for a computer system
Contains essential information every time the computer is turned on
Date and time
Amount of RAM
Type of keyboard
Content can be changed to reflect changes in the computer system
Page 164

14. Commonly Used Expansion Cards
Graphics cards
Sound cards
Modem cards
Network interface cards (NIC)
PC cards (PCMCIA cards)
TV tuner cards
Graphics cards - connect the system board to the monitor; convert internal electronic signals to video signals
Sound cards – convert electronic signals to audio signals and accept audio input from microphones
Modem cards – allow distant computers to communicate with each other by converting computer electronic signals to travel over phone lines
Network adapter cards (Key Term) (NICs or Network Interface Cards) - connect system unit to a cable (Key Term) attached to a network
TV turner cards (Key Term) - PC/TV combination cards led to high audio quality systems called home PCs
TV tuner converts video signal for viewing on computer monitor
PC cards - credit card-sized expansion cards for portable computers, also known as PCMCIA (Personal Computer Memory Card International Association) cards
Page 166

15. Bus Lines Connect parts of the CPU to each other
Data roadway for traveling bits
Measured as bus width
More lanes, faster traffic
Two basic categories
System buses
Expansion buses
Bus lines provide data pathways that connect various system components
A bus line is also called a bus (Key Term)
It is a pathway for bits representing data and instructions
Every computer has two basic categories of buses
System – connects the CPU to memory on the system board
Expansion – connects the CPU to slots on the system board
Page 170

16. Standard Ports Four common ports Serial ports Parallel ports USB ports
FireWire ports
Four common ports
Serial ports – data transported one bit at a time; mouse, keyboard, modem, etc.
Parallel ports – for external devices needing lots of data over short distances, like printers, scanners, external Zip drives
USB ports (universal serial bus) – gradually replacing serial and parallel ports; can connect multiple devices; faster than parallel
FireWire ports also known as high performance serial bus (HPSB) ports – for high-speed printers and video cameras; as fast as USB 2.0
Two most recent and significant developments in ports and buses:
PCI Express (PCIe) (Key Term) –
30 times faster than PCI
Contained in many of today’s most powerful microcomputers
Serial ATA (SATA) (Key Term) – used to connect both magnetic and optical disk drives to the system board with far greater speed than other buses and slots
Return
Page 170

17. Power Supply Computers require direct current (DC)
DC power provided by converting alternating current (AC) from wall outlets or batteries
Desktop computers use power supply units
Notebooks and handhelds use AC adapters
Cables are used to connect exterior devices to the system unit via the ports
Direct current (DC) powers computers
Desktop computers have a power supply unit located within the system unit
Notebook computers use AC adapters that are located outside the system unit
Page 172