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Chapter 4: The Components of the System Unit
OBJECTIVES:
After completing this chapter, you will be able to:
1. Differentiate among various styles of system units
2. Describe the components of a processor and bow they complete a machine cycle
3. Define a bit and describe how a series of bits represents data
4. Differentiate among the various types of memory
5. Describe the types of expansion slots and adapter cards
6. Explain the differences among a serial port, a parallel port, a USB port, and other ports
7. Describe how buses contribute to a computer’s processing speed
8. Identify components in mobile computers and mobile devices
9. Understand how to clean a system unit
CONTENTS:
THE SYSTEM UNIT
The Motherboard
PROCESSOR
The Control Unit
The Arithmetic Logic Unit
Machine Cycle
The System Clock
Comparison of Personal Computer Processors
Buying a Personal Computer
DATA REPRESENTATION
MEMORY
Bytes and Addressable Memory
Memory Sizes
Types of Memory
RAM
Cache ROM
Flash Memory
CMOS
Memory Access Times
EXPANSION SLOTS AND ADAPTER CARDS
PC Cards, Flash Memory Cards, and USB Flash Drives
PORTS AND CONNECTORS
Serial Ports
Parallel Ports
USB Ports
FireWire Ports
Special-Purpose Ports
BUSES
BAYS
POWER SUPPLY
MOBILE COMPUTERS AND DEVICES
PUTTING IT ALL TOGETHER
KEEPING YOUR COMPUTER CLEAN
COMPANIES ON THE CUTTING EDGE
AMD
Intel
TECHNOLOGY TRAILBLAZERS
Jack Kilby
Gordon Moore
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Whether you are a home user or a business user, you most likely will make the decision to purchase a new computer or upgrade an existing computer within the next several years. Thus, you should understand the purpose of each component in a computer. As Chapter 1 discussed, a computer includes devices used for input, processing, output, storage, and communications. Many of these components are part of the system unit.
The system unit is a case that contains electronic components of the computer used to process data. System units are available in a variety of shapes and sizes. The case of the system unit is made of metal or plastic and protects the internal electronic components from damage. All computers have a system unit (Figure 4-1).
On desktop personal computers, the electronic components and most storage devices are part of the system unit. Other devices, such as the keyboard, mouse, microphone, monitor, printer, scanner, PC video camera, and speakers, normally occupy space outside the system unit. On notebook computers, the keyboard and pointing device often occupy the area on the top of the system unit, and the display attaches to the system unit by hinges. The location of the system unit
FIGURE 4.1 All sizes of computers have a system unit.
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on a Tablet PC varies, depending on the design of the Tablet PC. Some models position the system unit below the keyboard, while others build the system unit behind the display. The sys tem unit on a PDA and smart phone usually consumes the entire device. On these mobile devices, the display often is built into the system unit.
At some point, you might have to open the system unit on a desktop personal computer to replace or install a new electronic component. For this reason, you should be familiar with the electronic components of a system unit. Figure 4-2 identifies some of these components, which include the processor, memory, adapter cards, ports, drive bays, and the power supply.
The processor interprets and carries out the basic instructions that operate a computer. Memory typically holds data waiting to be processed and instructions waiting to be executed. The electronic components and circuitry of the system unit, such as the processor and memory, usually are part of or are connected to a circuit board called the motherboard. Many motherboards also integrate modem and networking capabilities.
Adapter cards are circuit boards that provide connections and functions not built into the motherboard. Two adapter cards found in some desktop personal computers today are a sound card and a video card.
FIGURE 4-2 The system unit on a typical personal computer consists of numerous electronic components, some of which are shown in this figure. The sound card and video card are two types of adapter cards.
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Devices outside the system unit often attach to ports on the system unit by a connector on a cable. These devices may include a keyboard, mouse, microphone, monitor, printer, scanner, card reader/writer, digital camera, PC video camera, and speakers. A drive bay holds one or more disk drives. The power supply allows electricity to travel through a power cord from a wall outlet into a computer.
The motherboard, sometimes called a system board, is the main circuit board of the system unit. Many electronic components attach to the motherboard; others are built into it. Figure 4-3 shows a photograph of a current desktop personal computer motherboard and identifies some components that attach to it, including adapter cards, a processor chip, and a memory module. Memory chips are installed on memory cards (modules) that fit in a slot on the motherboard.
A computer chip is a small piece of semiconducting material, usually silicon, on which integrated circuits are etched. An integrated circuit contains many microscopic pathways capable of carrying electrical current. Each integrated circuit can contain millions of elements such as resistors, capacitors, and transistors.
FIGURE 4-3 Many electronic components attach to the motherboard in a desktop personal computer, including a processor chip, memory module, and adapter cards.
For more information, visit scsite.com/dcf2e/ ch4/weblink and then click Motherboards.
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The processor, also called the central processing unit (CPU), interprets and carries out the basic instructions that operate a computer. The processor significantly impacts overall computing power and manages most of a computer’s operations. On a personal computer, all functions of the processor usually are on a single chip. Some computer and chip manufacturers use the term microprocessor to refer to a personal computer processor chip.
Processors contain a control unit and an arithmetic logic unit (ALU). These two components work together to perform processing operations. Figure 4-4 illustrates how other devices that are connected to the computer communicate with the processor to carry out a task.
FIGURE 4.4 Most devices connected to the computer communicate with the processor to carry out a task. When a user starts a program, for example, its instructions transfer from a storage device to memory. Data needed by programs enters memory from either an input device or a storage device. The control unit interprets and executes instructions in memory, and the ALU performs calculations on the data in memory. Resulting information is stored in memory, from which it can be sent to an output device or a storage device for future access, as needed.
The control unit is the component of the processor that directs and coordinates most of the operations in the computer. The control unit has a role much like a traffic cop: it interprets each instruction issued by a program and then initiates the appropriate action to carry out the instruction.
The arithmetic logic unit (ALU), another component of the processor, performs arithmetic, comparison, and other operations. Arithmetic operations include basic calculations such as addition, subtraction, multiplication, and division. Comparison operations involve comparing one data item with another to determine whether the first item is greater than, equal to, or less than the other item. Depending on the result of the comparison, different actions may occur.
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For every instruction, a processor repeats a set of four basic operations, which comprise a machine cycle (Figure 4-5): (1) fetching, (2) decoding, (3) executing, and, if necessary, (4) storing. Fetching is the process of obtaining a program instruction or data item from memory. The term decoding refers to the process of translating the instruction into signals the computer can execute. Executing is the process of carrying out the commands. Storing, in this context, means writing the result to memory (not to a storage medium).
FIGURE 4-5 THE STEPS IN A MACHINE CYCLE
A student enters a math problem in to the memory of the computer
Step 1: The control unit fetches the math problem’s instructions and data from memory.
Step 2: The control unit decodes the math problem’s instructions and sends the instructions and data to the ALU.
Step 3: The ALU performs calculations on the data.
Step 4: The results of the math problems are stored in the memory.
The results in memory appear on the screen of the monitor
The processor relies on a small quartz crystal circuit called the system clock to control the timing of all computer operations. Just as your heart beats at a regular rate to keep your body functioning, the system clock generates regular electronic pulses, or ticks, that set the operating pace of components of the system unit.
The pace of the system clock, called the clock speed, is measured by the number of ticks per second. Current personal computer processors have clock speeds in the gigahertz range. Giga is a prefix that stands for billion, and a hertz is one cycle per second. Thus, one gigahertz (GHz) equals one billion ticks of the system clock per second. A computer that operates at 3.2 GHz has 3.2 billion (giga) clock cycles in one second (hertz). The system clock is one of the major factors that influence a computer’s speed. The faster the clock speed, the more instructions the processor can execute per second. Read Looking Ahead 4-1 for a look at the next generation of processing speeds.
For more information, visit scsite.com/ dcf2e/ch4/weblink and then click Clock Speed.
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LOOKING AHEAD 4-1
US Plans World’s Fastest Computer
Nine of the world’s ten fastest computers are located in the United States. The fastest computer is located in Japan, according to the Top500 Project, a group that monitors supercomputers throughout the world.
The U.S. Department of Energy is planning to have the United States regain the record as having the world’s fastest civilian computer. It is helping to fund a new supercomputer at a research laboratory in Oak Ridge, Tennessee, with $50 million in federal grants and assistance from Cray Corp., IBM Corp., and Silicon Graphics Inc.
Oak Ridge scientists predict the new computer will be able to perform 50 trillion calculations per second. The Japanese computer sustains 36 trillion calculations per second. For more information, visit scsite.com/dcf2e/ ch4/looking and then click Fastest Computer.
The leading processor chip manufacturers for personal computers are Intel, AMD (Advanced Micro Devices), IBM, Motorola, and Transmeta. These manufacturers often identify their processor chips by a model name or model number.
With its earlier processors, Intel used a model number to identify the various chips. After learning that processor model numbers could not be trademarked and protected from use by competitors, Intel began identifying its processors with names — thus emerged the series of processors known as the Pentium. Most high-performance PCs use some type of Pentium processor. Many notebook computers and Tablet PCs use a Pentium M processor. Less expensive, basic PCs use a brand of Intel processor called the Celeron. Two more brands, called the Xeon and Itanium processors, are ideal for workstations and low-end servers.
AMD is the leading manufacturer of Intel-compatible processors, which have an internal design similar to Intel processors, perform the same functions, and can be as powerful, but often are less expensive. Transmeta, also a manufacturer of Intel-compatible processors, specializes in processors for mobile computers and devices. Intel and Intel-compatible processors are used in PCs.
Apple computers use an IBM processor or a Motorola processor, which has a design different from the Intel-style processor. The PowerPC processor has a new architecture that increased the speed of the latest Apple computers.
In the past, chip manufacturers listed a processor’s clock speed in marketing literature and advertisements. Today, however, clock speed is only one factor that impacts processing speed. To help consumers evaluate various processors, manufacturers such as Intel and AMD now use a numbering scheme that more accurately reflects the processing speed of their chips.
If you are ready to buy a new computer, the processor you select should depend on how you plan to use the computer. If you purchase an IBM-compatible PC, you will choose an Intel processor or an Intel-compatible processor. Apple Macintosh and Power Macintosh computers have a Motorola or IBM processor. Current Apple processors include the PowerPC G4 and PowerPC G5.
Your intended use also will determine the clock speed you need. A home user surfing the Web, for example, may need only a 2 GHz processor, while an artist working with graphics or applications requiring multimedia capabilities such as full-motion video, may require at least a 3 GHz processor.
For detailed computer purchasing guidelines, read the Buyer’s Guide feature that follows Chapter 7. Read At Issue 4-1 for a related discussion. _____
At Issue 4-1: Computer Waste and the Environment: Whose Problem Is It?
Experts estimate that about 1 billion computers will be discarded by 2010. As technology advances and prices fall, many people think of computers as disposable items. But, disposing of old system units, monitors, and other computer components is a major problem. Computers contain several toxic elements, including lead, mercury, and barium. Computers thrown into landfills or burned in incinerators can pollute the ground and the air. One solution is to recycle old computers. Computers for Schools refurbishes donated computers and makes them available to schools and students at very low prices, and donors earn tax breaks. Some lawmakers prefer a more aggressive approach, such as setting up a recycling program that would be paid for by adding a $10 fee to a computer’s purchase price, or forcing computer makers to be responsible for collecting and recycling their products. Manufacturers already have taken steps. Several have reduced the amount of toxic material in their products, and some have set up their own recycling programs, for which users pay a fee. One manufacturer admits, however, that only seven percent of the computers it has sold have been recycled. What can be done to ensure that computers are disposed of safely? Should government, manufacturers, or users be responsible for safe disposal? Why? How can computer users be motivated to recycle obsolete equipment?
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Test your knowledge of pages 134 through 139 in Quiz Yourself 4-1.
Instructions: Find the true statement below. Then, rewrite the remaining false statements so they are true.
1. A computer chip is a small piece of semiconducting material, usually silicon, on which integrated circuits are etched.
2. Four basic operations in a machine cycle are: (1) comparing, (2) decoding, (3) executing, and, if necessary, (4) pipelining.
3. Processors contain a motherboard and an arithmetic logic unit (ALU).
4. The central processing unit, sometimes called a system board, is the main circuit board of the system unit.
5. The leading processor chip manufacturers for personal computers are Microsoft, AMD, IBM, Motorola, and Transmeta.
6. The system unit is a case that contains mechanical components of the computer used to process data. Quiz Yourself Online: To further check your knowledge of system unit styles, processor components, and machine cycles, visit scsite.com/dcf2e/ch4/quiz and then click Objectives 1 — 2.
FIGURE 4-6 A computer circuit represents the 0 or the 1 electronically by the presence or absence of an electronic charge.
FIGURE 4-7 Eight bits grouped together as a unit are called a byte. A byte represents a single character in the computer.
To understand fully the way a computer processes data, you should know how a computer represents data. Most computers are digital. They recognize only two discrete states: on and off. The two digits, 0 and 1, easily can represent these two states (Figure 4-6). The digit 0 represents the electronic state of off (absence of an electronic charge). The digit 1 represents the electronic state of on (presence of an electronic charge).
The computer uses a binary system because it recognizes only two states. The binary system is a number system that has just two unique digits, 0 and 1, called bits. A bit (short for binary digit) is the smallest unit of data the computer can process. By itself, a bit is not very informative. When 8 bits are grouped together as a unit, they form a byte. A byte provides enough different combinations of Os and is to represent 256 individual characters. These characters include numbers, uppercase and lowercase letters of the alphabet, punctuation marks, and others, such as the letters of the Greek alphabet.
The combinations of Os and is that represent characters are defined by patterns called a coding scheme. In one coding scheme, the number 4 is represented as 00110100, the number 6 as 00110110, and the capital letter F as 01000101 (Figure 4-7). Two popular coding schemes are ASCII and EBCDIC (Figure 4-8). The American Standard Code for Information Interchange (ASCII pronounced ASK-ee) scheme is the most widely used coding sys tem to represent data. Most personal computers and midrange servers use the ASCII coding scheme. The Extended Binary Coded Decimal Interchange Code (EBCDIC pronounced EB-see-dik) scheme is used primarily on mainframe computers and high-end servers.
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Coding schemes such as ASCII make it possible for humans to interact with a digital computer that processes only bits. When you press a key on a keyboard, a chip in the keyboard converts the key’s electronic signal into a scan code that is sent to the system unit. Then, the system unit converts the scan code into a binary form the com puter can process and is stored in memory. Every character is converted to its corresponding byte. The computer then processes the data as bytes, which actually is a series of on/off electrical states. When processing is finished, soft ware converts the byte into a human-recognizable number, letter of the alphabet, or special character that is displayed on a screen or is printed (Figure 4-9). All of these conversions take place so quickly that you do not realize they are occurring.
Standards, such as those defined by ASCII and EBCDIC, also make it possible for components in computers to communicate successfully with each other.
FIGURE 4-8 Two popular coding schemes are ASCII and EBCDIC.
FIGURE 4-9 HOW A LETTER IS CONVERTED TO BINARY FORM AND BACK
Step 1: [Illegible].
Step 2: The scan code for the capital letter T is sent to the system unit.
Step 3: The system unit converts the scan code for the capital letter T to its ASCII binary code (01010100) and stores it in memory for processing.
Step 4: After processing, the binary code for the capital letter T is converted to an image and displayed on the output device.
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Memory consists of electronic components that store instructions waiting to be executed by the processor, data needed by those instructions, and the results of processed data (information). Memory usually consists of one or more chips on the motherboard or some other circuit board in the computer.
Memory stores three basic categories of items: (1) the operating system and other system soft ware that control or maintain the computer and its devices; (2) application programs that carry out a specific task such as word processing; and (3) the data being processed by the application programs and resulting information. This role of memory to store both data and programs is known as the stored program concept.
A byte (character) is the basic storage unit in memory. When application program instructions and data are transferred to memory from storage devices, the instructions and data exist as bytes. Each byte resides temporarily in a location in mem ory that has an address. An address simply is a unique number that identifies the location of the byte in memory. The illustration in Figure 4-10 shows how seats in a concert hail are similar to addresses in memory: (1) a seat, which is identified by a unique seat number, holds one person at a time, and a location in memory, which is identified by a unique address, holds a single byte; and (2) both a seat, identified by a seat number, and a byte, identified by an address, can be empty. To access data or instructions in memory, the computer references the addresses that contain bytes of data.
FIGURE 4-10 Seats in a concert hail are similar to addresses in memory: a seat holds one person at a time, and a location in memory holds a single byte; and both a seat and a byte can be empty.
Manufacturers state the size of memory chips and storage devices in terms of the number of bytes the chip or device has available for storage (Figure 4-11). Recall that storage devices hold data, instructions, and information for future use, while most memory holds these items temporarily. A kilobyte (KB or K) is equal to exactly 1,024 bytes. To simplify memory and storage definitions, computer users often round a kilobyte down to 1,000 bytes. For example, if a memory chip can store 100 KB, it can hold approximately 100,000 bytes (characters). A megabyte (MB) is equal to approximately 1 million bytes. A gigabyte (GB) equals approximately 1 billion bytes. A terabyte (TB) is equal to approximately 1 trillion bytes.
The system unit contains two types of memory: volatile and nonvolatile. When the computer’s power is turned off, volatile memory loses its contents. Nonvolatile memory, by contrast, does not lose its contents when power is removed from the computer. Thus, volatile memory is temporary and nonvolatile memory is permanent. RAM is the most common type of volatile memory. Examples of nonvolatile memory include ROM, flash memory, and CMOS. The following sections discuss these types of memory.
FIGURE 4-1 1 Terms commonly used to define memory and storage sizes.
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Users typically are referring to RAM when discussing computer memory. RAM (random access memory), also called main memory, consists of memory chips that can be read from and written to by the processor and other devices. When you turn on power to a computer, certain operating system files (such as the files that determine how the Windows XP desktop appears) load into RAM from a storage device such as a hard disk. These files remain in RAM as long as the computer has continuous power. As additional programs and data are requested, they also load into RAM from storage.
The processor interprets and executes a program’s instructions while the program is in RAM. During this time, the contents of RAM may change (Figure 4-12). RAM can hold multiple programs simultaneously, provided the computer has enough RAM to accommodate all the programs.
Most RAM is volatile, which means it loses its contents when the power is removed from the computer. For this reason, you must save any items you may need in the future. Saving is the process of copying items from RAM to a storage device such as a hard disk.
FIGURE 4-12 HOW PROGRAM INSTRUCTIONS TRANSFER IN AND OUT OF RAM
Step 1: When you start the computer, certain operating system files are loaded into RAM from the hard disk. The operating system displays the user interface on the screen.
Step 2: When you start a Web browser, the program’s instructions are loaded into RAM from the hard disk. The Web browser and certain operating system instructions are in RAM. The Web browser window is displayed on the screen.
Step 3: When you start a word processing program, the program’s instructions are loaded into RAM from the hard disk....
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