the parts of a computer:
This part of the reading will examine the CPU, Buses, Controllers, and Main Memory. Other sections will examine input devices, output devices, and secondary memory.
The computer does its primary work in a part of the machine we cannot see, a control center that converts data input to information output. This control center, called the central processing unit (CPU), is a highly complex, extensive set of electronic circuitry that executes stored program instructions. All computers, large and small, must have a central processing unit. As Figure 1 shows, the central processing unit consists of two parts: The control unit and the arithmetic/logic unit. Each part has a specific function.
Before we discuss the control unit and the arithmetic/logic unit in detail, we need to consider data storage and its relationship to the central processing unit. Computers use two types of storage: Primary storage and secondary storage. The CPU interacts closely with primary storage, or main memory, referring to it for both instructions and data. For this reason this part of the reading will discuss memory in the context of the central processing unit. Technically, however, memory is not part of the CPU.
Recall that a computer's memory holds data only temporarily, at the time the computer is executing a program. Secondary storage holds permanent or semi-permanent data on some external magnetic or optical medium. The diskettes and CD-ROM disks that you have seen with personal computers are secondary storage devices, as are hard disks. Since the physical attributes of secondary storage devices determine the way data is organized on them, we will discuss secondary storage and data organization together in another part of our on-line readings.
Now let us consider the components of the central processing unit.
The Control Unit
The control unit of the CPU contains circuitry that uses electrical signals to direct the entire computer system to carry out, or execute, stored program instructions. Like an orchestra leader, the control unit does not execute program instructions; rather, it directs other parts of the system to do so. The control unit must communicate with both the arithmetic/logic unit and memory.
The Arithmetic/Logic Unit
The arithmetic/logic unit (ALU) contains the electronic circuitry that executes all arithmetic and logical operations.
The arithmetic/logic unit can perform four kinds of arithmetic operations, or mathematical calculations: addition, subtraction, multiplication, and division. As its name implies, the arithmetic/logic unit also performs logical operations. A logical operation is usually a comparison. The unit can compare numbers, letters, or special characters. The computer can then take action based on the result of the comparison. This is a very important capability. It is by comparing that a computer is able to tell, for instance, whether there are unfilled seats on airplanes, whether charge- card customers have exceeded their credit limits, and whether one candidate for Congress has more votes than another.
Logical operations can test for three conditions:
A computer can simultaneously test for more than one condition. In fact, a logic unit can usually discern six logical relationships: equal to, less than, greater than, less than or equal to, greater than or equal to, and not equal.
The symbols that let you define the type of comparison you want the computer to perform are called relational operators. The most common relational operators are the equal sign(=), the less-than symbol(<), and the greater-than symbol(>).
Memory is also known as primary storage, primary memory, main storage, internal storage, main memory, and RAM (Random Access Memory); all these terms are used interchangeably by people in computer circles. Memory is the part of the computer that holds data and instructions for processing. Although closely associated with the central processing unit, memory is separate from it. Memory stores program instructions or data for only as long as the program they pertain to is in operation. Keeping these items in memory when the program is not running is not feasible for three reasons:
How do data and instructions get from an input device into memory? The control unit sends them. Likewise, when the time is right, the control unit sends these items from memory to the arithmetic/logic unit, where an arithmetic operation or logical operation is performed. After being processed, the information is sent to memory, where it is hold until it is ready to he released to an output unit.
The chief characteristic of memory is that it allows very fast access to instructions and data, no matter where the items are within it. We will discuss the physical components of memory-memory chips-later in this chapter.
To see how registers, memory, and second storage all work together, let us use the analogy of making a salad. In our kitchen we have:
Now for a more technical example. let us look at how a payroll program uses all three types of storage. Suppose the program calculates the salary of an employee. The data representing the hours worked and the data for the rate of pay are ready in their respective registers. Other data related to the salary calculation-overtime hours, bonuses, deductions, and so forth-is waiting nearby in memory. The data for other employees is available in secondary storage. As the CPU finishes calculations about one employee, the data about the next employee is brought from secondary storage into memory and eventually into the registers.
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This part of the reading will examine the CPU, Buses, Controllers, and Main Memory. Other sections will examine input devices, output devices, and secondary memory.
The Central Processing Unit (CPU)
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| Figure 1: The Central Processing Unit |
Before we discuss the control unit and the arithmetic/logic unit in detail, we need to consider data storage and its relationship to the central processing unit. Computers use two types of storage: Primary storage and secondary storage. The CPU interacts closely with primary storage, or main memory, referring to it for both instructions and data. For this reason this part of the reading will discuss memory in the context of the central processing unit. Technically, however, memory is not part of the CPU.
Recall that a computer's memory holds data only temporarily, at the time the computer is executing a program. Secondary storage holds permanent or semi-permanent data on some external magnetic or optical medium. The diskettes and CD-ROM disks that you have seen with personal computers are secondary storage devices, as are hard disks. Since the physical attributes of secondary storage devices determine the way data is organized on them, we will discuss secondary storage and data organization together in another part of our on-line readings.
Now let us consider the components of the central processing unit.
The control unit of the CPU contains circuitry that uses electrical signals to direct the entire computer system to carry out, or execute, stored program instructions. Like an orchestra leader, the control unit does not execute program instructions; rather, it directs other parts of the system to do so. The control unit must communicate with both the arithmetic/logic unit and memory.
The arithmetic/logic unit (ALU) contains the electronic circuitry that executes all arithmetic and logical operations.
The arithmetic/logic unit can perform four kinds of arithmetic operations, or mathematical calculations: addition, subtraction, multiplication, and division. As its name implies, the arithmetic/logic unit also performs logical operations. A logical operation is usually a comparison. The unit can compare numbers, letters, or special characters. The computer can then take action based on the result of the comparison. This is a very important capability. It is by comparing that a computer is able to tell, for instance, whether there are unfilled seats on airplanes, whether charge- card customers have exceeded their credit limits, and whether one candidate for Congress has more votes than another.
Logical operations can test for three conditions:
- Equal-to condition. In a test for this condition, the arithmetic/logic unit compares two values to determine if they are equal. For example: If the number of tickets sold equals the number of seats in the auditorium, then the concert is declared sold out.
- Less-than condition. To test for this condition, the computer compares values to determine if one is less than another. For example: If the number of speeding tickets on a driver's record is less than three, then insurance rates are $425; otherwise, the rates are $500.
- Greater-than condition. In this type of comparison, the computer determines if one value is greater than another. For example: If the hours a person worked this week are greater than 40, then multiply every extra hour by 1.5 times the usual hourly wage to compute overtime pay.
A computer can simultaneously test for more than one condition. In fact, a logic unit can usually discern six logical relationships: equal to, less than, greater than, less than or equal to, greater than or equal to, and not equal.
The symbols that let you define the type of comparison you want the computer to perform are called relational operators. The most common relational operators are the equal sign(=), the less-than symbol(<), and the greater-than symbol(>).
- Registers: Temporary Storage Areas
Registers are temporary storage areas for instructions or data. They are not a part of memory; rather they are special additional storage locations that offer the advantage of speed. Registers work under the direction of the control unit to accept, hold, and transfer instructions or data and perform arithmetic or logical comparisons at high speed. The control unit uses a data storage register the way a store owner uses a cash register-as a temporary, convenient place to store what is used in transactions.
Computers usually assign special roles to certain registers, including these registers:- An accumulator, which collects the result of computations.
- An address register, which keeps track of where a given instruction or piece of data is stored in memory. Each storage location in memory is identified by an address, just as each house on a street has an address.
- A storage register, which temporarily holds data taken from or about to be sent to memory.
- A general-purpose register, which is used for several functions.
Memory is also known as primary storage, primary memory, main storage, internal storage, main memory, and RAM (Random Access Memory); all these terms are used interchangeably by people in computer circles. Memory is the part of the computer that holds data and instructions for processing. Although closely associated with the central processing unit, memory is separate from it. Memory stores program instructions or data for only as long as the program they pertain to is in operation. Keeping these items in memory when the program is not running is not feasible for three reasons:
- Most types of memory only store items while the computer is turned on; data is destroyed when the machine is turned off.
- If more than one program is running at once (often the case on large computers and sometimes on small computers), a single program can not lay exclusive claim to memory.
- There may not be room in memory to hold the processed data.
How do data and instructions get from an input device into memory? The control unit sends them. Likewise, when the time is right, the control unit sends these items from memory to the arithmetic/logic unit, where an arithmetic operation or logical operation is performed. After being processed, the information is sent to memory, where it is hold until it is ready to he released to an output unit.
The chief characteristic of memory is that it allows very fast access to instructions and data, no matter where the items are within it. We will discuss the physical components of memory-memory chips-later in this chapter.
To see how registers, memory, and second storage all work together, let us use the analogy of making a salad. In our kitchen we have:
- a refrigerator where we store our vegetables for the salad;
- a counter where we place all of our veggies before putting them on the cutting board for chopping;
- a cutting board on the counter where we chop the vegetables;
- a recipe that details what veggies to chop;
- the corners of the cutting board are kept free for partially chopped piles of veggies that we intend to chop more or to mix with other partially chopped veggies.
- a bowl on the counter where we mix and store the salad;
- space in the refrigerator to put the mixed salad after it is made.
The refrigerator is the equivalent of secondary (disk) storage. It can store high volumes of veggies for long periods of time. The counter top is the equivalent of the computer's motherboard - everything is done on the counter (inside the computer). The cutting board is the ALU - the work gets done there. The recipe is the control unit - it tells you what to do on the cutting board (ALU). Space on the counter top is the equivalent of RAM memory - all veggies must be brought from the fridge and placed on the counter top for fast access. Note that the counter top (RAM) is faster to access than the fridge (disk), but can not hold as much, and can not hold it for long periods of time. The corners of the cutting board where we temporarily store partially chopped veggies are equivalent to the registers. The corners of the cutting board are very fast to access for chopping, but can not hold much. The salad bowl is like a temporary register, it is for storing the salad waiting to take back to the fridge (putting data back on a disk) or for taking to the dinner table (outputting the data to an output device).
Now for a more technical example. let us look at how a payroll program uses all three types of storage. Suppose the program calculates the salary of an employee. The data representing the hours worked and the data for the rate of pay are ready in their respective registers. Other data related to the salary calculation-overtime hours, bonuses, deductions, and so forth-is waiting nearby in memory. The data for other employees is available in secondary storage. As the CPU finishes calculations about one employee, the data about the next employee is brought from secondary storage into memory and eventually into the registers.
The following table summarizes the characteristics of the various kinds of data storage in the storage hierarchy.
| Storage | Speed | Capacity | Relative Cost ($) | Permanent? |
| Registers | Fastest | Lowest | Highest | No |
| RAM | Very Fast | Low/Moderate | High | No |
| Floppy Disk | Very Slow | Low | Low | Yes |
| Hard Disk | Moderate | Very High | Very Low | Yes |
- How the CPU Executes Program Instructions
Let us examine the way the central processing unit, in association with memory, executes a computer program. We will be looking at how just one instruction in the program is executed. In fact, most computers today can execute only one instruction at a time, though they execute it very quickly. Many personal computers can execute instructions in less than one-millionth of a second, whereas those speed demons known as supercomputers can execute instructions in less than one-billionth of a second.
Before an instruction can be executed, program instructions and data must be placed into memory from an input device or a secondary storage device (the process is further complicated by the fact that, as we noted earlier, the data will probably make a temporary stop in a register). As Figure 2 shows, once the necessary data and instruction are in memory, the central processing unit performs the following four steps for each instruction:Figure 2: The Machine Cycle - The control unit fetches (gets) the instruction from memory.
- The control unit decodes the instruction (decides what it means) and directs that the necessary data be moved from memory to the arithmetic/logic unit. These first two steps together are called instruction time, or I-time.
- The arithmetic/logic unit executes the arithmetic or logical instruction. That is, the ALU is given control and performs the actual operation on the data.
- Thc arithmetic/logic unit stores the result of this operation in memory or in a register. Steps 3 and 4 together are called execution time, or E-time.
The control unit eventually directs memory to release the result to an output device or a secondary storage device. The combination of I-time and E-time is called the machine cycle. Figure 3 shows an instruction going through the machine cycle.
Each central processing unit has an internal clock that produces pulses at a fixed rate to synchronize all computer operations. A single machine-cycle instruction may be made up of a substantial number of sub-instructions, each of which must take at least one clock cycle. Each type of central processing unit is designed to understand a specific group of instructions called the instruction set. Just as there are many different languages that people understand, so each different type of CPU has an instruction set it understands. Therefore, one CPU-such as the one for a Compaq personal computer-cannot understand the instruction set from another CPU-say, for a Macintosh.
It is one thing to have instructions and data somewhere in memory and quite another for the control unit to be able to find them. How does it do this?Figure 3: The Machine Cycle in Action
The location in memory for each instruction and each piece of data is identified by an address. That is, each location has an address number, like the mailboxes in front of an apartment house. And, like the mailboxes, the address numbers of the locations remain the same, but the contents (instructions and data) of the locations may change. That is, new instructions or new data may be placed in the locations when the old contents no longer need to be stored in memory. Unlike a mailbox, however, a memory location can hold only a fixed amount of data; an address can hold only a fixed number of bytes - often two bytes in a modern computer.Figure 4: Memory Addresses Like Mailboxes
Figure 4 shows how a program manipulates data in memory. A payroll program, for example, may give instructions to put the rate of pay in location 3 and the number of hours worked in location 6. To compute the employee's salary, then, instructions tell the computer to multiply the data in location 3 by the data in location 6 and move the result to location 8. The choice of locations is arbitrary - any locations that are not already spoken for can be used. Programmers using programming languages, however, do not have to worry about the actual address numbers, because each data address is referred to by a name. The name is called a symbolic address. In this example, the symbolic address names are Rate, Hours, and Salary.
- The Benefits of Secondary Storage
Picture, if you can, how many filing-cabinet drawers would be required to hold the millions of files of, say, tax records kept by the Internal Revenue Service or historical employee records kept by General Motors. The record storage rooms would have to be enormous. Computers, in contrast, permit storage on tape or disk in extremely compressed form. Storage capacity is unquestionably one of the most valuable assets of the computer.
Secondary storage, sometimes called auxiliary storage, is storage separate from the computer itself, where you can store software and data on a semi permanent basis. Secondary storage is necessary because memory, or primary storage, can be used only temporarily. If you are sharing your computer, you must yield memory to someone else after your program runs; if you are not sharing your computer, your programs and data will disappear from memory when you turn off the computer. However, you probably want to store the data you have used or the information you have derived from processing; that is why secondary storage is needed. Furthermore, memory is limited in size, whereas secondary storage media can store as much data as necessary. Storage Speed Capacity Relative Cost ($) Permanent? Registers Fastest Lowest Highest No RAM Very Fast Low/Moderate High No Floppy Disk Very Slow Low Low Yes Hard Disk Moderate Very High Very Low Yes
The benefits of secondary storage can be summarized as follows:- Capacity. Organizations may store the equivalent of a roomful of data on sets of disks that take up less space than a breadbox. A simple diskette for a personal computer holds the equivalent of 500 printed pages, or one book. An optical disk can hold the equivalent of approximately 400 books.
- Reliability. Data in secondary storage is basically safe, since secondary storage is physically reliable. Also, it is more difficult for unscrupulous people to tamper with data on disk than data stored on paper in a file cabinet.
- Convenience. With the help of a computer, authorized people can locate and access data quickly.
- Cost. Together the three previous benefits indicate significant savings in storage costs. It is less expensive to store data on tape or disk (the principal means of secondary storage) than to buy and house filing cabinets. Data that is reliable and safe is less expensive to maintain than data subject to errors. But the greatest savings can be found in the speed and convenience of filing and retrieving data.
These benefits apply to all the various secondary storage devices but, as you will see, some devices are better than others. We begin with a look at the various storage media, including those used for personal computers, and then consider what it takes to get data organized and processed.- Magnetic Disk Storage
Diskettes and hard disks are magnetic media; that is, they are based on a technology of representing data as magnetized spots on the disk with a magnetized spot representing a 1 bit and the absence of such a spot representing a 0 bit.
Reading data from the disk means converting the magnetized data to electrical impulses that can be sent to the processor. Writing data to disk is the opposite: sending electrical impulses from the processor to be converted to magnetized spots on the disk. The surface of each disk has concentric tracks on it. The number of tracks per surface varies with the particular type of disk.
Diskettes
Made of flexible Mylar, a diskette can record data as magnetized spots on tracks on its surface. Diskettes became popular along with the personal computer.
The older diskette, 5-1/4 inches in diameter, is still in use, but newer computers use the 3-1/2 inch diskette (Figure 1). The 3-1/2 inch diskette has the protection of a hard plastic jacket, a size to fit conveniently in a shirt pocket or purse, and the capacity to hold significantly more data than a 5-1/4 inch diskette. Diskettes offer particular advantages which, as you will see, are not readily available with hard disk:Figure 1: Diskettes - Portability. Diskettes easily transport data from one computer to another. Workers, for example, carry their files from office computer to home computer and back on a diskette instead of in a briefcase. Students use the campus computers but keep their files on their own diskettes.
- Backup. It is convenient to place an extra copy of a hard disk file on a diskette.
- New software. Although, for convenience, software packages are kept on hard disk, new software out of the box may come on diskettes (new software also may come on CD-ROM disks, which we will discuss shortly).
Hard Disks
A hard disk is a metal platter coated with magnetic oxide that can be magnetized to represent data. Hard disks come in a variety of sizes.
Hard disk for mainframes and minicomputers may be as large as 14 inches in diameter. Several disks can be assembled into a disk pack. There are different types of disk packs, with the number of platters varying by model. Each disk in the pack has top and bottom surfaces on which to record data. Many disk devices, however, do not record data on the top of the top platter or on the bottom of the bottom platter.Figure 2: Hard Disk and Drive
A disk drive is a machine that allows data to be read from a disk or written on a disk. A disk pack is mounted on a disk drive that is a separate unit connected to the computer. Large computers have dozens or ever hundreds of disk drives. In a disk pack all disks rotate at the same time although only one disk is being read or written on at any one time. The mechanism for reading or writing data on a disk is an access arm; it moves a read/write head into position over a particular track. The read/write head on the end of the access arm hovers just above the track but does not actually touch the surface. When a read/write head does accidentally touch the disk surface, this is called a head crash and all data is destroyed. Data can also be destroyed if a read/write head encounters even minuscule foreign matter on the disk surface. A disk pack has a series of access arms that slip in between the disks in the pack. Two read/write heads are on each arm, one facing up for the surface above it and one facing down for the surface below it. However, only one read/write head can operate at any one time.
In some disk drives the access arms can be retracted; then the disk pack can be removed from the drive. Most disk packs, however, combine the disks, access arms, and read/write heads in a sealed module called a Winchester disk. Winchester disk assemblies are put together in clean rooms so even microscopic dust particles do not get on the disk surface.
Hard disks for personal computers are 5-1/4 inch or 3-1/2 inch disks in sealed modules and even gigabytes are not unusual. Hard disk capacity for personal computers has soared in recent years; capacities of hundreds of megabytes are common and gigabytes are not unusual. Although an individual probably cannot imagine generating enough output-letters, budgets, reports, and so forth-to fill a hard disk, software packages take up a lot of space and can make a dent rather quickly. Furthermore, graphics images and audio and video files require large file capacities. Perhaps more important than capacity, however, is the convenience of speed. Personal computer users find accessing files on a hard disk is significantly faster and thus more convenient than accessing files on a diskette.
Removable Storage: Zip Disks
Personal computer users, who never seem to have enough hard disk storage space, may turn to a removable hard disk cartridge. Once full, a removable hard disk cartridge can be replaced with a fresh one. In effect, a removable cartridge is as portable as a diskette, but the disk cartridge holds much more data. Removable units also are important to businesses concerned with security, because the units can be used during business hours but hidden away during off hours. A disadvantage of a removable hard disk is that it takes longer to access data than a built-in hard drive.Figure 3: Iomega Zip Disk
The most popular removable disk media is the Zip drive from Iomega (Figure 3). Over 100's of millions have been sold, making it the de facto standard. The disk cartridges look like a floppy disk, but are slightly bigger in all dimensions. Older Zip disks hold 100MB, newer ones hold 250MB and cost $8-$10 a piece (Floppies hold 1.4MB and cost around $2). The drive sells for around $80- $125. Many new PCs come with Zip drives built in addition to floppy drives. Zip disks are a great way to store large files and software programs.
Hard Disks in Groups
A concept of using several small disks that work together as a unit is called a redundant array of inexpensive disks, or simply RAID. The group of connected disks operates as if it were just one large disk, but it speeds up reading and writing by having multiple access paths. The data file for, say, aircraft factory tools, may be spread across several disks; thus, if the computer is used to look up tools for several workers, the computer need not read the data in turn but instead read them at the same time in parallel. Furthermore, data security is improved because if a disk fails, the disk system can reconstruct data on an extra disk; thus, computer operations can continue uninterrupted. This is significant data insurance.
How Data Is Organized on a Disk
There is more than one way of physically organizing data on a disk. The methods we will consider here are the sector method and the cylinder method.
The Sector Method
In the sector method each track is divided into sectors that hold a specific number of characters. Data on the track is accessed by referring to the surface number, track number, and sector number where the data is stored. The sector method is used for diskettes as well as disk packs.
Zone Recording
The fact that a disk is circular presents a problem: The distances around the tracks on the outside of the disk are greater than that of the tracks or the inside. A given amount of data that takes up 1 inch of a track on the inside of a disk might be spread over several inches on a track near the outside of a disk. This means that the tracks on the outside are not storing data as efficiently.
Zone recording involves dividing a disk into zones to take advantage of the storage available on all tracks, by assigning more sectors to tracks in outer zones than to those in inner zones. Since each sector on the disk holds the same amount of data, more sectors mean more data storage than if all tracks had the same number of sectors.
The Cylinder Method
A way to organize data on a disk pack is the cylinder method. The organization in this case is vertical. The purpose is to reduce the time it takes to move the access arms of a disk pack into position. Once the access arms are in position, they are in the same vertical position on all disk surfaces.
To appreciate this, suppose you had an empty disk pack on which you wished to record data. You might be tempted to record the data horizontally-to start with the first surface, fill track 000, then fill track 001, track 002, and so on, and then move to the second surface and again fill tracks 000, 001, 002, and so forth. Each new track and new surface, however, would require movement of the access arms, a relatively slow mechanical process.
Recording the data vertically, on the other hand, substantially reduces access arm movement. The data is recorded on the tracks that can be accessed by one positioning of the access arms-that is, on one cylinder. To visualize cylinder organization, pretend a cylindrically shaped item, such as a tin can, were figuratively dropped straight down through all the disks in the disk pack. All the tracks thus encountered, in the same position on each disk surface, comprise a cylinder. The cylinder method, then, means all tracks of a certain cylinder on a disk pack are lined up one beneath the other, and all the vertical tracks of one cylinder are accessible by the read/write heads with one positioning of the access arms mechanism. Tracks within a cylinder are numbered according to this vertical perspective: A 20-surface disk pack contains cylinder tracks numbered 0 through 19, top to bottom.
- Optical Disk Storage
The explosive growth in storage needs has driven the computer industry to provide cheaper, more compact, and more versatile storage devices with greater capacity. This demanding shopping list is a description of the optical disk, like a CD. The technology works like this: A laser hits a layer of metallic material spread over the surface of a disk. When data is being entered, heat from the laser produces tiny spots on the disk surface. To read the data, the laser scans the disk, and a lens picks up different light reflections from the various spots.
Optical storage technology is categorized according to its read/write capability. Read-only media are recorded on by the manufacturer and can be read from but not written to by the user. Such a disk cannot, obviously, be used for your files, but manufacturers can use it to supply software. Applications software packages sometimes include a dozen diskettes or more; all these could fit on one optical disk with plenty of room to spare. The most prominent optical technology is the CD-ROM, for compact disk read-only memory. The disk in its drive is shown in Figure 3.
CD-ROM has a major advantage over other optical disk designs: The disk format is identical to that of audio compact disks, so the same dust-free manufacturing plants that are now stamping out digital versions of Mozart or Mary Chapin Carpenter can easily convert to producing anything from software to an encyclopedia. Furthermore, CD-ROM storage is large -up to 660 megabytes per disk, the equivalent of over 400 3-1/2 inch diskettes.Figure 3: Compact Disk (CD) and Drive)
When buying a computer the speed of the CD-ROM drive is advertised using an "X" factor, like 12X, or 24X. This indicates the speed at which the CD can transfer data to the CPU - the higher the X factor, the faster the CD.Modern computers now offer a write CD drive or, CD-RW as an option. CD-RW is a write-once, read-many media. With a CD-RW drive, you can create your own CDs. This offers an inexpensive, convenient, safe way to store large volumes of data such as favorite songs, photographs, etc.
DVDs
Digital Versatile Disk (DVD) drives are now widely available in computers as well as home entertainment centers. DVD-ROM drives can read data, such as stored commercial videos for playing. DVD-RW allow DVDs to be created on a computer.
The DVD is a flat disk, the size of a CD - 4.7 inches diameter and .05 inches thick. Data are stored in a small indentation in a spiral track, just like in the CD. DVD disks are read by a laser beam of shorter wave-length than used by the CD ROM drives. This allows for smaller indentations and increased storage capacity. The data layer is only half as thick as in the CD-ROM. This opens the possibility to write data in two layers. The outer gold layer is semi transparent, to allow reading of the underlying silver layer. The laser beam is set to two different intensities, strongest for reading the underlying silver layer.Figure 4: DVD Disk and Drive
A 4.7 GB side of a DVD can hold 135 minutes top quality video with 6 track stereo. This requires a transmission rate of 4692 bits per second. The 17 GB disk holds 200 hours top quality music recording.
DVD movies are made in two "codes." Region one is USA and Canada, while Europe and Asia is region two. When you play movies, your hardware (MPEG decoder. MGEG is the data coding for movies similar to JPEG for pictures.) must match the DVD region. The movies are made in two formats, each with their own coding.
The DVD drives come in 2X, 4X, etc. versions, like the CD-ROM's.
The DVD drives will not replace the magnetic hard disks. The hard disks are being improved as rapidly as DVD, and they definitely offer the fastest seek time and transmission rate (currently 5-10 MB/second). No optic media can keep up with this. But the DVD will undoubtedly gain a place as the successor to the CD ROM and is playing an important role in the blending of computers and entertainment centers.
- Magnetic Tape Storage
We saved magnetic tape storage for last because it has taken a subordinate role in storage technology. Magnetic tape looks like the tape used in music cassettes plastic tape with a magnetic coating. As in other magnetic media, data is stored as extremely small magnetic spots. Tapes come in a number of forms, including l/2-inch-wide tape wound on a reel, l/4-inch- wide tape in data cartridges and cassettes, and tapes that look like ordinary music cassettes but are designed to store data instead of music. The amount of data on a tape is expressed in terms of density, which is the number of characters per inch (cpi) or bytes per inch (bpi) that can be stored on the tape.
The highest-capacity tape is the digital audio tape, or DAT, which uses a different method of recording data. Using a method called helical scan recording, DAT wraps around a rotating read/write head that spins vertically as it moves. This places the data in diagonal bands that run across the tape rather than down its length. This method produces high density and faster access to data.
Two reels are used, a supply reel and a take-up reel. The supply reel, which has the tape with data on it or on which data will be recorded, is the reel that is changed. The take-up reel always stays with the magnetic tape unit. Many cartridges and cassettes have the supply and take-up reels built into the same case.
Tape now has a limited role because disk has proved the superior storage medium. Disk data is quite reliable, especially within a sealed module. Furthermore, as we will see, disk data can be accessed directly, as opposed to data on tape, which can be accessed only by passing by all the data ahead of it on the tape. Consequently, the primary role of tape today is as an inexpensive backup medium. - Backup Systems
Although a hard disk is an extremely reliable device, a hard disk drive is subject to electromechanical failures that cause loss of data. Furthermore, data files, particularly those accessed by several users, are subject to errors introduced by users. There is also the possibility of errors introduced by software. With any method of data storage, a backup system a way of storing data in more than one place to protect it from damage and errors is vital. As we have already noted, magnetic tape is used primarily for backup purposes. For personal computer users, an easy and inexpensive way to back up a hard disk file is to simply copy it to a diskette whenever it is updated. But this is not practical for a system with many files or many users.
Personal computer users have the option of purchasing their own tape backup system, to be used on a regular basis for copying all data from hard disk to a high-capacity tape. Data thus saved can be restored to the hard disk later if needed. A key advantage of a tape backup system is that it can copy the entire hard disk in minutes, saving you the trouble of swapping diskettes in and out of the machine.
A rule of thumb among computer professionals is to estimate disk needs generously and then double that amount. But estimating future needs is rarely easy. Many users, therefore, make later adjustments like adding a removable hard disk cartridge to accommodate expanding storage needs. To quote many a computer user, "I just couldn't envision how I could use all that disk space. Now I can imagine even the extra disk filling up."
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