Architecture (Несколько текстов для зачёта), страница 18

Both types of players spin the discs to access data as they read the data with a laser device. CD-ROM players only spin the disc to access a sector of data and copy it into main memory for use by the computer, while audio CDs spin throughout the time that the audio recording is read out, directly feeding the signal to an audio amplifier.

The most important distinguishing feature among CD-ROM players is their speed, which indicates how fast they can read data from the disc. A single-speed CD-ROM player reads 150,000 bytes of data per second. Double-speed (2X), triple-speed (3X), quadruple-speed (4X), six-times speed (6X), and eight-times speed (8x) CD-ROM players are also widely available.

Other important characteristics of CD-ROM players are seek time and data transfer rate. The seek time (also called the access time) measures how long it takes for the laser to access a particular segment of data. A typical CD-ROM takes about a third of a second to access data, as compared to a typical hard drive, which takes about 10 milliseconds (thousandths of a second) to access data. The data transfer rate measures how quickly data is transferred from the disk media to the computer’s main memory.

The computer industry also manufactures blank, writeable compact discs, called WORMs (Write-Once-Read-Many), that users can record data onto for one-time, permanent storage using personal WORM recording units. Another technology that allows the user to write to a compact disc is the magneto-optical (MO) disk, which combines magnetic and optical data storage. Users can record, erase, and save data to these disks any number of times using special MO drives.

Information Storage and Retrieval

I

INTRODUCTION

Information Storage and Retrieval, in computer science, term used to describe the organization, storage, location, and retrieval of encoded information in computer systems. Important factors in storing and retrieving information are the type of media, or storage device, used to store information; the media’s storage capacity; the speed of access and information transfer to and from the storage media; the number of times new information can be written to the media; and how the media interacts with the computer.

II

TYPES OF INFORMATION STORAGE

Information storage can be classified as being either permanent, semipermanent, or temporary. Information can also be classified as having been stored to or retrieved from primary or secondary memory. Primary memory, also known as main memory, is the computer’s main random access memory (RAM). All information that is processed by the computer must first pass through main memory. Secondary memory is any form of memory other than the main computer memory, including the hard disk, floppy disks, CD-ROMs, and magnetic tape.

A

Permanent Storage

Information is stored permanently on storage media that is written to only once, such as ROM (read-only memory) chips and CD-ROMs (compact disc read-only memory). Permanent storage media is used for archiving information or, in the case of ROM chips, for storing basic information that the computer needs to function that cannot be overwritten.

B

Semipermanent Storage

Semipermanent information storage is also often used for archival purposes, but the media used can be overwritten. A common example of a semipermanent storage material is a floppy disk. The magnetic material that serves as the storage media in a floppy disk can be written to many times when a removable tab is in place. Once this tab is removed, the disk is protected from further overwriting; the write-protection may be bypassed by placing a piece of tape over the hole left by the removal of the tab.

C

Temporary Storage

Temporary information storage is used as intermediate storage between permanent or semipermanent storage and a computer’s central processing unit (CPU). Temporary storage is in the form of memory chips called RAM. Information is stored in RAM while it is being used by the CPU; it is then returned to a more permanent form of memory. RAM chips are known as volatile memory because they must have power supplied to them continuously or they lose the contents of their memory.

III

HOW INFORMATION STORAGE AND RETRIEVAL WORKS

All information processed by a computer can be expressed as numbers in binary notation. These binary numbers are strings of bits, or 1s and 0s, that are grouped together in sequences of eight called bytes. The two values a bit can have are physically stored in a storage media in a variety of ways. For instance, they can be represented by the presence or absence of electrical charge on a pair of closely spaced conductors called a capacitor, by the direction of magnetization on the surface of a magnetic material, or by the presence or absence of a hole in a thin material, such as a plastic disk.

In computer systems, the CPU processes information and maintains it temporarily in RAM in the form of strings of bits called files. When this temporary information needs to be stored permanently or semipermanently, the CPU finds unused space on the storage media—generally a hard drive, floppy disk, or magnetic tape. It then instructs a device capable of making changes to the media (called a read/write head) to start transmitting bits. The read/write head and its associated electronics convert each bit to the equivalent physical value on the media. The recording mechanism then moves to the next location on the media capable of storing a bit.

When the CPU needs to access some piece of stored information, the process is reversed. The CPU determines where on the physical media the appropriate file is stored, directs the read/write head to position itself at that location on the media, and then directs it to read the information stored there.

A

Storage Media

Information is stored on many different types of media, the most common being floppy disks, hard drives, CD-ROMs, and magnetic tape. Floppy disks are most often used to store material that is not accessed frequently or to back up files contained on a computer’s hard drive. Hard drives are most often used to store information, such as application programs, that is frequently accessed by the user. CD-ROMs are a type of storage medium that is capable of being written to only once but read many times, hence the name read-only memory. CD-ROMs are useful for storing a large amount of information that does not need to be changed or updated by the user. They are usually purchased with information already written to them, although special types of CD drives, called WORM (write once, read many) drives, allow the user to write data to a blank CD once, after which it is a CD-ROM. Magnetic tape is most commonly used in situations where large databases of information need to be stored.

A1

Floppy Disks

A floppy disk is a thin piece of magnetizable material inside a protective envelope. The size of the disk is usually given as the diameter of the magnetic media, with the two most common sizes being 5.25 inch and 3.5 inch. Although both sizes are called floppies, the name actually comes from the 5.25-inch size, in which both the envelope and the disk itself are thin enough to bend easily.

Both sizes of floppies are removable disks—that is, they must be inserted into a compatible disk drive in order to be read from or written to. This drive is usually internal to, or part of, a computer. Inside the drive, a motor spins the disk inside its envelope and a read/write head moves over the surface of the disk on the end of an arm called an actuator. The head in the floppy drive is much like that in a tape recorder. To record information, the head magnetizes a small area on the surface of the disk in a certain direction. To read information stored on the disk, the disk controller—circuitry that controls the disk drive—directs the actuator to the location of the information on the disk. The head then senses the direction of magnetization of a small area on the disk and translates this into a signal that gets stored in RAM until the CPU retrieves it. Most floppy drives today are double sided, with one head on each side of the disk. This doubles the storage capacity of the disk, allowing it to be written to on either side.

Information is organized on the disk by dividing the disk into tracks and sectors. Tracks are concentric circular regions on the surface of the disk; sectors are pie-shaped wedges that intersect each of the tracks, further dividing them. Before a floppy disk can be used, the computer must format it by placing special information on the disk that enables the computer to find each track and sector.

A2

Hard Drives

Hard drives consist of rigid circular platters of magnetizable material sealed in a metal box with associated read/write heads. They are usually internal to a computer. Most hard drives have multiple platters stacked on top of one another, each with its own read/write heads. The media in a hard drive is generally not removable from the drive assembly, although external hard drives do exist with removable hard disks. The read/write heads in a hard drive are precisely aligned with the surfaces of the hard disks, allowing thousands of tracks and dozens of sectors per track. The combination of more heads and more tracks allows hard drives to store more data and to transfer data at a higher rate than floppy disks.

Accessing information on a hard disk involves moving the heads to the right track and then waiting for the correct sector to revolve underneath the heads. Seek time is the average time required to move the heads from one track to some other desired track on the disk. The time needed to move from one track to a neighboring track is often in the 1 millisecond (one-thousandth of a second) range, and the average seek time to reach arbitrary tracks anywhere on the disk is in the 6 to 15 millisecond range. Rotational latency is the average time required for the correct sector to come under the heads once they are positioned on the correct track. This time depends on how fast the disk is revolving. Today, many drives run at 60 to 120 revolutions per second or faster, yielding average rotational latencies of a few milliseconds.

If a file requires more than one sector for storage, the positions of the sectors on the individual tracks can greatly affect the average access time. Typically, it takes the disk controller a small amount of time to finish reading a sector. If the next sector to be read is the neighboring sector on the track, the electronics may not have enough time to get ready to read it before it rotates under the read/write head. If this is the case, the drive must wait until the sector comes all the way around again. This access time can be reduced by interleaving, or alternatively placing, the sectors on the tracks so that sequential sectors for the same file are separated from each other by one or two sectors. When information is distributed optimally, the device controller is ready to start reading just as the appropriate sector comes under the read-write heads.

After many files have been written to and erased from a disk, fragmentation can occur. Fragmentation happens when pieces of single files are inefficiently distributed in many locations on a disk. The result is an increase in the average file access time. This problem can be fixed by running a defragmentation program, which goes through the drive track by track and rearranges the sectors for each file so that they can be accessed more quickly.

Unlike floppy drives, in which the read/write heads actually touch the surface of the material, the heads in most hard disks float slightly off the surface. When the heads accidentally touch the media, either because the drive is dropped or bumped hard or because of an electrical malfunction, the surface becomes scratched. Any data stored where the head has touched the disk is lost. This is called a head crash. To help reduce the possibility of a head crash, most disk controllers park the heads over an unused track on the disk when the drive is not being used by the CPU.

A3

CD-ROMs

While magnetic material is the dominant media for read/write information storage (files that are read from and written to frequently), other media have become popular for more permanent storage applications. One of the most common alternative information storage mediums is the CD-ROM. CD-ROMs are plastic disks on which individual bits are stored as pits burned onto the surface of the disk by high-powered lasers. The surface of the disk is then covered with a layer of reflecting material such as aluminum. The computer uses a CD-ROM drive to access information on the CD-ROM. The drive may be external to, or part of, the computer. A light-sensitive instrument in the drive reads the disk by watching the amount of light reflected back from a smaller laser positioned over the spinning disk. Such disks can hold large amounts of information, but can only be written to once. The drives capable of writing to CD-ROMs are called write once, read many (WORM) drives. Due to their inexpensive production costs, CD-ROMs are widely used today for storing music, video, and application programs.

A4

Magnetic Tape

Magnetic tape has served as a very efficient and reliable information storage media since the early 1950s. Most magnetic tape is made of mylar, a type of strong plastic, into which metallic particles have been embedded. A read/write head identical to those used for audio tape reads and writes binary information to the tape. Reel-to-reel magnetic tape is commonly used to store information for large mainframe or supercomputers. High-density cassette tapes, resembling audio cassette tapes, are used to store information for personal computers and mainframes.

Magnetic tape storage has the advantage of being able to hold enormous amounts of data; for this reason it is used to store information on the largest computer systems. However, magnetic tape has two major shortcomings: It has a very slow data access time when compared to other forms of storage media, and access to information on magnetic tape is sequential. In sequential data storage, data are stored with the first bit at the beginning of the tape and the last bit at the end of the tape, in a linear fashion. To access a random bit of information, the tape drive has to forward or reverse through the tape until it finds the location of the bit. The bits closest to the location of the read/write head can be accessed relatively quickly, but bits far away may take considerable time to access. RAM, on the other hand, is random access, meaning that it can locate any one bit as easily as any other.

A5

Other Types of Storage Media

Variations in hard and floppy disk drive technology are used in read-mostly drives, in which the same drive media may be written to multiple times, although at much slower rates than data can be read. In magneto-optical (MO) drives, a strong laser heats up and re-orients metallic crystals in the surface of the MO disk, effectively erasing any information stored on the disk. To write to the MO disk, an electromagnetic head similar to that in a floppy drive polarizes, or orients, the magnetic crystals in one of two directions while the laser is on, thus storing information in a binary form. To read the disk, a light-sensitive instrument reads the light from a separate, lower-power laser that reflects light from the crystals. The crystals polarize the reflected light in one of two directions depending on which way they point.