Computer storage Fundamentals

Data are processed and stored in a computer system through the presence or absence of electronic or magnetic signals in the computer's circuitry in the media it users. This is called a " two-state" or binary representation of data, since the computer and the media can exhibit only two possible states or conditions. This binary characteristic of computer circuitry and media is what makes the binary number system the basis for representing data in computers. Thus, for electronic circuits, the conducting (ON) state represents the number one, while the nonconducting (OFF) state represents the number zero. For magnetic media, the magnetic field of a

magnetized spot in one direction represents a one, while magnetism in the other direction represents a zero.

The smallest element of data is called a bit, or binary digit, which can have a value of either zero or one. The capacity of memory chips is usually expressed in terms of bits. A byte is a basic grouping of bits that the computer operates as a single unit. Typically, it consists of eight bits and represents one character of data in most computer coding schemes. Thus, the capacity of a computer's memory and secondary storage devices is usually expressed in terms of bytes. Computer codes such as ASCII (American Standard Code for Information Interchange) use various arrangements of bits to form bytes that represent the numbers zero through nine, the letters of the alphabet, and many other characters.

Storage capacities are frequently measured in kilobytes (KB), megabytes (MB), gigabytes (GB), or terabytes (TB).

The primary storage (main memory) of your computer consists of microelectronic semiconductor memory chips. Memory chips with capacities of about 300 gigabytes are now common. The capacity of a computer is the quantity of data that its memory unit can hold. Plug-in memory circuit boards containing 16 or 32 megabytes or more of memory chips can be added to your PC to increase its memory capacity. Specialized memory can help improve your computer's performance. Examples include external cache memory of 256 or 512 kilobytes to help your microprocessor work faster, or a video graphics accelerator card with 2 megabytes or more of RAM for faster and clearer video performance. Removable credit-card-size and smaller " flash memory" RAM cards can also provide several megabytes of erasable direct access storage for PDAs or handheld PCs.

Some of the major attractions of semiconductor memory are its small size, great speed, and temperature resistance. One major disadvantage of most semiconductor memory is its volatility. Uninterrupted electric power must be supplied or the contents of memory will be lost. Therefore, emergency transfer to other devices or standby electrical power (through battery packs or emergence generators) is required if data are to be saved. Another alternative is to permanently " burn in" the contents of semiconductor devices so that they cannot be erased by a loss of power.

Thus, there are two basic types of semiconductor memory: random access memory (RAM) and read only memory(ROM).

RAM: random access memory. This is a poor term, since all main memory has the property of random access.A better term would be read- write memory; the most likely reason why this term is not used is that its acronym, RWM, is unpronounceable.

Users work with the random-access-memory (RAM). RAM is a collec­tion of cells and we can access any cell at any given time, hence the name random access memory.

RAM is often referred to as main memory or primary storage. RAM usually occupies a number of chips connected together. It is used to store programs and data being used by these programs while the computer is on. When the computer is turned off any information is erased from the RAM. That is why RAM is often called volatile while ROM is a non-volatile type of memory.

As we have already mentioned this is a volatile memory. Because your most current computer work is stored in RAM, this is why you lose your most recent typing at the computer when your computer crashes or the electricity goes out. The need often arises, however, to store some programs permanently in main memory. The bootstrap program that starts up the computer when it is first turned on is a case in point. Some computers provide permanent storage for operating system and frequently used programs such as language processors and word processors, thus assuring that these will be ready for immediate use when the computer is turned on.

To facilitate quick access for processing, a typical modern computer has a main memory connected by a memory bus directly to the processor. Random access memory is much faster to read from and write to than the other kinds of storage devices in a computer such as the hard disk, and CD-ROM.

Read-only-memory (ROM) is a chip or a group of chips containing programs, tables of constants and instructions for the CPU. Only the CPU has access to this memory. For example, when we turn on the computer, the CPU gets its first instructions from the ROM. These instructions are burnt into the chip with the help of special equipment before the chip is installed inside the computer. Users do not have access to the ROM, so they cannot change these instructions.

Nonvolatile random access memory chips are used for permanent storage. ROM can be read but not erased or overwritten. Frequently used control instruction in the control unit and programs in primary storage (such as parts of the operating system) can be permanently burned in to the storage cells during manufacture and cannot be erased once they are programmed or loaded with specific instructions. This is sometimes called firmware, which is " hard" software. Variations include FROM (programmable read only memory) and EPROM (erasable programmable read only memory) that can be permanently or temporarily programmed after manufacture.

Computer chips are designed to serve several different kinds of memory needs. Though RAM and ROM are the most common forms of computer memory chips, there are other forms of memory:

EPROM is Erasable Programmable Read Only Memory. These chips allow the user to store data that will not be lost when electrical power to the computer stops, but through various techniques can be quickly erased to start over again. This is known as nonvolatile memory. These are sometimes referred to as Flash chips because they can be erased in a flash, yet hold data when the power is off.

MRAM is Magnetic Random Access Memory, which uses the magnetic spin of electrons instead of power refreshing to hold the electrical signal of one or zero. This is another form of nonvolatile memory which allows: immediate start up without booting software from a hard drive; greatly reduce battery usage; store more data in the same chip space; and provide faster access to data. When purchasing additional RAM for a computer, the computer manual that ships with your computer will tell you which kind you need. These chips might be titled SIMMs or DIMMs chips and new types will emerge.

Comments:

media (мн. від medium) носії (даних); матеріали, які зберігають дані в різноманітній формі або дозволяють передавати їх через себе

primary storage основна (первинна) пам'ять комп'ютера

plug-in memory убудована схемна плата

circuit board

resistance опір

video graphics графічний прискорювач

accelerator card

standby electrical power резервне (запасне) електропостачання bootstrap program програма (початкового) завантаження firmware умонтоване ПЗ

I. Match words in the text with their definitions.

1. Spin A. Representation of facts, concepts or instruc­tions in a formalized manner suitable for com­munication, interpretation or processing.

2. Bit B. To make easy or easier; help.

3. ROM C. It can be read but not erased or overwritten.

4. RAM D. Chips that allow the user to store, erase or

correct data.

5. EPROM E. The smallest element of data.

6. To facilitate F. Chips which are used for permanent storage.

7. Data G. Fast turning movement.

II. Identify whether the following statements are true or false.

Use the model:

1) Student A: For electronic circuits the conducting state represents the number zero, the nonconducting state represents the number one. — Student B: Yes, that is true.

2) S. A: One major advantage of most semiconductor memory is its volatility. — S.B: No, that is wrong! Volatility is one of the disadvantages because uninterrupted electric power must be supplied not to lose the contents of memory.

1. A byte is a basic grouping of bits that the computer operates as a single unit.

2. Removable credit-card-size and smaller “flash memory” RAM cards can also provide several megabytes of permanent direct access storage for PDAs or handheld PCs.

3. Emergency transfer to other devices or standby electrical power (through battery packs or emergence generators) is not necessary if data are to be saved.

4. Nonvolatile random access memory chips are used for permanent storage.

5. When we turn on the computer, the CPU gets its first instructions from the RAM.

6. ROM can be read, erased and overwritten.

7. Users have access to the ROM, so they can change the instructions.

III. Complete the following sentences using words and

expressions given below.

1. The capacity of memory chips is usually expressed in terms of....

2. The primary storage of a computer consists of microelectronic semi­conductor

3. Specialized memory can help improve your computer’s....

4. Some of the major attractions of semiconductor are its small size, great speed and

5. Emergency transfer to other devices or standby electrical power is required if data are to....

6. MRAM uses the magnetic spin of... instead of power refreshing to hold the electrical signal of one or....

temperature resistance bits zero memory chips electrons performance be saved

IV. Read and translate the text. Replace the Ukrainian words into their English variants given after the text.

We can think of main memory as the computer’s chalkboard in that it is used for тимчасового зберігання during the course of a computation. The following are some characteristics of main memory:

— Fast Access. The CPU can transfer a data item to or from main memory in less than a millionth of a second. A typical час доступу is one fifth of a millionth of a second.

— Прямий доступ. The access time for each item in main memory is the same, regardless of the order in which the items were stored or the місцезнаходження of each item in memory.

— Moderate Capacity. Typically, small computers have main memory capacities у межах from 128 MB to 4 GB. The trend in recent years has been for the cost of memory chips to drop and their capacity to increase, making ever larger main memory possible.

— Енергозалежність. The most commonly used type of main memory is volatile — the stored data is lost when the electrical power is removed. Normally, this means that everything stored in main memory is lost when the computer is turned off.

location temporary storage

ranging volatility

access time random access

V. Answer the following questions.

1. What makes the binary number system the basis for representing data in computers?

2. In what terms is the capacity of computer’s memory expressed?

3. What is the difference between the computer and any adding machine?

4. What does the primary storage of your computer consist of?

5. What are the advantages of semiconductor memory?

6. What is the major disadvantage of semiconductor memory?

7. What types of semiconductor memory can you define?

CACHE MEMORY

The cache memory was introduced as the first attempt at using memories of different speeds. The problem was to increase speed of instruction execution. The analysis of programs showed that in the majority of programs only few variables are used frequently, so only few memory cells are frequently accessed. The solution was to store this frequently used data in a special memory with the higher speed. This type of memory is called a cache memory. For example, on a typical 100-megahertz system board, it takes the CPU as much as 180 nanoseconds to obtain information from main memory, compared to just 45 nanoseconds from cache memory.

When the program is executed, some of the variables are held in the cache memory. The control unit interprets the instruction and looks for the necessary data in the cache memory first. If the data is there, it is processed; otherwise the control unit looks for the data in RAM. A more sophisticated cache memory keeps acount of number of accesses made to each variable. These counts are compared at regular intervals and the most frequently used variables are moved to the cache.

The cache memory system is managed by an ’intelligent’ circuit called the cache memory controller. When a cache memory controller retrieves an instruction from RAM, it also takes back the next several instructions to cache. This occurs because there is a high probability that the adjacent instruction will also be needed.

To speed up the computers even more, some CPUs (e.g. 80486 and Pentiums) have got built-in cache memory. In this case, there will be two cache memories: one — built-in and another — external (to the CPU). The built-in cache memory is also referred to as level 1 cache or LI or primary cache. It is located inside the CPU. External cache is referred to as level

2 cache or L2 or secondary cache and is located on the motherboard. The capacity of built-in cache is between 8 and 32K, depending on the microprocessor. The capacity of external cache ranges in size from 64K to 1M.

When CPU chips do not contain internal cache, the external cache, if present, would actually be the primary (LI) cache. Some secondary caches can be expanded, some cannot.

Some advertisements specify the type of the secondary cache installed as write-back or associative.

Write-back cache holds off writing to the hard disk until there is a lull in CPU activity. This gives an advantage in speed but there is a danger that data can be lost if the power fails.

Associative cache describes an alternative architecture to direct mapped memory, and is generally faster than direct mapped cache.

Cache Speed and RAM Speed

In Pentium systems, 20ns cache SRAM is generally used for 50-60MHz system boards (using the Pentium 75/90/100/120), and 15ns cache SRAM is normally utilized for 66MHz system boards (using the Pentium 100/133). Cache SRAM at speeds up to 8ns has recently become available, although rare and expensive.

Comments:

cache memory кеш-пам'ять; над швидкодіюча оперативна пам'ять,

яка слугує для буферизації команд і/або даних із повільної пам'яті перед їхньою обробкою про­цесором або операціями вводу-виводу

variable змінна; у програмуванні — поіменована ділянка пам'я-

ті, якій програмно присвоюються різні значення. controller контролер; мікросхема, плата чи блок, які здійснюють

керування внутрішнім або периферійним пристроєм та обмін даними між ним і комп'ютером

associative cache асоціативний кеш; тип кеш-памяті, розроблений,

щоб усунути проблему зіткнень, характерну для кеша з прямим записуванням

SRAM (Static RAM) статичний ОПЗ; дорожчий і швидший (із часом до­ступу до 5 не) динамічний ОПЗ

I. Match words from the text with their definitions.

1. To obtain A. Extremely unusual or uncommon.

2. Adjacent B. To increase in size, number, volume, degree,

etc.

3. To expand C. To become the owner of, esp. by means of effort

or planning.

4. Rare D. Very close; touching or almost touching.

II. Identify whether the following statements are true or false.

Use the model:

1) Student A: Some secondary caches can be expanded, some cannot. — Student B: Yes, that is true.

2) S. A: The control unit interprets the instruction and looks for the necessary data in RA M first. — S. B: No, that is false. It interprets the instruction and

looks for the necessary data in the cache memory first. If the data is there, it

is processed; otherwise the control unit looks for the data in RAM.

1. The cache memory was introduced to increase speed of instruction execution.

2. The analysis of programs showed that all the memory cells are frequently accessed.

3. After the program had been executed, some of the variables are held in the cache memory.

4. A cache memory controller retrieves instructions from ROM.

5. The capacity of built-in cache is between 8 and 32K, depending on the microprocessor.

6. Associative cache is generally slower than direct mapped cache.

III. Join the sentences with the proper variant in the right

column.

1. Write back cache holds off a) although rare and expensive.

writing to the hard disk

2. A more sophisticated cache b) ranges in size from 64K to 1M.

memory keeps

3. The built-in cache memory c) an alternative architecture to direct

mapped memory.

4. Associative cache describes d) until there is a lull in CPU activity.

5. Cache SRAM at speeds up e) a count of number of accesses made

to 8ns has recently become to each variable.

available

6. The capacity of external cache f) is located inside the CPU.

IV. Answer the following questions.

1. What was the reason of the cache memory introduction?

2. What are the functions of the control unit in case of cache memory?

3. What can a more sophisticated cache memory do?

4. What is called a cache memory controller?

5. What types of cache memory help to speed up the computer’s performance?

6. What cache holds off writing to the hard disk until there is a lull in CPU activity?

7. What cache describes an alternative architecture to direct mapped memory?

8. What cache SRAM is generally used for system boards?

Read the text.

In order to make room for the new entry on a cache miss, the cache generally has to evict one of the existing entries. The heuristic that it uses to choose the entry to evict is called the replacement policy. The fundamental problem with any replacement policy is that it must predict which existing cache entry is least likely to be used in the future. Predicting the future is difficult, especially for hardware caches which use simple rules amenable to implementation in circuitry, so there are a variety of replacement policies to choose from and no perfect way to decide among them. One popular replacement policy, LRU, replaces the least recently used entry.

When data is written to the cache, it must at some point be written to main memory as well. The timing of this write is controlled by what is known as the write policy. In a write-through cache, every write to the cache causes a write to main memory. Alternatively, in a write-back or copy-back cache, writes are not immediately mirrored to memory. Instead, the cache tracks which locations have been written over (these locations are marked dirty). The data in these locations is written back to main memory when that data is evicted from the cache. For this reason, a miss in a write­back cache will often require two memory accesses to service: one to read the new location from memory and the other to write the dirty location to memory. This is also called batch mode.

There are intermediate policies as well. The cache may be write- through, but the writes may be held in a store data queue temporarily, usually so that multiple stores can be processed together (which can reduce bus turnarounds and so improve bus utilization).

The data in main memory being cached may be changed by other entities, in which case the copy in the cache may become out-of-date or stale. Alternatively, when the CPU updates the data in the cache, copies of data in other caches will become stale. Communication protocols between the cache managers which keep the data consistent are known as cache coherenct protocols.

Find the English equivalents in the text.

Витісняти (дані із кеш-памяті) Методика заміщення

Кеш із прямим записуванням Кеш із зворотним записом

Змінені (дані) Застарілі (дані)

Протоколи синхронізації кеша

Алгоритм видалення елементів, які найдовше не використовувались

FLASH MEMORY DEVICES

A memory card, also known as a smart card or multimedia card, is one of the external storage media of a personal computer and multifunction terminal electronic equipment. A semiconductor memory card has an internalized non-volatile memory, which is accessed by a connected device, enabling the semiconductor memory card to be used as a recording medium. The most widely used solid-state memory devices include flash- memory chips configured on a small removable memory card, and are commonly referred to as flash memory cards. A flash memory is a type of EEPROM that can be erased and reprogrammed in blocks instead of one byte at a time.

Compared with other memory devices, flash memory devices have many advantages, which include their non-volatility, speed, ease of erasure and reprogramming, small physical size and related factors. A flash memory is a semiconductor memory showing low power consumption and maintaining stored information even when power is off. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Flash memories can program, read and erase data for multiple times, and data stored therein can be maintained even if the power applied thereto is off. Flash memory devices do not require refreshing and can store data indefinitely after the power is removed. Flash memory devices are applicable for multiple operations of data writing, reading and erasing. Flash memory devices have achieved a commercial success in an electronic industry because they are able to store data for a relatively long time even without a power supply. Flash memory has been widely used for high volume data storage in devices such as personal computers, personal digital assistants (PDAs), cellular phones, digital TV, digital camcorder, digital camera, and MP3 players.

Flash memory brings convenience to information industries for storage of data. The multimedia network technology has developed to the extent that digital contents such as video information are distributed via a communication network such as the Internet. For example, traditional film cameras have been widely replaced by digital cameras capable of recording images that may be directly downloaded to and stored on personal computers. The pictures recorded by digital cameras can easily be converted to common graphics file formats such as joint photographic experts group {JPEG), graphic interchange format (GIF) or bitmap (BMP), and sent as e-mail attachments or posted on web pages and online photo albums. Many digital cameras are also capable of capturing short video

clips in standard digital video formats, for example, moving picture experts group (MPEG), which may also be directly downloaded and stored on personal computers or notebook computers. Small flash-memory cards have been designed that have a connector that can plug into a specialized reader, such as for compact-flash, secure-digital, memory stick, or other standardized formats.

Flash memory cards are formed in card shapes in which memory chips are sealed in card type outer shells. Flash memory cards are designed in many standards such as an SD (Secure-Digital) card, a multimedia card, a Smartmedia, a compact flash (CF) card, and a memory stick. SD card is an extension of the earlier MultiMediaCard (MMC) format. SD cards are hot-swappable, allowing the user to easily insert and remove SD cards without rebooting or cycling power. Information equipment such as personal computers, PDAs, digital cameras, and cellular phones are generally provided with a card slot for mounting a flash memory card.

More recently, flash memory cards are being designed that contain a USB connector which is constructed that the flash memory card isinsertable and extractable in a direction parallel to a mounting wiring board through the slot port. USB flash-memory drives and devices have been developed to transport data from one host to another, replacing floppy disks. A USB- flash card can have a capacity of more than ten floppy disks in an area not much larger than a large postage stamp.

Comments:

smart card картка пам'яті, мікропроцесорна картка

refreshing відновлення змін зображення//заміна частини або

всього зображення на екрані за допомогою виводу нових графічних даних; регенерація//періодичне пере записування зображення на дисплеї для опе­ративного відображення

MP3 player технологія ущільнення звуку МРЗ//формат для збе­рігання та пересилання сильно ущільнених циф­рових музичних та аудіо файлів

JPEG (Joint Photo- об'єднана група експертів із машинного оброблення

graphic Experts Group) фотографічних зображень//робоча група зі стан­дартів цифрових відео- та мультиплікаційних зоб­ражень; алгоритм ущільнення нерухомого зобра­ження

GIF Graphics Interchange формат обміну графічними даними

Format BMP формат ВМР//стандартний формат растрових гра-

(від bitmap) фічних файлів

reboot перезавантаження; перезавантажити // перезапуск

комп'ютера з клавіатури

I. Match words from the text with their definitions.

1. Removable A. Something that can be put or placed in (some­

thing else).

2. Consumption B. Directed towards or concerning a particular per­

son or group.

3. Applicable C. The act of consuming.

4. Insertable D. Been able to be pulled or taken away.

5. Extractable E. Able to be taken away (from a place) taken to

another place.

II. Identify whether the following statements are true or false.

Use the model:

1) Student A: The semiconductor memory card is used as a recording medium. — Student B: Yes, it is true.

2) S. A: SD cards are built-in. — S. B: No, it is false. These cards are hot- swappable, allowing the user to easily insert and remove them.

1. A memory card is one of internal storage media of a personal computer and multifunction terminal electronic equipment.

2. A flash memory is a type of EPROM that can be erased and reprogrammed in blocks instead of one byte at a time.

3. A flash memory is a semiconductor memory maintaining stored information even when power is off.

4. Flash memory devices require regular refreshing.

5. Flash memory cards are designed only in one standard.

6. USB connector is constructed that the flash memory card is insertable and extractable to a mounting wiring board through the slot port.

III. Complete the following sentences using words and

expressions given below.

1. Flash memory devices have achieved a commercial success in an electronic industry because they are... to store data for a relatively long time.

2. Traditional film cameras have been widely... by digital cameras capable of recording images that may be downloaded to and stored on personal computers.

3. A semiconductor memory card has an internalized non-volatile memory, which is accessed by a connected device, enabling the semiconductor memory card to be used as a recording....

4. Many digital cameras are also capable of... short video clips in standard digital video formats.

5. The pictures recorded by digital cameras can easily be... to common graphics file formats.

6. SD cards are hot-swappable, allowing the user to easily insert and remove SD cards without... power.

cycling able medium converted replaced capturing

IV. Answer the following questions.

1. What is one of external storage media of a personal computer?

2. What enables the semiconductor memory card to be used as a recording medium?

3. What type of memory is flash memory?

4. What advantages do flash memory devices have comparing with other memory devices?

5. What conveniences does flash memory bring to information industries?

6. What standards of flash memory cards design are available?

7. What is a USB connector?

Read the text.

The flash memory cards have the advantages of small size, large capacities, quick access and easy to install and carry. The use of non­volatile memory systems such as flash memory storage systems is increasing due to the compact physical size of such memory systems, and the ability for non-volatile memory to be repetitively reprogrammed.

A major advantage of flash over ROM is that the memory contents of flash may be changed after the device is manufactured. The ability to repetitively reprogram non-volatile memory included in flash memory storage systems enables flash memory storage systems to be used and reused. Flash memory has found wide acceptance in many types of computers, including desktop computers, mobile phones and hand held computers. Flash memory is also widely used in digital cameras and portable digital music players.

The flash memory is divided into a NAND-type flash memory having a large memory capacity and a NOR-type flash memory having a high

processing speed. The NAND-type flash memory, which attains high integration and has a replaceable hand disk, is mostly used to store high integration voices or images. Flash memory storage systems may include flash memory cards and flash memory chip sets. Flash memory chip sets generally include flash memory components and a controller component. A flash memory chip set may be arranged to be assembled into an embedded system. In the case of an MP3 player, a number of digitized audio files in the form of MP3 files are stored in a storage medium included in or coupled to the player in a fashion deemed most suitable to that user.

Find the English equivalents in the text.

кишеньковий ПК носій даних

висока швидкість обробки убудована система

енергозалежна пам’ять переваги

DEFINITION OF A HARD DISK

A hard disk drive (HDD) is a digital data storage device that writes and reads data via magnetization changes of a magnetic storage disk along concentric tracks. Computer hard disk drives have become a standard data storage component of modern computer systems and are used in other modern consumer electronics as well. Their proliferation can be directly attributed to their low cost, high storage capacity and high reliability, in addition to wide availability, low power consumption, high data transfer speeds and decreasing physical size.

A hard disk drive contains rigid, disk-shaped platters usually constructed of aluminium or glass. Unlike floppy disks, the platters cannot bend or flex — hence the term hard disk. In most hard disk drives, the platters cannot be removed; for that reason, IBM calls them fixed disk drives. Hard disk drives used to be called Winchester drives. This term dates back to the 1960s, when IBM developed a high-speed hard disk drive that had 30M of fixed-platter storage and 30M of removable-platter storage. The drive had platters that spun at high speeds and heads that floated over the platters while they spun in a sealed environment. That drive, the 30-30 drive, soon received the nickname Winchester after the famous Winchester 30-30 rifle. After that time, drives that used a high-speed spinning platter with a floating head also became known as Winchester drives. The term

has no technical or scientific meaning; it is a slang term, and is considered synonymous with hard disk.

A hard drive uses spinning disks with heads that move over the disks and store data in tracks and sectors. The mechanism for reading and writing data on a disk is called an access arm. A track is a concentric ring of information, which is divided into individual sectors that normally store 512 bytes each. The nearer to the circumference of the magnetic disk, the higher the track recording density can be made.

Hard disks usually have multiple platters, each with two sides on which data can be stored. Most drives have at least two or three platters, resulting in four or six sides, and some drives have up to 11 or more platters. The identically positioned tracks on each side of every platter together make up a cylinder. A hard disk drive normally has one head per platter side, and all the heads are mounted on a common carrier device, or rack. The heads move in and out across the disk in unison; they cannot move independently because they are mounted on the same rack.

The heads in most hard disks do not and should not touch the platters during normal operation. When the heads are powered off, however, they land on the platters as they stop spinning. While the drive is on, a very thin cushion of air keeps each head suspended a short distance above or below the platter. If the air cushion is disturbed by a particle of dust or a shock, the head may come into contact with the platter spinning at full speed. When contact with the spinning platters is forceful enough to do damage, the event is called a head crash. The result of a head crash may be anything from a few lost bytes of data to a totally trashed drive. Most drives have special lubricants on the platters and hardened surfaces that can withstand the daily “takeoffs and landings” as well as more severe abuse.

Magnetic drives, such as hard disk drives, operate by using electromagnetism. This basic principle of physics states that as electric current flows through a conductor, a magnetic field is generated around the conductor. This magnetic field then can influence magnetic material in the field. When the direction of the flow of electric current is reversed, the magnetic field’s polarity also is reversed. An electric motor uses electromagnetism to exert pushing and pulling forces on magnets attached to a rotating shaft.

Another effect of electromagnetism is that if a conductor is passed through a changing magnetic field, an electrical current is generated. As the polarity of the magnetic field changes, so does the direction of the electric current flow. For example, a type of electrical generator used in automobiles, called an alternator, operates by rotating electromagnets past

coils of wire conductors in which large amounts of electrical current can be induced. The two-way operation of electromagnetism makes it possible to record data on a disk and read that data back later.

The overwhelming majority of the data necessary to operate the hardware and software of the computer is stored on the hard disk drive. The larger the capacity of the hard disk drive, the more data the computer can store. The storage capacity of hard drive is measured in megabytes, gigabytes, or terabytes.

The flow of data to and from the hard disk is managed by a collection of electronic circuits called the hard disk controller. The controller can be built into the disk drive or it can be a separate board in an expansion slot. For personal computers two types of controllers are common, IDE and SCSI. Integrated drive electronics (IDE) controllers can operate one or two hard disk drives. Small computer system interface, or SCSI (pronounced scuzzy), controllers can support multiple disk drives or mix of other SCSI- compatible devices.

Comments:

proliferation швидке збільшення

access доступ

circumference коло

head crash аварія голівок//непередбачуване зіткнення рухомих

магнітних голівок із поверхнею жорсткого диска, що призводить до псування даних або відмови на­копичувана

to exert здійснювати

alternator альтернатор, генератор змінного струму