INPUT, OUTPUT AND STORAGE

MAGNETIC DISC

 

A magnetic hard disc unit consists of rotating metal disc platters on which data is recorded. Several of these platters, each of about 5 inches to 14 inches in diameter, are mounted on a vertical shaft forming a disc pack. Both surfaces of these platters in a pack are used for recording except for the top and bottom platters. The top and bottom platters have data recorded only on their inner surfaces.

 

An assembly of access arms moves in and out between the disc platter for purpose of data recording and retrieval. Read/write heads attached to the access arms are used for the reading and writing of data on disc. The read/write head do not actually touch the surface of the discs. There is a thin layer of air between the head and the disc surface that allows the read/write head to float less than a hair's breadth from the disc surface. This is called the flying head principle.

 

There are two types of disc unit. One is the fixed disc unit which the disc is permanently mounted on the disc unit and non-removable. The second type is the exchangeable disc unit, sometimes also called removable disc unit which are portable and replaceable.

 

Data Recording/Retrieval on Magnetic Disc

 

Data is recorded on concentric rings called tracks which are found on the surface on the disk platter. Each track is subdivided into pie shape sectors. A disc platter may also be divided into blocks, where each block may be equal to one or more sectors. To prepare a disc pack for the storage of data, the tracks and sectors will have to be labeled with the proper disc addresses.

The size of the sector may be decided by the manufacturer at hardware level by specifying the location of each sector with slots around the disc. This is called hard sectoring. On the other hand, the size of the sector that is determined by software formatting is called soft sectoring.

 

Cylinder

 

In a disc pack, all the tracks that are in vertical alignment is called a cylinder. Hence a cylinder is the column of equal radial tracks from all the different recording surfaces and is the total amount of storage accessible by all the read/write head on a disk pack without needing physical movement.

 

This is very important concept as the data records in a file should be stored in the same cylinder whenever possible in order to minimize the amount of head movement during access. If the file of records is too large to be accommodated in the same cylinder, provision must be made for the excess records to be stored in the adjacent cylinder. Sometimes a cylinder is also termed as seek area.

 

Access Times And Response Times Concepts

 

An important measure of the speed and efficiency of a disk drive is the access time and response time. The two times are measured as follows:-

 

          Access Time = Seek Time + Rotational Delay

 

Seek time is the time taken for the read/write head to position itself on the track where the desired record is found. Rotational delay or latency is the time taken for the desired record to be positioned under the read/write head.

 

          Response Time = Access Time + Data Transfer Time

 

In simple terms, it is the total time taken from the moment an input or output command is issued to the time when it is successfully completed.

 

In order to speed up response times on disc, some disc packs have one read/write head for every track. These types of disk units called fixed head discs. The conventional disk platter are called moving head disk. In fixed head discs, there is no physical movement during access, hence seek time is eliminated.

HARD DISC DRIVES

 

Hard disks provide larger and faster secondary storage capabilities than diskettes. Hard disks consist of one or more rigid platters coated with an oxide material that allows data to be magnetically recorded on the surface of the platters.

 

The platters are usually made of aluminum but some newer disks use glass or ceramic materials. Most hard disk permanently mounted inside the computer and are not removable like diskettes. On hard disks, the platters, the read/write heads, and the mechanism for moving the heads across the surface of the disk are enclosed in an airtight, sealed case. This help to ensure a clean environment for the disk.

 

Hard Disk Storage Capacity

 

Hard disks contain a spindle on which one or more disk platters re mounted. Each surface of a platter can be used to store data. Thus, if one platter is used in the drive, two surfaces are available for data. Naturally, the more platters the more data that can be stored on the drive. Like a diskette, hard disks must be formatted before they can be used to store data.

 

Access time for a hard disk is between ten to twenty milliseconds. This is significantly faster than for a diskette because of two reasons. First, a hard disk spins ten to twenty times faster than a diskette drive. Second, a hard disk is always spinning, whereas a diskette only starts spinning when a read or write command is received.

 

The storage capacity of hard drives is measured in megabytes or millions of bytes of storage. Common sizes for personal computers range from 100MB to 500MB  of storage and even larger sizes are available.

Storing Data On Hard Disk

Storing data on hard disks is similar to storing data on diskettes. Hard disks rotate at a high speed, usually 3,600 to 7,200 revolutions per minute. Hard disk read/write heads are attached to access arms that swing out over the disk surface to the correct track.

 

The read/write heads float on a cushion of air and do not actually touch the surface of the disk. The distance between the head and the surface varies from approximately ten to twenty millionths of an inch. The close tolerance leaves no room for any type of contamination. If some form of contamination is introduced or if the alignment of the read/write heads is altered by something accidentally jarring the computer, the disk head can collide with and damage the disk surface, causing a loss of data. This event is known as a head crash.

FLOPPY DISC DRIVES

A diskette consists of a circular piece of thin mylar plastic (the actual disk), which is coated with an oxide material similar to that used on recording tape. Because they were flexible, they were often called floppy disks, or floppies, terms that are still used. Today, diskettes are used as a principal secondary storage medium for personal computers. This type of storage is convenient, reliable and inexpensive. Diskettes are available in two different sizes, 3 1/2 inch and 5 1/4 inch. The size indicates the diameter (width) of the diskette.

 

On 3 1/2 diskette, the circular piece of plastic is enclosed in a rigid plastic shell and a piece of metal called the shutter covers the reading and writing areas. When the 3 ½ inch diskette is inserted into a disk drive, the drive slides the shutter to the side to expose a portion of both sides of the recording surface.

 

On a 5 ¼ inch diskette, the circular piece of plastic is enclosed in a flexible, square protective jacket. The jacket has an opening on each side so that a portion of the diskette’s surfaces are exposed for reading and writing.

Formatting : Preparing A Diskette For Use

 

Before a diskette can be used for secondary storage, it must be formatted. The formatting process prepares the diskette so it can store data and includes defining the tracks, cylinders, and sectors on the surface of a diskette.

 

A track is a narrow recording band forming a full circle around the diskette. A cylinder is defined as all tracks of the same number. For example, track 0 on side 1 of the diskette and track 0 on side 2 of the diskette would be called cylinder 0. A sector is a pie-shaped section of the diskette.

 

The number of tracks and sectors created on a diskette when it is formatted varies based on the capacity of the diskette, the capabilities of the diskette drive being used, and the specifications in the operating system software that does the formatting. 5 ¼ inch diskettes are formatted with 40 or 80 tracks and 9 or 15 sectors on the surface of the diskette. 3 ½ inch diskettes are usually formatted with 80 tracks and either 9, 18, or 36 sectors on each side.

 

To protect data from being accidentally erased during formatting or other writing operations, diskettes have ‘write-protection’ features. A 5 ¼ inch diskette has a write-protect notch. This notch is located on the side of the diskette. To prevent writing to a diskette, you cover this notch with a small piece of removable tape. Before writing data onto the diskette, the diskette drive checks the notch. If the notch is open, the drive will proceed to write on the diskette. If the notch is covered, the diskette drive will not write on the diskette. 

 

On the 3 ½ inch diskettes, the situation is reversed. Instead of a write-protect notch, there is a small window in the corner of the diskette. A piece of plastic in the window can be moved to open and close the window. If the write-protect window is closed, the drive can write on the diskette. If the window is open, the drive will not write on the diskette.

 

 

 

OPTICAL DISCS – CD and DVD

 

Enormous quantities of information are stored on optical disks by using a laser to burn microscopic holes on the surface of a hard plastic disk. A lower power laser reads the disk by reflecting light off the disk surface. The reflected light is converted into a series of bits that the computer can process.

 

A full-size, 14-inch optical disk can store 6.8 billion bytes of information. The smaller disks, just under five inches in diameter, can store more than 800 million bytes, or approximately 550 times the data that can be stored on a high-density 31/2 inch diskette. That is enough space to store approximately 400,000 pages of typed data.

 

The smaller optical disk is called a CD-ROM, an acronym or compact disk read-only memory. They use the same laser technology used for the CD-ROM disks that are become popular for recorded music.

 

Most optical disks are pre-recorded and cannot be modified by the user. Optical disk devices that provide for one-time recording are called WORM devices, an acronym for write once, read many. Erasable optical disk drives are just starting to be used.

 

MAGNETIC TAPE DEVICES

 

The fundamental principal of how data is stored is one that is similar to the cassette tapes we use in daily life for the storage and recording of music. Data is stored on a thin film of ferric oxide on the tape in the form of magnetized spots.

 

A typical magnetic tape is generally 2,400 to 3,600 feet long, but other sizes also exist. Most tapes are half an inch wide and made of mylar or polyester and are wound on reels. The amount of data that can be stored on magnetic tape is enormous compared to punched cards or paper tape. Data that is punched on an 80 column card can be stored on as little as 1/10 inch of tape.

Tape density or recording density refers to the number of characters that can be recorded per unit length of tape, usually in inches. Standard recording density is usually 1600 bytes per inch (BPI), but higher recording densities are not uncommon. A 6250 BPI tape is commonly used. One inch of tape with a recording density of 1600 BPI can hold the equivalent of 20 punched cards. After tape file is processed and is no longer needed, it can be reused for storing of other data by erasing and writing over the old file.

 

A single magnetic tape unit can perform both input and output functions. All magnetic tape drives require 2 tape reels. The one containing the tape to be read or written is called the file reel, while the other is called the take up reel. The use of vacuum columns is to ensure that there is enough slack between both reels so that the tape will not be torn apart during high speed reading and writing.

 

Data Recording/Retrieval on Magnetic Tape

 

The read/write heads assembly consists of erase, write and read heads in that order. During a write operation, the tape is first erased, then the data is written. Immediately after write operation, a read operation is performed to ensure that the data corresponds to what was written. This is called read after write check.

 

Tape File Security

 

In order to prevent accidental data erasure from tape, several measures can be taken. The first measure is to have the tape reel prominently labeled. This is called external labeling. The second measure is to use internal labels. These labels are special information recorded on the tape itself and are thus not visible to the human eyes. During processing, the internal labels are checked and verified by the appropriate control software. Internal labels contained information such as name of the files maintained on a tape, their expiry dates, length of file, length of records etc.. The internal label consists of the header label found at the beginning of each file and a trailer label record found at the end of the file.

The third measure is to use a write permit ring which can be removed from the tape to prevent accidental overwriting of data. The ring is also sometimes called the file protect ring. Operators remember the function of the write permit ring by using the rhythm ‘no ring no write’.

 

Data security on tape is maintained by having generations of backup files. When a magnetic tape file is updated the data is output onto a new tape reel. The new tape reel then becomes the son file and the old tape is called the father tape. Thus, the most recent version of the file is called the son file, and the previous version, the father with the grandfather being the oldest of the 3 generations. If for some reasons the data on the son-file was erased or corrupted, the file can be re-generated from the father, and if that were not possible, the grandfather file can be used.

 

Sequential organization

 

For the records on file to be stored in an organized fashion, the records are usually stored in some kind of order. For instance, the employee payroll file might have the data records stored in ascending order of employee number, beginning from the very first employee to the last. Such a file ordered on a particular field, in this case employee number is called a sequential file. The field which was used as the basis of the ordering of the records in the file is called the record key, key field, or key for short.

 

The key is used as a means of identifying a record in a computer file. A record key is by definition a field within a record designated as the bases for ordering the file or as a means of identifying a specific record.

 

In order to access the records, records are read and up-dated one after another according to their key sequence beginning from the first record in the file to the last. This type of access is known as sequential access.