Memory (i)

MAGNETIC MEMORY

Floppy Disk, hard disk, and magnetic tape use similar technology for reading and writing data. The surface of these devices is coated with iron oxide which reacts to the magnetic field. The orientation of a magnetic field is used to represent data. To write data, the head is magnetized by passing current through a coil which generates the magnetic field in the iron as the head passes over the disk or tape. The head alters the polarity of an iron oxide particle to encode the bit. To read data from a magnetic surface the head passes over the disk or tape while no current is flowing through the electromagnetic head. Since the storage medium has the magnetic field , the storage medium charges the magnet in the head which causes a small current flow through the head in one direction of flow since the storage medium passes by the head and data is sent from READ/WRITE head into the memory.

_{^{Source:commons.wikimedia.org Fig: Magnetic Memory}}

Before the computer can use a magnetic disk’s surface must be magnetically mapped, so that the computer can go directly to a specific point onto without searching through data. The process of mapping a disk or the process of creating new tracks and sectors on a disk is called formatting or initializing.

MAGNETIC DISK

The Magnetic disk is the most common secondary storage devices in a computer system. Generally, it is a random access device. It contains circular disks, which is made of metal or a thin plastic coated with iron oxide on both sides. It allows the recording of data in the form of magnetized spots. The data are stored on the disks in a number of a concentric circle called tracks. Tracks are divided into sectors. All the tracks have the same number of sectors. The most common magnetic disks are floppy disk and hard disk.

_{^{Source:museum.ipsj.or.jp Fig: Magnetic Disk}}

Advantages of Magnetic Disk

• Magnetic disk supports direct access of data and making them more suitable for a wider range of applications.
• Due to direct access property, a magnetic disk is often used by multiple users simultaneously as a shared devices. For example, Winchester disk pack are often used as on-line secondary storage devices storing data from multiple users of a computer system.
• The magnetic disk is suitable for both on-line and off-line storage of data. For example, Winchester disk and disk pack are often used as on-line secondary storage devices whereas floppy disk and zip disk are used as off-line secondary storage.
• An additional cost benefit of the magnetic disk can be erased and reused many times.
• The magnetic disk is less vulnerable data corruption due to careless handling or unfavorable temperature and humidity conditions.

Limitations of Magnetic Disk

• Although magnetic disk can be used for both types of applications (random as well as sequential data processing applications), use of magnetic disk is less efficient for sequential applications.
• It is more difficult to maintain the security of information stored on the magnetic disk.
• It is less portable.
• It must be stored in a dust-free environment.

HARD DISK

The Hard disk is a magnetic disk that is used as a secondary memory for mass storage of data permanently. It stores programs, data operating system, compilers, assemblers, the application program and database. The term hard disk is used to distinguish it from a soft disk, or floppy disk. Hard disk holds more data and is faster than floppy disk according to data access rate.

A single hard disk usually consists of several platters. Each platter requires two reads/write heads one for each side. All the read/ write head are attached to a single access arm so that they can move independently. It is generally made up of aluminum and is coated on both sides with the special iron oxide to store data in the form of magnetized dots. The standard side of the disk is an a3.5-inch diameter. The platter is mounted on a stack on a spindle driven but the motor is connected to the spindle. It rotates at a very high speed in between 3066 rpm to 15000 rpm (revolution per minute) or more. Its track and bit densities are much higher than that of the floppy disk. A hard disk may have more than 10000 tracks per surface and bit density 15000 bits per inch of track. The average access time is about 15ms.

FLOPPY DISK

A floppy disk is a removable round flat piece of Mylar plastic that stores data and programs as magnetized spots. More specially, data is stored as electromagnetic charges, these electromagnetic charges follow standard patterns of data representation. The disk is contained in a square paper envelope or plastic case to protect it from being touched by human hands. The diskette is sometimes called floppy because the disk within the envelope or case is flexible, not rigid. It is a removable disk and used for small-scale data transfer from one computer to another. Its access time is about 150-250ms.

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_{^{Source:pixabay.com Fig: Floppy Disk}}

Difference between hard disk and floppy disk

A floppy disk is a small plastic storage device typically capable of no more than 1.44 Megabytes of storage. It is also now rather outdated, thanks to advancements in today's technologies. Generally, floppy disks are capable of reaching 250-300 bit/s read/write speeds.A hard disk is a mechanical drive with multiple disks capable of storing 8Terabytes if not more worth of data. Disk drives are capable of reaching read-write speeds of 300MB/s.Both work under the same principle, a disk has various sectors where information is saved so it can be accessed at a later date. The difference is that a floppy relies on a floppy disk reader to spin its disk, whereas a hard drive is fully enclosed container that has it’s reader enclosed as well. A floppy disk is a single plastic film disk, whereas a hard disk can have multiple metal disks. Floppy disks were made for transportation, whereas hard disks were not made to be as portable, though smaller 2.5″ laptop-sized hard drives can be put into an enclosure and transported.

MAGNETIC TAP

magnetic tap is a medium formagnetic recording, made of a thin, magnetizable coating on a long, narrow strip ofplastic film. It is a plastic tape coated with a substance that can be magnetized. Data are represented by the magnetized or non-magnetized spots. Today, it is used mainly to provide backup or duplicates storage. It is a sequential access type of storage devices which are suitable for backup storage. The high-speed magnetic tape is employed for hard disk backup. It is also used for transporting data from one computer to another. Its main drawback is to store data sequentially. With an addition to this, it needs a complex mechanism to keep its speed constant and to start and stop the tape without breaking it.

A magnetic tape is made of Mylar plastic coated with magnetic material only on one side of the tape. The magnetic tape is available in ½ inch, ¼ inch, 8mm and 3mm wide. At present, it is available in the cassette form, which is called cartridge tape. The storing capacity can be 500 MB, 2GB, 4GB or higher. Data density is 120 Kbits per inch. Speed is 200 inch per second and data transfer rate is 240 KB/sec.

MAGNETIC DRUM

The Magnetic drum is a magnetic data stored devices which were the early form of computer’s memory. It was widely used in the 1950s-1960s. It was invented by Gustav Tauschek of Australia in 1932.

_{^{Source:www.reddit.com Fig: Magnetic Drum}}

For many machines, it was used as the main working memory with data programs being loaded onto or off the drum using media such as paper tape or punched cards. It was so commonly used for the main working memory that, those computers were often referred to as drum machines. It was later replaced as the main working memory by magnetic core memory and a variety of other systems, which were faster as they had no moving parts. It lasted until semiconductor memory was developed.

The Magnetic drum is coated with magnetic oxide (iron oxide) on the drum. It is basically used to store a large amount of binary information. Each track in the drum is controlled by one read/ write head. As the drum rotates, the binary data can be available one track is talked as timing track through which reading and writing related operation are controlled.

FLASH MEMORY

_{^{Source:www.datarecoveryexpert.ca Fig: Flash Memory}}

Flash memory is used in small portable computers. Flash memory, or flash RAM cards, consists of circuitry on credit-card-size. cards that can be inserted into slot connecting to the motherboard. Unlike standard RAM chips, flash memory is nonvolatile. That is, it retains data even when the power is turned off. Flash memory can be used not only to simulate main memory but also to supplement or replace hard disk drives for permanent storage.

Bibliography

Shrestha, Prachandra Ram, et al. Computer Essentials. Kathmandu: Asmita's Publication, 2014

Dilli , S. P., Singh, k. E., Khadka , D., Bhatta, K. D., Baral, N., Saud, R. S., . . Dangi, R. (2015). Fundamental of Computer Science. Kathmandu: KEC publication and Distribution.