Memory And Storage Devices: Magnetic Memory

Magnetic Memory

We have seen various types of semiconductor RAMs. These high-speed semiconductor storage devices [i.e., RAMs] are expensive. So we need some inexpensive media for storage. Magnetic memory is a good alternative for this purpose. Also the semiconductor memory has the following limitations:

Limited Capacity: Semiconductor (primary) memory of today’s computer is not sufficient, since most of the data processing organization requires a large volume of data.

Volatile Memory: Semiconductor memory is volatile in nature. But nowadays, the computer system needs to store data on a permanent basis.

Thus, there is a need for an additional memory, which should be inexpensive, non-volatile in nature, and having large capacity. Magnetic material is inexpensive and quite long lasting material, so it is an ideal choice for us. Magnetic memory is a permanent non-­volatile memory.

Any Modern Computer Uses The Following Two Types Of Magnetic Memory

1. 1. Hard Disks.
   2. Floppy Disks.

Magnetic Disks are the most common form of secondary storage because they provide fast access and high storage capacities at a reasonable cost.

Storage Mechanisms: Magnetic Disk drives contain metal disks that are coated on both sides with an iron oxide recording material. Several disks are mounted together on a vertical shaft which typically rotates the disks at speeds of 3600 to 7600 revolutions per minute [rpm].

Electromagnetic read/write heads are positioned by access arms between the slightly separated disks to read and write data on concentric, circular tracks. Data are recorded on tracks in the form of tiny magnetized spots to form the binary digits of common computer codes. Thousands of bytes can be recorded on each track, and there are several hundred data tracks on each disk surface, which provides billions of storage positions for your software and data.

There are basically two types of magnetic disk arrangements, one having a removable disk cartridge and the other having a fixed disk unit. Removable disk devices are popular because they are transportable and can be used as backup copies of your data.

Data Organizations: A magnetic disk is a surface device, which stores data on its surface. Its surface is divided into circular concentric tracks. The number of tracks on a disk range up to 800. Each track is divided into sectors [normally 10-­100]. These sectors should be either fixed or variable length sectors. The division of track into equal sized blocks or pages is set by the operating system during disk formatting. The number of bytes stored in each sector is kept the same. Therefore, all tracks store the same amount of data, typically ranging from 4­-50k bytes.

The number varies but there [Often 200 or more] range up to 800 is often 8 or more sectors per track. Magnetic disks are semi­-random devices. A track on a disk is selected in a random fashion, but data is written to or read from a sector in a serial fashion.

 

• Hard­-Disk Drives (HDD)

1- Hard Disks are on­line storage devices. The term online means that the device [Hard­-Disk] is permanently connected to the computer system and when the computer is on, the device [Hard ­Disk] is available to store information or to give information.

2- HDD stores programs, data, operating system, compiler, assemblers, application programs, etc.

3- HDD contains magnetic disks, access arms, and read/write heads into a sealed, air filtered enclosure. This technique is known as the Winchester Technique.

4- Winchester Disk is another name for “Hard Disk Drive”. There are two stories behind the name Winchester Disks; one is that the disk was developed at IBM’s facility at Winchester, New York State; that had 30MB of fixed storage and 30MB of removable storage; the other is that the first model number was given as 3030, which is also the model number of the well­-known Winchester Rifle popular in the Wild West. Although modern disk drives are faster and hold more data, the basic technology is the same, so Winchester has become synonymous with hard.

5- Thus, the Winchester disk is a sealed “hard disk” having rotation speed typically 7200 rpm. Disk is having 5000 to 10000 concentric tracks per centimeter and about 100000 bits per centimeter around the circumference.

6- The read/write head reads data from the disk and writes data to the disk. A disk is mounted [or stacked] on the disk drive, which has the motor that rotates it. Hard­-Disks together with read/write heads, access mechanisms, and driving motors to constitute a unit called Hard-­Disk­ Drive [HDD] Unit. The whole unit of HDD is fixed.

7- Hard Disk is also known as a platter. It can not be removed or inserted into an HDD unit. Some disks have single platter e.g., floppy disk.

8- To increase the storage capacity several Hard Disks [platters] are mounted [stacked] vertically, normally at a distance of an inch. This is known as disk pack or multi­-platter configuration.

9- A set of corresponding tracks in all surfaces of a disk pack [i.e., the tracks with the same diameter on the various surfaces] is called a cylinder. The concept of cylinder is very important because data stored on the same cylinder can be retrieved much faster that the data were distributed among different cylinders.

There are several disk drives [C, D, F, etc.] in a computer, which are connected to a disk controller. The controller converts instructions received from the computer [software] to electrical signals to operate disks. The Disk controller accepts commands from the computer and positions the read/write head of the specified disk for reading or writing. For reading or writing operations on a disk pack, the computer must specify the drive number, cylinder number, surface number, and sector number. Drive number must be specified, because a controller normally controls more than one drive.

Access Time On Magnetic Disk: Magnetic disks are semi­-random devices. A track on a disk is selected in random fashion, but data is written to or read from a sector in serial fashion. In order to access information from a disk, the disk address of the desired data has to be specified. The disk address is specified in terms of track number, surface number, and the sector number. Information is always written from the beginning of a sector and can be read only from the tracks beginning of the track.

As soon as the read/write command is received by the disk controller, the read/write heads are first positioned onto the specified track number [or cylinder] by moving the arm assembly in the proper direction. The time required to position the read/write head over the proper track is called the seek time.

Seek Time [Ts]:  

1- The time required to move the read/write head on a specific [address] track.

2- Seek time varies depending on the position of the arm assembly when a read/write command is received.

3- Seek time will be maximum if the arm assembly is positioned on the outer most track and the track to be reached is the inner most track and it will be zero if the arm assembly is already on the desired track.

4- The average seeks time is thus specified for most systems which is generally between a few milliseconds to fractions of a second.

Note that seek time is associated only with the movable-­head system. For a fixed-­head system, it is always 0 because there is ahead for each track and no movement of head is required for accessing a particular track.

Once the heads are positioned on the desired track, the head on the specified surface is activated. Since the disk is continuously rotating, this head should wait for the desired data [specified sector] to come under this head. This rotational waiting time i.e., the time required to bring the needed data [i.e., starting position of the addressed sector] under the read/write head is called the latency time.

Latency Time or Search time: Time required to bring the needed data under the R/W head. Latency time is also a variable and depends on the following two things:

1. 1. Distance of the desired data from the initial position of the head on the specified track.
   2. The rotational speed of the disk.

The average seeks time is thus normally specified for most systems which is generally of the order of 10 to 15 milliseconds.

The total access time for a disk is equal to the seek time plus the latency time [Access Time = Seem Time + Latency Time]. The average access time for most disk systems is usually between 10 to 100 milliseconds.

 

• Floppy Disks

Floppy disks or magnetic diskettes consist of polyester film disks covered with an iron oxide compound. The disk is not a hard plate, rather it is a very thin piece of flexible polyester [plastic material] film, and hence it is called a Floppy Disk. A single disk is mounted and rotates freely inside a protective flexible cover or hard plastic jacket. We have two different sizes of floppy disks: [1] 3.5 Inches. [2] 5.25 Inches.

The 3.5 Inch Floppy Disk, having a capacity of 1.44 MB, is the most widely used version. A floppy disk rotates at 360 rpm. Its average access time is 150­-250 ms.

The storage capacity of a 5.25 Inch Floppy Disk is 1.2 MB. It uses 80 tracks per surface, 96 TPI [Tracks Per Inch], 15 sectors per track, rotating speed of 300 rpm, and store 512 bytes per sector. Both sides of the disk store information.

Floppy disks are now outdated, in place of it, now Universal Serial Bus (USB) Drive which are also known as Pen Drive are used. Pen Drives are having a larger capacity, which varies from 1 GB to 10 GB.

 

• Magnetic Tapes

Magnetic tape is sequential access type secondary storage device. It is used for backup in servers, workstations, and large computers. The main advantage of magnetic disks, these are cheaper and since these are removable from the drive, they provide unlimited storage capacity [20 GB to 150 GB].

The read/write heads of magnetic tape drives record data in the form of magnetized spots on the iron oxide coating of the plastic tape. Magnetic tape devices include tape reels and cartridges in mainframes and midrange systems, and small cassettes or cartridges for PCs.

The main drawback of magnetic tapes is that it store information sequentially. A file or a particular information stored on a magnetic tape cannot be accessed directly on a random basis as it is done in the case of Hard ­Disks or Floppy Disks. These devices are slower, but due to low cost, they are still widely used for massive data warehouse and other business storage requirements.

 

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