3.2.2 Magnetic Memory 

In the above section 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. Also semiconductor memory has the following limitations:

1)     Limited Capacity: Semiconductor (primary) memory of today‟s computers is not sufficient, since most of the data processing organizations deal with a large volume of data.

2)     Volatile Memory: Semiconductor memory is volatile in nature. But there is always a need to store data on a permanent basis. 

Thus there is a need of additional memory, that is inexpensive, non-volatile in nature and has large capacity. Magnetic material is inexpensive and long lasting, so it is an ideal choice for us. Magnetic memory is a permanent non-volatile, type of memory. Now-adays, we are not using floppy disk.

A modern computer uses the following two types of magnetic memory: 

(i)    Magnetic Disks: Hard disks and Floppy disks. 

(ii) Magnetic Tapes : Magnetic disks are the most common form of secondary storage because they provide fast access and high storage capacities at a reasonable cost.

Storage Mechanism: 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 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 Memory System 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 can 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. 


The numbers vary but there are often 200 or more ranging up to 800 is 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 serial fashion. 

Hard-Disk Drives (HDD)

  • Hard disks are on-line 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 retrieve information.  
  • HDD stores programs, data, operating system, compiler, assemblers, application programs etc. 
Storage Organization in HDD 
  • HDD contains magnetic disks, access arms and read/write heads into a sealed, air filtered enclosure. This technique is known as Winchester technique.
  • 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 disk.
  • Thus Winchester disk is a sealed “hard disk” having rotation speed typically 7200 rpm. A disk has 5000 to 10,000 concentric tracks per centimeter and about 100,000 bits per centimeter around circumference. Figure 3.12 illustrates a portion of Winchester disk.


  • 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. Harddisks together with read/write heads, access mechanism and driving motor constitute a unit called hard-disk-drive (HDD) unit. The whole unit is fixed. 
  • Hard disk is also known as platter. It can not be removed or inserted into a HDD unit. Some disks have a single platter e.g. floppy disk. 
  • 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. 
  • 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 (see Figure 3.13). Here the concept of cylinder is very important because data stored on the same cylinder can be retrieved much faster than if it were distributed among different cylinders. 

Relationship among Capacity, density and speed

Suppose a HDD (or disk pack) having n plates, has: 
 m=2n= total number of recording surfaces
 t= tracks per surface
 p= Sectors per track
 s=bytes per sector,
 Ï€=3.14 then 
  • Storage capacity of the disk=(m*t*p*s) bytes
  • If d is the diameter of the disk, the density of the recording is: Density=(s*p)/(Ï€*d) byte/inches
Example 1:     A 2.5 inch diameter disk pack has 6 plates (12 recording surfaces), 256 sectors per track, 5250 tracks per surface, 512 byes per sector. Thus disk capacity = 12×5250×256×512=8,257,536,000 bytes = 7.69 GB and recording density= (512×256)/(3.14×2.5)=16697 bytes/inch.

Example 2:    What will be the storage capacity of a 2.5 inch diameter disk pack having 8 plates, 400 sectors per track, 2820 tracks per surface where 512 bytes of data can be stored per sector. 

Solution:     Total number of recording surface (m) = 2n = 2*8=16 Storage Capacity=16*2820*400*512 = 9240576000 bytes = 8.6 GB

There are several disk drives (C,D,F etc.) in a computer, which are connected to a disk controller. The controller converts instructions received form 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. The drive number must be specified, because a controller normally controls more than one drive. Table 3.6 shows a disk address format for a disk controller of 8 drives, each disk pack having 250 cylinders, 12 surface and 256 sectors.


Access time on a 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 track beginning.

Notes--The time required to position the read/write head over proper track is called the seek time 

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 proper track is called the seek time.

Seek time (Ts): The time required to move the read/write head on a specific (address) track. 
  • Seek time varies depending on the position of the arm assembly when a read/write command is received. 
  • 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 one and it will be zero if the arm assembly is already on the desired track.
  • The average seek time is thus specified for most systems which is generally between few milliseconds to fractions of a second.
Note that seek time is associated only with movable-head system. For a fixed-head system, it is always 0 because there is a head for each track and no head movement is required for accessing a particular track.

Notes--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

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. 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 (tL) 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 parameters: 
  • Distance of the desired data from the initial position of the head on the specified track.
  • Rotational speed of the disk.
The average seek 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 = Seek time + Latency time
The average access time for most disk systems is usually between 10 to 100 milliseconds

Pen Drive
Now-a-days a Pen Drive is available as a very convenient and flexible data storage medium which can store up to 256 GB data. It can be used for the same purposes as floppy-disks or CD-ROMs. Pen Drives are a smaller, faster, durable and more reliable storage medium. Compared to floppy disks or CD-ROMs it has thousands of times more data storage capacity. It is a portable USB flash memory device. It is integrated with a Memory System USB (Universal Serial Bus) interface. It can be used to quickly transfer data from one system to another. The pen drive derives its name from the fact that many of these devices resemble a small pen or pencil in shape and size. Flash drives implement the USB mass storage device class so it is possible for modern operating systems to read and write from them without installing the device driver software. Some computers can even boot up from flash drives. 

Magnetic Tapes 
 A Magnetic tape is a sequential access type secondary storage device. It is used for backups in servers, workstations, and large computers. The main advantages of magnetic tapes are that they 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 they store information sequentially. A file or some particular information stored on a magnetic tape cannot be accessed directly on random basis as is possible in the case of hard-disks or floppy disks. These devices are slower, but due to their low cost, they are still widely used for massive data warehouse and other business storage requirements. 

Notes--The storage capacity of a tape is measured by multiplying its length and data recording density.

Storage Capacity = data recording density * length 

Example 1 : If a tape length is 3400 feet long and has a data recording density of 900 bpi (bytes per inch) its storage capacity will be 3400*12 inches *900 bpi =36720000 bytes. 

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