PC Computer Hardware
Storage Devices
Magnetic Memory ( Secondary Memory)
The secondary memory is a mass storage memory. Also the information stored in the secondary memory is never read directly by CPU. It is first transferred to primary memory before being read .Also it can only accept as well as output device .
Magnetic Tapes
magnetic tapes memories are similar to the commonly used audio tape recorders . It has been already discussed in Input devices.
Magnetic Hard disks
When the power to a PC is switched off , the contents of memory are lost . It is the PCs hard disk serves as non volatile, bulk storage medium and as the repository for a user 's documents, files and application .
Magnetic disk are used as an input or output device , and for computers auxiliary memory. A magnetic disk is similar to gramophone record but is coated with ferromagnetic materials and rotes at very high speed up to 5400 or 7200 rpm . in the magnetic disk we can have direct access to any information, irrespective of its location on disk.
Hard disks are rigid platters, composed of substrate and a magnetic medium, The substrate - the platter's base material- must be non magnetic and capable of being machined to a smooth finish. It is made either of aluminum alloy or a mixture of glass and ceramic. To allow data storage , both sides of each platter are coated with a magnetic medium - formerly magnetic oxide , but now , almost exclusively , a layer of metal called a thin film medium. This stores data in magnetic patterns, with each platter capable of storing a billion or so bits per square inch(bpsi) of platter surface,
In disk pack with 10 platters there are 20surfaces . out of 20 surfaces , the top most surface and the bottom most surface are not used for recording. Thus there is 18surface on which information is recorded . Cylinder thus consists of 18 tracks in such a disk pack. A track is divided into sectors . Read and write operation on a disk start at sector boundaries. Typically 512 byte are stored per sector. There are 50 sector per track, 400 track per surface and 18 surface
The total capacity of disk is
512 x 50 x 400 x 18 = 18,43,20,000 bytes
or
512 x 50 x 400 x 18 x 8 = 1,47,45,60, 000 bits ( 1 byte= 8bits)
The density of a recording on a tracks is of the order of
(50 x 512 x 8) / (12 x p) = 5500 bits inch
for a disk pack of diameter 12 inches.
There's read / write head for each side of each platter , mounted on arms which can move them towards the central spindle or towards the edge. The arms are moved by the head actuator, which contains a voice- coil- an electromagnetic coil that can move magnet very rapidly. Loudspeaker cones are vibrated using a similar mechanism.
The heads are designed to touch the platters when the disk stops spinning . that is when the drive is powered off,. During the spin down period, the airflow diminishes until it stops completely, when the head hand gently on the platter surface, to a dedicated spot called the landing zone(LZ). T he LZ is dedicated to providing a parking spot for the read/ write heads, and never contains data . The time taken to reach the specified cylinder from the landing zone is known as the seek time (T). The seek time varies depending upon the position of the arm assembly when the read- write command is received by the controller. It is normally measured in millisecond . The maximum seek time is the time taken by the head assembly to reach the innermost cylinder from the outermost cylinder or vice versa. The average seek time is usually specified on the disk and it is of the order of 30 milliseconds.
Once the hard assembly is positioned on the specified cylinder, the head corresponding to the specified surface is switched. After selecting the head, there is further delay because the specified sector has to reach the read write head. This rotation delay is variable. The average rotational delay equals half the time taken by the disk to rotate once. This time is known as latency time(Tl). E.g for a disk rotating at 4800 rpm
Tl=0.5 x 60/ 4800=6.25 milliseconds
The sum of average latency and seek time is known as average access time.
Now after reaching the exact location of data, data is read by the head . The rate at which information is read from the disk is known as transfer rate. In calculating the transfer rate , the speed of the rotation of disk play an important role . for a disk rotating at 4800 rpm , one full revolution take 1/4899 minute , in one revolution, one track, which has say 50 sectors , is read. (one sector contains 512 bytes). Thus the number of bytes read per revolution is thus
50 x12 = 25600 bytes
=25600 x 8 bits
Thus transfer rate =25600 x 4800 /60 per second
= 2048000 bytes per second
=2000MB per second
Time to read one sector =512/2000 x 1024 second
= 0.25 millisecond
Thus Average Access time =30+6.25 millisecond
=36.25 millisecond.
