Write a short notes on magnetic disk, magnetic tape and optical disk.
Write a short notes on magnetic disk, magnetic tape and optical disk.
Quick answer
A magnetic disk is the main secondary storage of a computer. It has one or more circular platters coated with magnetic material. The platters spin at a fixed speed, such as 7200 rpm. Each surface is divided into concentric circles called tracks, and each track is divided into sectors, which are the smallest units read or written. A read/write head floats over each surface on an arm. The head magnetizes spots to write and senses them to read.
Magnetic disk
The disk is a direct access device. The head first moves to the right track, and then it waits for the sector. The access time is
$$\text{access time} = \text{seek time} + \text{rotational latency} + \text{transfer time}$$
The seek time is the time to move the arm to the track. Rotational latency is the wait for the sector to come under the head, on average half a rotation. At 7200 rpm one rotation takes $60/7200 = 8.33$ ms, so the average latency is about $4.17$ ms. Disks are cheap, large and non-volatile, but slow compared to memory.
Magnetic tape
A magnetic tape is a long plastic strip coated with magnetic material, wound on reels or in a cartridge. Data is written in tracks along the length of the tape. It is a sequential access device. To reach a record, the tape must be wound past all the records before it. So the access is slow, but once the right position is reached the data transfer is fast. Tape is very cheap per bit and holds a large amount of data. It is used for backup and archives. Its drawback is that random access is not possible.
Optical disk
An optical disk stores data as tiny pits and flat areas (lands) along a spiral track. A laser beam is focused on the surface. The reflected light is different for a pit and a land, and a sensor reads this as 0 or 1. Common types are as follows.
| Type | Meaning | Typical capacity |
|---|---|---|
| CD-ROM | read only, factory made | about 700 MB |
| CD-R, DVD-R | write once | 700 MB, 4.7 GB |
| CD-RW, DVD-RW | can be erased and rewritten | 700 MB, 4.7 GB |
| Blu-ray | uses a blue laser | 25 GB or more |
Optical disks are cheap, easy to carry and last long, which is good for software and movies. They are slower than a magnetic disk.
All three are non-volatile, so data stays when power is off. Magnetic media can also be rewritten many times, but they can be damaged by strong magnetic fields, heat and dust.
Comparison
| Point | Magnetic disk | Magnetic tape | Optical disk |
|---|---|---|---|
| Access | direct | sequential | direct |
| Speed | fast | slow | medium |
| Use | main storage | backup | distribution, archive |
In a computer system, the disk is used as the main file store, the tape as an off-line backup, and the optical disk to carry and share software and media.
Short on time? The 1, 5 and 10 mark versions of this answer, flashcards and practice sets come with a program pass.
Up nextWith a neat schematic diagram, explain about DMA controller and its mode of data transfer.