Types of Storage Disks 

I/O DEVICES:

There are many kinds of input and output devices that are used to manage and maintain the computer network system. The structure of some devices and how the operating system co-ordinates these devices are discussed below:

Storage Disks:

Today, all computer systems have disks for storing information permanently. A disk is the most important and commonly used I/O device. There are many kinds of disks such as floppy disk, hard disk, CD-ROM, DVD, etc. Data on disks may be stored magnetically or optically (using laser technology).

Hard Disk:

Today, hard disk is the most commonly used storage device in personal computers and laptops computers. Most application programs and operating systems require hard disk for their installations. A hard disk contains more than one platter. Information is stored on both sides of the platter, however, some multi-patter disks do not use the topmost and bottommost surface.

Explanation:

The hard disks and drive from a single unit. For this reason, a hard disk is also known as a fixed disk drive. It includes the hard disk and a motor that rotates the platter. Each platter has two read/write heads, one for each side for reading and writing data. The hard disk has also arms that move the read /write heads to the proper location on the platter to read and write data. All the read/write heads move together. The entire unit is enclosed in an airtight. sealed case. Normally, the hard disk remains fixed within the system unit.

The disk or platter has a flat circle shape. The computer network surface of the disk is logically divided into circular tracks, which are subdivided into sectors of disks. The set of concentric tracks (i.e tracks with the same radius) of different platters is known as a cylinder.

However, in hard disk the basic storage unit is sector. All sectors contain the same number of bytes. Usually, in a single sector, there are 512 bytes. The total capacity of a disk can be calculated by multiplying together, the following:

• Several cylinders, which is the same as the number of tracks per surface.
• Several reading/write heads.
• Several sectors per track.
• Several bytes per sector.

Hard Disk Performance:

The performances of a disk depend upon the following factors:

Seek Time:

Seek time is the time required to move a read/write head to a particular cylinder or tracks. Seek time is variable because it will take more time to reach track 300 from track 1 than to reach track 100. Therefore, seek time is expressed as average seek time.

Rotational Latency:   

Once a cylinder or tracks is selected, the disk controller waits for the unit the read/write heads reach an appropriate sector. The time requires to reach the heads at the beginning of an appropriate sector is called rotational latency. It is also called rotational delay.

Transfer Time:

Transfer time is the time required to transfer data, read from the disk into the main memory. Alternatively, it is the time required to read or write data on the disk from the main memory.

Access Time:

Access time is the sum of seek time, rotational delay, and transfer time in a computer system. Mathematically, it is written as:

Access Time=Average Seek Time +Rotational latency +Transfre Time

Once the read/write head is in position, the read/write operating is performed. Typically, a disk can transfer several megabytes of data per second. Therefore, the value of seek time and rotational latency is in milliseconds.

Device Drivers:

A device driver is a special system software that controls the functioning of the devices controller. The device driver understands the device controller and presents a uniform network architecture interface to the device of the computer system technology. Each device driver handles one or more closely related devices.

The device driver can be thought of as a translator of any type that is a language translator. It accepts high-level commands such as ''retrieve block 106'' from disk. Then it translates the high-level commands into low-level that are used by the hardware controller, which interface the I/O device to the computer system. For example, the basic computer network file system issues a generic command to the appropriate device driver of the system to read and write special physical blocks on the disk which include sectors...

Explanation:

Please note that different kinds of devices may be connected to a computer system. It is not practical to include all the drivers for all the possible devices in the operating system. The device driver of a particular device s installed properly. It means that the configuration of the operating system is changed to include the new driver into it. When a driver is installed, the image of the operating system on the disk is also modified. Therefore, the system must be restarted to use the new image. Some systems are automatically updated when new changes occur.

we have known that each controller has one more device registers used to give it commands. Device drivers issue these commands and check that they are carried out properly, Some controllers can handle only one command at a time, while other controllers can accept multiple commands that are added into a queen. The controller takes action on these commands one by one without the help of the operating system.

Thus, the disk drive is only part of the operating system that knows how many registers that disk controller has and what they are used for, It knows about sectors, tracks, cylinders, heads, arm motion, and all the other mechanics of making the disk work properly. 

Device Controller:

An electronic device in the form of a chip or circuit board that controls the functioning of an I/O device is called the device controller or I/O Controller or adapter. The operating system directly deals with the device controller in a system network technology.

Cycle Stealing:

The process of taking the system bus away from the CPU and giving it to the I/O device is known as cycle stealing.