OPERATING SYSTEM

Unit-4: Device management

Device management in operating system known as the management of the I/O devices such as a keyboard, magnetic tape, disk, printer, microphone, USB ports, scanner, etc.as well as the supporting units like control channels.

Technique of device management in the operating system

An operating system or the OS manages communication with the devices through their respective drivers. The operating system component provides a uniform interface to access devices of varied physical attributes. For device management in operating system:

Keep tracks of all devices and the program which is responsible to perform this is called I/O controller.
Monitoring the status of each device such as storage drivers, printers and other peripheral devices.
Enforcing preset policies and taking a decision which process gets the device when and for how long.
Allocates and Deallocates the device in an efficient way.De-allocating them at two levels: at the process level when I/O command has been executed and the device is temporarily released, and at the job level, when the job is finished and the device is permanently released.
Optimizes the performance of individual devices.

Types of devices

The OS peripheral devices can be categorized into 3: Dedicated, Shared, and Virtual. The differences among them are the functions of the characteristics of the devices as well as how they are managed by the Device Manager.

Dedicated devices:-

Such type of devices in the device management in operating system are dedicated or assigned to only one job at a time until that job releases them. Devices like printers, tape drivers, plotters etc. demand such allocation scheme since it would be awkward if several users share them at the same point of time. The disadvantages of such kind f devices s the inefficiency resulting from the allocation of the device to a single user for the entire duration of job execution even though the device is not put to use 100% of the time.

Shared devices:-

These devices can be allocated o several processes. Disk-DASD can be shared among several processes at the same time by interleaving their requests. The interleaving is carefully controlled by the Device Manager and all issues must be resolved on the basis of predetermined policies.

Virtual Devices:-

These devices are the combination of the first two types and they are dedicated devices which are transformed into shared devices. For example, a printer converted into a shareable device via spooling program which re-routes all the print requests to a disk. A print job is not sent straight to the printer, instead, it goes to the disk(spool)until it is fully prepared with all the necessary sequences and formatting, then it goes to the printers. This technique can transform one printer into several virtual printers which leads to better performance and use.

input/output devices

input/Output devices are the devices that are responsible for the input/output operations in a computer system.

Basically there are following two types of input/output devices:

Block devices
Character devices

Block Devices

A block device stores information in block with fixed-size and own-address.

It is possible to read/write each and every block independently in case of block device.

In case of disk, it is always possible to seek another cylinder and then wait for required block to rotate under head without mattering where the arm currently is. Therefore, disk is a block addressable device.

Character Devices

A character device accepts/delivers a stream of characters without regarding to any block structure.

Character device isn’t addressable.

Character device doesn’t have any seek operation.

There are too many character devices present in a computer system such as printer, mice, rats, network interfaces etc. These four are the common character devices.

Input/Output Devices Examples

Here are the list of some most popular and common input/output devices:

Keyboard
Mouse
Monitor
Modem
Scanner
Laser Printer
Ethernet
Disk

Storage devices

There are two types of storage devices:-

Volatile Storage Device –
It looses its contents when the power of the device is removed.
Non-Volatile Storage device –
It does not looses its contents when the power is removed. It holds all the data when the power is removed.

Secondary Storage is used as an extension of main memory. Secondary storage devices can hold the data permanently.
Storage devices consists of Registers, Cache, Main-Memory, Electronic-Disk, Magnetic-Disk, Optical-Disk, Magnetic-Tapes. Each storage system provides the basic system of storing a datum and of holding the datum until it is retrieved at a later time. All the storage devices differ in speed, cost, size and volatility. The most common Secondary-storage device is a Magnetic-disk, which provides storage for both programs and data.

In this hierarchy all the storage devices are arranged according to speed and cost. The higher levels are expensive, but they are fast. As we move down the hierarchy, the cost per bit generally decreases, where as the access time generally increases.

The storage systems above the Electronic disk are Volatile, where as those below are Non-Volatile.
An Electronic disk can be either designed to be either Volatile or Non-Volatile. During normal operation, the electronic disk stores data in a large DRAM array, which is Volatile. But many electronic disk devices contain a hidden magnetic hard disk and a battery for backup power. If external power is interrupted, the electronic disk controller copies the data from RAM to the magnetic disk. When external power is restored, the controller copies the data back into the RAM.

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