DEVICE MANAGEMENT

 DEVICE MANAGEMENT 

  Functions: 

The roles of OS in computer are to manage and control I/O operations and devices.

These I/O devices can be grouped to 3 categories:-

1. Human readable: - These types of I/O devices establish communication between user and computer, e.g. Keyboard, Mouse etc.

2. Machine readable: - Suitable for communication with electronic equipment, e.g. disk drives, sensors, controllers etc.

3. Communication: - Suitable for communication with remote devices, e.g. MODEM.

Data rate:

Data rate varies between I/O devices to devices. Out of all devices, keyboard is the least data transmission rate. Graphics display is the highest data transmission rate.

Unit of transfer:

 Data may be transferred as a stream of bytes or characters e.g. terminal (keyboard) or in large blocks (a disk).

Organisation of I/O function:

1. Programmed I/O

2. Interrupt driven I/O

3. Direct Memory access

IO hardware:

1] A device communicates with a computer system by sending singles over a cable, the device communicates with the machine via a connection point, called port.(serial port, parallel port)

2] If one or more devices use a common set of wires the connection is called a Bus.

3] The PCI (peripheral component interconnect) Bus connects the processor-memory sub-system through fast devices, and an expansion bus that connects relatively slow devices such as keyboard, serial port and parallel.

4] SCSI (small computer system interface) is plugged into SCSI controller.

5] A controller is a collection of electronic that can operate a port a bus or a device. The processor communicates with the controller by reading and writing bit pattern in register.

Disk Structure:

1] A hard disk is a collection of platter, each dick platter have circular shape like ac, two surface of a platter is covered with a magnetic material

2] a read-write head located just above each surface of every platter. The space of platter is logically divided into circular track, tracks are subdivided into sectors.

Disk performance Parameters:

1] Seek Time: The time required to reach the desire track by read-write head is the seek time.

The liner formula for seek time is:

                          Ts (seek time)= m*n+s

  (n=number of tracks, m= constant that depends on the disk drive, s= startup time)

2] Rotational Delay: The time is required to desire sector by the read write head is the Rotational delay.

(generally, Avg rotational delay is between 100-200ms)

3] Transfer Time: The transfer time depends on the rotation speed of the disk.

The liner formula for Transfer time is:

                               T = b/r *n

(T= transfer rate, b= number of bytes transfer, n = number bits on track,   r = Rotation speed of revolution per sec   )

 

Thus the total Avg access time is (Ta) = Ts + ½r+(b*n)/r

Disk  Scheduling  Algorithm

There are no. of disk scheduling algorithm available, out of those highly used everywhere, those are-

1] FCFS (first come first serve) Scheduling

2] SSTF (shorted seek time first) Scheduling

3] Scan scheduling

4] C-scan Scheduling

5] Look scheduling

 

FCFS Scheduling: First come first served, it is simplest Disk Scheduling algorithm of all but it does not provide the fastest service.

Q) Consider a disk queue with request for I/O

   87,170,40,150,36,72,66,15(the disk head is initially at cylinder 60)

Find average head movement?

Using FCFS

Total head movement:-

60-87+87-170+170-40+40-150+150-36+36-72+72-66+66-15

27+83+130+110+114+36+6+51

= 557 cylinders

Avg. head movement are 557/8 cylinders = 69.625 cylinders

Using SSTF

This Algo. selects the with the minimum seek time then the current head position

Total head movement:-

60-66+66-72+72-87+87-40+40-36+36-14+14-150+150-170

6+15+47+4+21+135+20

=254 cylinders

Avg. head movement are 254/8 cylinders = 31.75

 Scan Scheduling: The scan algorithm is called elevator algorithm. In this the disk arm stars at one end of the disk and moves towards the other end, while in the meantime all requests are servicing until it gets the other end of the disk. At the other end the direction of the head movement is reversed and servicing continues.

 C Scan Scheduling: The main drawback in scan scheduling is that the waiting time is not uniform. In this algorithm head moves from one end to other end of the disc servicing the request along the way.

Note: The average head movement in C-Scan scheduling is more than scan scheduling. But it provides more uniform wait rate.

 Look Scheduling: In the C-Scan Scheduling the disk arm moves across the full width of disk but in Look Scheduling algorithm the arm goes only as far as the final request in each direction. Then it reverses the direction immediately without going to the end of the disk.