1. Introduction What is an operating system bootstrap
different objectives of OS
functions of OS
OS as manager

2. Objectives of OS
Convenience.
Efficiency.
Ability to evolve.

3. Evolution of OS Different generation of computers. Serial processing.
Simple batch system
multi-programmed batch system, IBSYS ‘60.
Timesharing OS
Parallel systems, Distributed systems, Real time systems, Embedded systems

4. Process
Program under execution. It is an dynamic entity consisting of the sequence of events created as a result of execution of instructions. Is has a beginning, a life time and an end. It includes the current activity as represented by PC, stack containing temporary data (Parameter, return address, temporary variables). The same code of program can result in many processes. A program may have several conditional branches, depending on data, CPU may follow different paths in the address space. The same program code can be shared by more than one user at a time. This type of memory sharing will result in different processes.

5. Process life cycle Five state model Scheduled Admitted Interrupt
Running
New
Ready
I/O or event wait
exit
I/O or event completion
Terminate
Wait

6. Process life cycle Seven state model Suspend New Admit Admit
Scheduler dispatch
Activate
Interrupt
Running
Ready
Ready, suspend
I/O or event wait
Suspend
exit
I/O or event completion
Event complete
Terminate
Activate
Wait, suspend
Wait
Memory release

7. Transitions
newready,suspend (process creation duty with blocked process)
newready if the new process is of high priority
wait wait suspend, if no space for new process
readyready suspend, if the wait queues are empty
ready suspend ready, ready queue is empty
wait suspend  ready suspend, event for what process waited is completed
wait suspend  wait, if the process is of high priority and memory becomes available.(poor performance)
Running  ready suspend, a high priority process preempts the process and no memory available.

8. Process control block
The structure maintained by the operating system that represents a process
Typical elements of PCB
Process identification
Process ID
Parent process ID
User identifier
Processor state information
User visible registers
Control registers
Stack pointers

9. PCB Process ID Registers Memory limits List of open files
Registers
Memory limits
List of open files
Program counter
Process state
Pointer

10. Process control block (contd.)
Process control information
Scheduling information
Process state
Priority
Scheduling related information
Event waiting
Data structuring
Pointers to other processes in queues
Inter process communication
Process control information
Process privileges
Memory management
Resource ownership and utilization

11. Schedulers Long term Short term Medium term
Controls degree of multiprogramming
Good mixture of I/O and CPU bound process
Short term
CPU scheduler
Medium term
swapping

12. CPU and I/O burst CPU burst I/O burst or wait
Process can be viewed as an alternating sequence of CPU and I/O burst.
Starts and ends with CPU burst
CPU burst
I/O

13. Context switching
Executing a new process needs to save the current state of the currently executing process, that includes the contents of different registers.
It is a pure overhead to the system.
It is dependent on hardware.
DECSYSTEM-20 uses different set of registers. Contex switching here is a simply the change of pointer to a new set of registers.
If the no of process is high, then needs the saving in memory

14. Scheduling criteria CPU utilization Throughput Turnaround time
Waiting time
Response time

15. Category of scheduling algorithm
Preemptive
Non-preemptive

16. When to make scheduling decision
Running  wait
Terminate
Running  ready
Wait  ready
Only in Preemptive

17. Scheduling algorithms
First in first out (FIFO)
Shortest job first (SJF)
Shortest remaining time next(SRTN)
Highest response ration next(HRTN)
Round Robin (RR)
Priority Scheduling
Multilevel queue
Multilevel feedback queue

18. FIFO Process P1 P2 P3 P4 P5 Arrival time 0 0 2 8 12 Length 10 9 5 5 4P3 2 P4 8 P5 12 P1 P2 P3 P4 P5 10 19 24 29 33
Process P1 P2 P3 P4 P5 Average
Waiting time
Turnaround time

19. SJF Process P1 P2 P3 P4 P5 Arrival time 0 0 2 8 12 Length 10 9 5 5 4P3 2 P4 8 P5 12 P2 P3 P5 P4 P1 9 14 18 23 33
Process P1 P2 P3 P4 P5 Average
Waiting time
Turnaround time

20. SRTN Process P1 P2 P3 P4 P5 Arrival time 0 0 2 8 12 Length 10 9 5 5 4P3
0 2
7 5 7 P3 P4 8 5 P1 33 P5
8 12
1 4 P4 13 12 4 P2 23 P5 17 8 P2 6 P2 2 12 P4
Process P1 P2 P3 P4 P5 Average
Waiting time
Turnaround time

21. HRRN RR=(L+W)/L P5 33 Process P1 P2 P3 P4 P5 Arrival time 0 0 2 8 12
Length
Process P1 P2 P3 P4 P5
Arrival time
Length
P1 P2 P2 P3 2 P4 8 9
2 8
5 5 P4 29 P1 24 P3 P5 12 14
8 12
5 4
RR 1 1
RR 4.2 4 RR RR
RR 5.2
Process P1 P2 P3 P4 P5 Average
Waiting time
Turnaround time

22. Round Robin Process P1 P2 P3 P4 P5 Arrival time 0 0 2 8 12
Length
Process P1 P2 P3 P4 P5
Arrival time
Length P1 P3 5 2
5 0 2
5 9 5 P2 P4 10 8 P1 20 P3 P5 15 12
8 12
5 4 P2 29
P1P2 P4 25 P5 33
Process P1 P2 P3 P4 P5 Average
Waiting time
Turnaround time

23. Round Robin with Time slice 10 &2P1 P2 P4 P3 P5
P1 P2 2 8 12 10 19 24 29 33
With longer time slice RR will be converted to FIFO P1 P2 P3 P4 P5
With very short time slice in RR, considerable amount of time will be wasted on Context Switching.

24. Problem
Draw the Gantt chart and find the average waiting time for the following set of process, scheduled with --
Highest response ratio next algorithm
Round Robin algorithm with time slice 5.
P1 P2 P3 P4 P5 P6
Arrival time
Length

25. Multilevel Queue Scheduling
Priority Scheduling
Multilevel Queue Scheduling
Ready queue 1
Ready queue 2
Ready queue N 
Highest priority 
Lowest priority

26. Multilevel Feedback Queue Scheduling
Different ready queues with priority, Process’s priority is dynamic
Ready queue 1
If the process can’t complete within time slice of the Q, then degrade the priority of the process.
If process’s waiting time in a Q exceeds the limit, then improve its priority.
Ready queue 2
Ready queue 3
Ready queue 3

27. Example At time 7 Q1(4,--) P5(7,6,0) Q2(8,10) P3(7,2,0) Q3(16,20)
Process p1 p2 p3 p4 p5 p6 p7 p8
A/T
Priority
Len
At time 7
Q1(4,--) P5(7,6,0)
Q2(8,10) P3(7,2,0)
Q3(16,20)
Q4(--,30) P1(0,18,7), P4(5,20,2)
At time 3
Q1(4,--)
Q2(8,10) P3(2,6,1)
Q3(16,20)
Q4(--,30) P1(0,18,3)
At time 13
Q1(4,--)
Q2(8,10) P5(11,2,2)
Q3(16,20) P6(8,10,5)
Q4(--,30) P1(0,18,13), P4(5,20,8)
At time 11 (time expires for P5)
Q1(4,--)
Q2(8,10) P3(7,2,4), P5(11,6,0)
Q3(16,20) P6(8,10,3)
Q4(--,30) P1(0,18,11), P4(5,20,6)
At time 0
Q1(4,--) P2(0,3,0)
Q2(8,10)
Q3(16,20)
Q4(--,30) P1(0,18,0)
At time 39(time expires for P6, P4 is promoted)
Q1(4,--)
Q2(8,10)
Q3(16,20) P7(23,1,16),P1(30,18,9),P8(31,2,8), P4(35,20,0),P6(39,2,0)
Q4(--,30)
At time 51 (P8 is promoted)
Q1(4,--)
Q2(8,10) P8(51,2,0)
Q3(16,20) P4(35,20,16),P6(39,2,12), P1(51,7,0)
Q4(--,30)
At time 40
Q1(4,--)
Q2(8,10)
Q3(16,20) P1(30,18,10),P8(31,2,9), P4(35,20,5),P6(39,2,1)
Q4(--,30)
At time 59 (P6 is promoted)
Q1(4,--)
Q2(8,10) P6(59,2,0)
Q3(16,20) P1(51,7,8), P4(59,14,0)
Q4(--,30)
At time 53
Q1(4,--)
Q2(8,10)
Q3(16,20) P4(35,20,18),P6(39,2,14), P1(51,7,2)
Q4(--,30)
At time 61
Q1(4,--)
Q2(8,10)
Q3(16,20) P1(51,7,10), P4(59,14,2)
Q4(--,30)
At time 68
Q1(4,--)
Q2(8,10)
Q3(16,20) P4(59,14,9)
Q4(--,30)
At time 31(time expires for P8, P6and P1 are promoted)
Q1(4,--)
Q2(8,10) P6(28,10,3)
Q3(16,20) P7(23,1,8), P1(30,18,1),P8(31,2,0)
Q4(--,30) P4(5,20,26)
At time 15
Q1(4,--)
Q2(8,10) P7(14,9,1)
Q3(16,20) P6(8,10,7)
Q4(--,30) P1(0,18,15), P4(5,20,10)
At time 23(time expires for P7)
Q1(4,--)
Q2(8,10) P8(21,10,2)
Q3(16,20) P6(8,10,15), P7(23,1,0)
Q4(--,30) P1(0,18,23), P4(5,20,18) P2 P3 P5 P7 P8 P6 P1 P4