Functions of Operating Systems

1. FUNCTIONS OF OS
MODULE 1

2. FUNCTIONS OF OS
 Process management
 Memory management
 File management
 Device management
 Security management
 User interface

3. FILE MANAGEMENT
 Provides a hierarchical structure for
the entire data.
 Provides a set of commands to
perform various operations on the
file.
 The file manager
 Controls access to files.
 Supervises the creation, deletion, and
modification of files.
 Controls the naming of files.
 Supervises the storage of files.
 Responsible for archiving and
backups.

4. USER INTERFACE
 Provides 2 types of user interface
 CLI
 Provides an interactive terminal.
 User can communicate with the
system through the terminal.
 Commands are used to initiate a
program.
 GUI
 Provides users with pictures,
icons, buttons and dialog boxes.
 User sends instructions by moving
a pointer on the screen.

5. SECURITY MANAGEMENT
 Provides internal security to the
system .
 Can be provided by using
 User Authentication
 Back up of data

6. DEVICE MANAGEMENT
 Managing the various devices
connected to the system.
 OS communicates with the
hardware by using the
software called device driver.
 Spooling
 Refers to storing jobs in a
buffer, where a device can
access them when it is
ready.
 Provides a waiting station
for devices.
 Eg: Print spooling.

7. PROCESS MANAGEMENT
 Manages all processes that are running on a
computer.
 It is responsible for
 Process creation and deletion.
 Process suspension and resumption.
 Process scheduling
 Resource allocation

8. PROCESS STATE
 New
 The process is being created.
 Ready
 The process is waiting to be assigned
to a process.
 Running
 Instructions are being executed.
 Waiting
 The process is waiting for some
event to occur.
 Terminated
 The process has finished execution.

9. PROCESS CONTROL BLOCK (PCB)
 A data structure maintained by
the OS for each process
 It contains
 Process ID
 Process state
 Program counter
 CPU scheduling information
 I/O status information

10. CPU SCHEDULING
 Process of allocating CPU time among all processes.
 Scheduling can be performed with the help of
schedulers.
 For scheduling purposes, different queues are used.
 Job queue
 Set of all jobs in the system.
 Ready queue
 Set of all processes waiting for the CPU.
 Device queue
 Set of processes waiting for an I/O device.

11. CPU SCHEDULING QUEUES

12. CPU SCHEDULERS
 Long-term scheduler (job scheduler)
 Selects job from the job queue and loads it into the main
memory for execution.
 Short-term scheduler (CPU scheduler)
 Selects a process from the ready queue and allocates CPU to
it.
 Medium term scheduler
 Decides whether to introduce a process from running state
to a waiting state or a waiting state to the ready state.

13. CPU SCHEDULING
 CPU scheduling decisions may take place when a
process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4. Terminates
 Conditions 1 & 4 comes under non-preemptive
scheduling.
 All other conditions come under preemptive
scheduling.

14. CPU SCHEDULING
 Non-preemptive scheduling
 Once the CPU has been allocated to a process, the
process keep the CPU until it release the CPU either by
terminating or by switching to waiting state.
 Preemptive scheduling
 Scheduler can remove a process from the running state
to the other state in order to allow other processes to
run.

15. PROCEDURES FOR CPU SCHEDULING
 First Come First Served (FCFS)
 Round Robin Scheduling

16. First Come First Served (FCFS)
 All processes are enter in a queue.
 New process arrived is placed at the end of the queue.
 Process at the start of the queue is dispatched to the
processor.
 Falls under non-preemptive scheduling.
 If a process takes a very long time to complete, then other
processes have to wait .

17. Round Robin Scheduling
 Processes are allocated CPU time on a turn basis.
 Each process is allocated a fixed time to use the CPU.
 The time is known as time slice or quantum period.
 Falls under preemptive scheduling.

18. DEADLOCK

19. DEADLOCK
 A set of blocked processes each
holding a resource and waiting to
acquire a resource held by another
process in the set.

20. DEADLOCK
 Various conditions are
o Mutual exclusion
 Only one process at a time can use a resource.
 Hold and wait
 A process holding at least one resource is waiting to acquire
additional resources held by other processes.
 No preemption
 A resource can be released only after that process has
completed its task.
 Circular wait
 There must be a circular chain of 2 or more processes where
each is waiting for a resource held by the next member of the
chain.

21. MEMORY MANAGEMENT
 The part of the OS that performs memory management is
known as Memory Manager.
 Memory Manager is responsible
 Keeping track of which parts of memory are currently
being used and by whom.
 Decide which processes are to be loaded into memory
when memory space becomes available.
 Allocate and de-allocate memory space as needed

22. MEMORY MANAGER
 The main task of Memory Manager is
 Memory Relocation
 Memory Protection and Sharing

23. MEMORY RELOCATION
 Swapping
 Moving a process temporarily out of memory to a backing
store and then brought back to memory for continued
execution.

24. MEMORY ALLOCATION
 OS allocates a portion of the primary memory to each
process for its own use.
 Methods used for memory allocation
 First Fit
 Allocate the first hole that is big enough.
 Best Fit
 Allocate the smallest hole that is big enough.
 Worse Fit
 Allocate the largest hole.

25. MEMORY ALLOCATION
Fragmentation
Process of splitting the primary
memory into segments as the
memory is allocated and de-
allocated.

26. PAGING
 A memory management scheme that allows processes
to be stored non-contiguously in memory.
 Physical memory is divided into fixed sized blocks
called page frames.
 Logical memory is divided into fixed sized blocks
called pages.
 When a program is loaded into memory, each page is
loaded into a page frame and the frame can be residing
anywhere in the memory.
 OS maintains a page table for keeping track of the
pages of the process.

27. PAGING
 Logical address is divided into 2 parts
 Page number (p)
 Page Offset (d)
 To map the logical address to physical address in memory a
mapping table called page table is used.
 To map a given logical address to a physical address, Os
first extracts the page no and offset.
 If the page no is valid, the system uses page no to find the
corresponding page frame no in the page table.
 The page frame no is attached to the page offset to form
the physical address in memory

28. PAGING
CPU p d
page table
f
f d
physical
memory
Page No

29. VIRTUAL MEMORY
 A memory larger than the physical memory placed on
the hard disk.
 Data is stored on the VM and can be loaded into main
memory whenever needed.
 The process of swapping pages from VM to main
memory is known as page –in or swap-in.
 In VM systems, the page table of each process stores
an additional bit to identify the location of the page.
 If the bit is 1, the page is in the main memory
otherwise in the virtual memory.

30. VIRTUAL MEMORY
 Wherever a page reference is made, the Os checks the
page table for that page.
 If the page is not in the main memory, a page fault
occurs and control is passed to the page fault routine
in the Os.
 Page fault routine checks whether the virtual address
of that page is valid or not.
 If it is valid, locates the free frame in the memory and
allocate the process.
 If it is not valid, it terminates the process.

31. EMBEDDED SOFTWARE
 A software written to control machines or devices.