The operating system performs key functions like scheduling, synchronization, and memory management. Scheduling involves allocating time to programs and switching between threads. Synchronization ensures mutual exclusion so only one process accesses shared resources at a time using techniques like semaphores. Memory management uses virtual memory to allow processes to access more memory than physically available by swapping pages between disk and RAM.

1. Operating System

2. Operating System
Functions of OS
• scheduler (time allocation to programs) ,
• synchronizer ( avoid conflicts for a
resource at the same time)
• memory manager (space allocation to
programs)

3. Scheduling
• Scheduling Algo
• basically utilize IO time by scheduling another
task
• Switching from one thread to another (A --> B )
can occur when
a) A is complete
b) A is blocked on IO
c) Timer interrupt forcefully preempts A

4. SCHEDULER

5. Synchronization
• Synchronization is needed as process need to
exchange information using a shared region in memory.
• However, this shared region cannot be accessed in any
manner. They needed to be accessed in correct
sequence

6. Synchronization
The code accessing Shared Data is called Critical
Section
No more than one process can access shared data
at a given time
Critical Sections must obey mutual exclusion

7. Synchronization
• How to ensure Mutual exclusion (only
one process is in critical section)
• Disable interrupts
• Test and set instructions
• Semaphore

8. Synchronization
• Semaphores
• Used to solve Producer Consumer
problems
• Producer consumer problems occur for
example in networks (sending and
receiving packets), unix shell IO

9. Synchronization
• Producer Code
down(empty_slot)
put item in buffer
up(full_slot)

10. Synchronization
• The above solution works for a single producer
consumer
• When multiple producers and consumers are
present , then a race condition can happen
resulting in writing to the same slot by two
different producers
• To overcome this problem, we need to make
sure that only one producer is executing "put
item in buffer". So we need to execute mutual
exclusion within a critical section

11. Synchronization
• down(empty_slot)
• down(mutex)
• put item in buffer
• up(mutex)
• up(full_slot)

12. Memory Management
• How to run process when not enough
physical memory?
• How to protect one process from
accessing another’s memory

13. Memory Management
• How to run process when not enough physical memory?
• How to protect one process from accessing another’s
memory
VIRTUAL MEMORY provides illusion of
more memory
Each process access only a window of
memory – due to Locality of Reference
Only a small portion of address space must
be in physical memory

14. Memory Management
• Each layer acts as “backing store” for layer above
disk storage
main memory
cache
registers
size
speed cost

15. vpn …
vpn 3
vpn 2
vpn 1
vpn 0
P1
Virtual
pages
Frame # Present bit
… …
Blk 1031 0
Blk 1030 0
Fr # 15 1
Blk 1030
…
Blk 1031
Fr #15
Fr #16
Fr # 16 1
…
PAGE TABLE

17. Memory Management
Page Replacement