Operating System

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)

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 IOc) Timer interrupt forcefully preempts A

SCHEDULER

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

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

Synchronization

• How to ensure Mutual exclusion (only one process is in critical section)

• Disable interrupts

• Test and set instructions

• Semaphore

Synchronization

• Semaphores

• Used to solve Producer Consumer problems

• Producer consumer problems occur for example in networks (sending and receiving packets), unix shell IO

Synchronization

• Producer Code

down(empty_slot)

put item in buffer

up(full_slot)

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

Synchronization

• down(empty_slot)

• down(mutex)

• put item in buffer

• up(mutex)

• up(full_slot)

Memory Management

• How to run process when not enough physical memory?

• How to protect one process from accessing another’s memory

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

Memory Management

• Each layer acts as “backing store” for layer above

disk storage

main memory

cache

registers

sizespeed cost

vpn …

vpn 3

vpn 2

vpn 1

vpn 0

P1Virtualpages

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

Memory ManagementPage Replacement