A quick refresher on concepts of Operating System which will help people build up on modern technologies like Cloud etc

1. Operating Systems
HIMANSHU PANT

2. What
 Broker between user and computer hardware. Manages computer hardware.
 Ubiquitous for all computing environments – be it PC , mainframes , IoT devices , smart devices,
cloud computing devices etc.
 Resource allocator – Acts as manage of a computing system’s resources . Does prioritization of
resource requests to resolve conflicts.
 Control program – manages the execution of user programs to prevent errors.
 The single program which is running at all times in a computing environment.
 All code runs on top of OS so knowledge of OS is crucial and important to secure, efficient,
effective programming.

3. Fitment of OS

4. Types of computer system architecture
 Single processor architecture(SPA)
 One CPU , single processing core
 Have other special purpose processors which are device specific.
 Multi-processor architecture(MPA)
 Two or more processors with single/multiple core CPU
 Benefit increased throughput , though not a linear increase compared to SPA

5. Modes of operation
 Typically two modes of operation in computing system – user mode and kernel mode.
 Mode bit in H/W of computer indicates the current mode of operation.
 User mode used for common user operations.
 Kernel mode is for privileged instructions.
 Protects OS from malicious users by classifying some instructions as privileged instruction.
 Systems with virtualization have different mode for showing when virtual machine manager is in control.
 Privileges more than user but less than kernel mode
 Used to create and manage VMs without interfering with kernel of host machine.

6. OS as Resource Manager
 Process manager –
 Process – an active instance of a passive entity called program. A unit of work in a system.
 OS responsible for –
 Creating and deleting user/kernel processes.
 Scheduling processes
 Suspending and resuming processes
 Process synchronization and communication
 Memory Manager
 Keeps track of memory usage
 Allocating and freeing memory

7. OS as Resource Manager
 Cache manager
 OS doesn’t deal with hardware based cache.
 Replacement policy for cache management.
 I/O system manager
 Abstract complexity of hardware I/O from user via device driver communiction.

8. OS as Resource Manager
 File system manager
 File system a logical abstraction of physical properties of storage.
 Manages CRUD operations on file
 Controls user access rights
 File organization – directories and sub-directories
 Mass Storage management – Device management for secondary storage

9. Virtualization
 Allows abstraction of hardware of single physical computer into multiple distinct execution
environment.
 User of VM see various running OSes like different applications in single processor system.
 OS running as application within other application.
 Emulation – Simulating hardware via software used when different CPU has to simulated. High
performance cost.
 Virtual Machine Manager ( VMM) runs on top of host OS and controls the guest OS

10. Interacting with OS
 Multiple ways in which users can interact with OS
 CLI ( Command line Interface) – Special program that is running when user first logs in. Systems can
have multiple interpreters like Bourne , Korn etc. Typically preferred by power users as it makes their
activity easily repeatable and loggable.
 GUI ( Graphical user interface) – User friendly graphical interface ( windows , mouse) is used to interact
with OS.
 Touchscreen interface – Typically used in modern portable devices like mobiles , tablets etc.

11. Process
 Program in execution
 Made up of following sections
 Text section – Executable code of process
 Data Section – Global data
 Heap section – Dynamically allocated memory
 Stack section – Memory keeping track of run time stats
 Program != Process . Program is static/passive entity , process active entity
 Multiple process running same program considered separate.

12. Process
 States of a process –
 New – Process being created
 Running – Process being executed
 Waiting – Process waiting for some input
 Ready - Process ready to be assigned to some processor.
 Terminated – Process finished execution.
 Number of processes which can run at a time == Number of processors (cores)

13. Operations on Process
 Process can be created by other process ( parent process).
 Process identified by a numeric identifier called PID.
 Process is terminated when finishes executing its final statement.

14. Process Communication
 Concurrently operating processes can be –
 Independent – Having no data sharing between each other
 Cooperating – Having data sharing between them
 Inter process communication done via –
 Shared Memory – Faster than message passing
 Message passing – Useful for small amounts of data, preferable for distributed system
 Communication can be of two logical kinds -
 Direct – Each process must know name of sender or receiver.
 Indirect – Process communication done via ports.

15. Thread
 Basic unit of CPU utilization.
 Finer unit of processing than process.
 Threads (of same process) run in shared memory space.
 Less resource intensive way to do multiple instances of processing.

16. Benefits of multi-threading
 Increased Responsiveness – Similar to pseudo conversational concept of OLTPs
 Allows Resource sharing – Allow default sharing of memory of the process to which they
belong
 Economical in resource usage – Share resources of the overarching process so economical to create.
 Scalability

17. Concurrency vs Parallelism
 Concurrency – Running multiple tasks in quick succession to give the illusion of mult—processing.
 Parallelism – Multiple tasks actually running at a single point of time in parallel.

18. Types of Parallelism
 Data Parallelism – Data spread across multiple cores to facilitate fast processing.
 Task Parallelism – Tasks spready across multiple cores.
Not mutually exclusive , most common usage is hybrid of both

19. CPU Scheduling