Os Unit 1 Review-1(3).docx

Operating Systems Review ( Unit 1 and Unit 2) Points to Remember: 1. Definition of OS:  An operating system (OS) is software that manages computer hardware resources (CPU, Memory, Input/ Output devices).  OS is an interface between user and hardware. 2. Purpose (Goal) of Operating system:  To use computer hardware efficiently.  To make application software portable and versatile.  To provide isolation, security and protection among user programs. 3. Multi Programming: Multiple Programs are running in the same time period. When one program is waiting for I/O transfer, another program utilizes the CPU.

4. Views of Operating System:  OS as a Resource Manager: - OS manages all computer resources (CPU, Memory, Input/ Output devices, files and Application Programs). It permits resource sharing, provides protection and improves system performance.  OS as a Controller: Controls execution of user programs and operation of I/O devices.  OS as a Command Executer (Shell): It executes commands entered by a user.  OS as a Virtual Machine. 5. Types of Operating systems:  Batch Operating System  Time Sharing Operating System  Real Time Operating System  Distributed Operating System  Networked Operating System 6. Batch Operating System:  User don’t interact with computer, submit the job to Computer operator,  Similar jobs are batched and run as a group. 7. Time sharing OS:  Multiple jobs are executed by the CPU by switching between them, switching occur so frequently so the user receive an immediate response. 8. Distributed OS:  It is software over a collection of independent, networked computers. But, the users unaware of multiplicity of machines.

9. Networked OS:  NOS allows file sharing and printer access among multiple computers in a network (LAN). But, the users aware of multiplicity of machines. 10. Components of Operating Systems:

11. Process management activities(functions) of OS:  A process is a program in execution. The operating system is responsible for i. Creation, deletion, suspension and resuming of process. ii. Allocating resources (CPU, Memory, files, I/O devices) to process. iii. Reclaiming resources when that process is completed. 12. Memory management activities(functions) of OS:  Keeping track of which parts of memory are currently being used and by whom.  Allocating memory to process and reclaiming memory when process is over. 13. File Management Activities of OS:  Creating and deleting files and directories  Manipulate files.  Mapping files onto secondary storage  File Backup. 14. Secondary Storage management Activities of OS:  Free-space management  Storage allocation  Disk scheduling 15. Important services provided by an operating systems :  Program execution  I/O operations  Accounting  Communications  Error detection  Protection 16. System Calls:  System calls provide the interface between a running program and the operating system. 17. Interrupt:  It is a signal from a Hardware device or from a program within the computer.  Hardware interrupt: It is usually generated by hardware devices (e.g. a timer, keyboard, network card) to indicate that some device needs OS service.  Software Interrupt: It is usually generated by programs. It causes a change in the normal flow of instruction execution. 18. Dual Mode Operation:  Dual-mode operation allows OS to protect itself and other system components (Hardware and Software).

 19. Difference between Monolithic and Micro Kernel Monolithic Kernel Micro Kernel 1 Kernel is a single large process running Kernel is broken down into separate processes, entirely in a single address space. known as servers. Some of the servers run in kernel space and some run in user-space. 2 Examples: Unix, Linux. Examples: Mac OS X and Windows NT. 3 Disadvantages: Hard to understand. Hard to modify. Unreliable: a bug anywhere causes a system crash.

Advantages: Easier to extend a microkernel, Easier to port OS to new architectures,

20. Layered Approach in Operating system Design  Operating system is divided into many layers (levels)  Bottom layer (layer 0) is hardware and highest layer (layer N) is the user interface.  Each lower layer provides services to its higher layer next to it.  Advantage o Modularity o Easy debugging o Easy Maintenance.

21. Virtual Machine:  A virtual machine (VM) provides;  Virtual Hardware  Virtual Software  Virtual Operating-system.

Advantages:  Multiple operating systems can run on the same physical platform  If one OS crash, it does not affect others. It is easy to replace failed OS with fresh one.  Migrating running VM to different computer is possible.

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