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system calls, single user, multiuser os structure
- 1. Basics of Operating Systems System calls, Single user, Multiuser OS Structure • HOME PREVIOUS TOPIC NEXT • PREVIOUS QUESTION PAPERS FOR OS • CPP TUTORIALS 1
- 2. Recap In the last class, you have learnt • Operating System Services 2
- 3. Objectives On completion of this period, you would be able to know • Definition of system call • Types of system calls • Single user operating system structure • Multi user operating system structure 3
- 4. System calls • User writes a program for a specific purpose, using a specific language • The program that is written uses the hardware as an aid to accomplish the task • The kernel ( part of operating system and considered as heart of operating system ) controls and streamlines the access to the hardware 4
- 5. System calls • The applications that a user writes normally run on top of an operating system • In most operating systems, a program cannot access the resources directly • The programs run by the user, request the operating system to do a specific task on behalf of a program 5
- 6. System calls • For example, if the program wants to open a file, it request the operating system to “please open the file with file name” “/usr/read.txt” • The kernel replies to the request suitably • It opens the file if the calling application has permissions • If the application does not have permissions, the operating system simply ignores the request, giving back an error 6
- 7. System calls • The process of calling the kernel is referred to as system call invocation • Call to the kernel “please open the file” is known as a system call • The kernel calls appropriate device drivers to access the hardware • Figure (1) in next slide shows working of a system call 7
- 8. User program System call Kernel has control over Hardware resources Device driver Device Fig(1) System Call 8
- 9. Definition of System Call • System call can be defined as a method by which a program makes a request to the operating system • There are different system calls for performing different kinds of tasks 9
- 10. Types of System Calls Process control • end, abort • load, execute • create process, terminate process • get process attributes, set process attributes • wait for time • wait event, signal event • allocate and free memory 10
- 11. File Management • create file, delete file • open, close • read, write, reposition • get file attributes, set file attributes 11
- 12. Device Management • request device, release device • read, write, reposition • get device attributes, set device attributes • logically attach or detach devices 12
- 13. Information Maintenance • get time or date, set time or date • get system data, set system data • get process, file or device attributes • set process, file or device attributes 13
- 14. Communications • create, delete communication connection • send, receive messages • transfer status information • attach or detach remote devices 14
- 15. Operating System Structure • Operating system is a large and complex program • It has to be designed carefully so that it will function normally and can be modified easily • Generally operating system will not be implemented as a single system • Operating System will be divided into number of modules • Each module will occupy some portion of the system • Each module will have set of inputs, outputs and definite tasks to perform. The module may take assistance of other modules for performing the tasks 15
- 16. Operating System Structure • The structure of an operating system is dictated by the model employed in building them • An operating system model is a broad frame work that unifies many features and services • Operating systems are broadly classified into two categories based on their structure – Monolithic operating system – Layered operating system 16
- 17. Monolithic Operating System ( Single User ) • The components of monolithic operating system are organized randomly and any module can call any other module • The applications in the monolithic operating system are separated from the operating system itself • Operating System code runs in privileged mode (kernel mode) with access to system data and to the hardware 17
- 18. Monolithic Operating System ( single user ) • Applications runs in non privileged mode (called user mode) With a limited set of interfaces available with limited access to the system data • The monolithic operating system structure with separate user and kernel processor is shown in the Fig (2) in next slide 18
- 19. Application Application program program User mode Kernel mode System service OS Procedures Hardware Fig:2 Monolithic operating system 19
- 20. Layered Operating System ( Multiuser ) • The components of layered operating system are organized into modules and layers are placed on top of one another • Each module provide a set of functions that other module can call • Interface functions at any particular level can invoke services provided by lower layers but not the other way round • The layered operating system structure with hierarchical organization of modules is shown in Fig ( 3 ) in next slide 20
- 21. Application Application program program User mode Kernel mode System service File system Memory and I/O Device Management Processor scheduling Hardware Fig:3 Layered operating system 21
- 22. Advantages of Layered Structure • One advantage of a layered operating system structure is that each layer of code is given access to only the lower-level interface • That is in this approach, the Nth layer can access services provided by the (N-1)th layer and provide services to the (N+1)th layer • This structure also allows the operating system to be debugged starting at the lowest layer, adding one layer at a time until the whole system works correctly 22
- 23. Advantages of Layered Structure • Layering also make it easier to enhance the operating system • one entire layer can be replaced without affecting other parts of the system • Layered operating system deliver low application performance in comparison to monolithic operating system 23
- 24. Summary • The process of calling the kernel is referred to as system call invocation • The call to the kernel “please open the file” is known as a system call • The kernel calls appropriate device drivers to access the hardware • Operating system is a large and complex program • System call can be defined as a method by which a program makes a request to the operating system 24
- 25. Frequently Asked Questions 1. What is system call 2. What are the various types of systems calls 3. Explain the working of system calls 4. Explain Single user Operating System structure and Multiuser Operating System structure 25
- 26. Quiz 1. The components of layered operating system are organized into modules and layers them one on top of the other • Yes • No 26
- 27. Quiz 2. The components of monolithic operating system are organized randomly and any module can call any other module • Yes • No 27
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