Basic concepts of OS OS Structures

1. Basic concepts of OSOS Structures G.Anuradha

2. Basic concepts of OS • Processes • Deadlocks • Memory management • Input/output • Files • Security

3. Processes • Process is a program in execution • Associated with each process we have • Address space (executable program+data+stack) • List of memory locations (with max and min memory locations) • Registers(stack pointer, program counter, hardware registers) • Eg: Time Sharing System

4. Processes Contd. • Whenever a process is suspended temporarily the current state of the process is stored in a process table. • Interprocess communication is the Communication between different interrelated process • When communicating between processes of different computers a alarm signal is send to notify lost messages.

5. Processes Contd. • In UNIX processes, every user who logs in has a Unique Identification (UID). • Every process started has the UID of the person who started it • A child process has the UID as that of parent • Members of the same group can have Group Identification(GID) • One UID called Superuser has all the administrative powers.

6. Deadlocks

7. Deadlocks Contd… • When two or more processes are interacting, they can sometimes get themselves into a stalemate situation they cannot get out of. Such a situation is called a deadlock. • Processes in a computer can experience an analogous situation in which they cannot make any progress

8. Memory Management • Main memory holds the executing program • In simple OS only one executing program resides in the main memory • In sophisticated OS more than one executing program can reside in memory • To prevent them from interfering with each our some protection mechanism has to be provided by the OS

9. Memory Management Contd… • Managing the address space of the processes. • The executable program address space is less than the main memory address space for the program to reside in the memory • If the program address space is more than main memory address space a concept of virtual memory is used • Virtual Memory-OS keeps part of address space in memory and part in disk and shuttles between both

10. Input/Output • OS has to manage the input output devices like keyboards, monitors, printers…. • Every OS has I/O subsystem for managing the I/O devices • Some I/O software are device independent but some device drivers are specific to particular I/O devices

11. Files • One major function of the OS is to hide the peculiarities of the disks and other I/O devices and present the programmer with a nice, clean abstract model of device-independent files • System calls are needed • create files • remove files • read files • write files.

12. Files Contd… • OS has a concept of directory for grouping files together(UNIX) the path for file CS101 is /Faculty/Prof.Brown/Courses/CS101.

13. Files Contd… • Every file within the directory hierarchy is specified by its path name from the root directory. • Each process has a current working directory • Before a file can be read or written, it must be opened, at which time the permissions are checked • If permitted the system returns a file descriptor else an error code is returned

14. Special Files • Special files are provided in order to make I/O devices look like files. • Two kinds of special files exist • block special files :- used to model devices that consist of a collection of randomly addressable blocks (disks) • character special files:-used to model printers, modems, and other devices that accept or output a character stream.

15. Pipes • A pipe is a sort of pseudofile that can be used to connect two processes • If processes A and B wish to talk using a pipe, they must set it up in advance. • When process A wants to send data to process B, it writes on the pipe as though it were an output file. • Process B can read the data by reading from the pipe as though it were an input file.

16. Security • OS has to manage the system security so that files, are accessible to authorized users. • In UNIX security is provided by 9-bit binary protection code • The protection code consists of three 3-bit fields • Owner • Members of the owner’s group • Everyone • rwxr-x--x • r read • w write • x execute

17. Shells(exclusive for UNIX) • OS carries out system calls • Although Shells are not part of OS it makes use of many OS Features • When user logs in a shell is started • Shell has terminal as standard input and standard output • It starts out by typing the prompt, a character such as a dollar sign, which tells the user that the shell is waiting to accept a command

18. OS-Structure • Simple structure • Layered approach • Microkernels • modules

19. Simple Structure • Do not have a well-defined structure • MS-DOS – written to provide the most functionality in the least space • Not divided into modules • Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

20. MS-DOS Layer Structure(Simple Structure) initially written to provide the most functionality in the least space started small and grew beyond its original scope levels not well separated: programs could access I/O devices directly excuse: the hardware of that time was limited (no dual user/kernel mode)

22. Simple Layered Approach(UNIX) • Enormous functionality crammed into the kernel below the system call interface • No encapsulation but total visibility across system • Made of thick monolithic layers More functionality combined into one level system call interface to the kernel

23. Fully Layered Approach • The operating system is divided into a number of layers (levels), each built on top of lower layers. • The bottom layer (layer 0), is the hardware; The highest (layer N) is the user interface. • With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers • THE system (by Dijkstra), MULTICS, GLUnix, VAX/VMS

32. Solaris Modular Approach • Advantages :- • More flexible than a layered system • More efficient than the micro kernel approach because there is no message passing

34. Mac OS X Structure • Uses a hybrid structure • Mach provides • Memory management • Remote Procedure Calls • Interprocess communication facilities • Thread Scheduling • BSD(Berkeley Software Distribution) provides • Command line interface • Support for networking and file systems • Implementation of POSIX API

35. Types of OS: Operating System can also be classified as,- • Single User Systems • Multi User Systems

36. Single User Systems: • Provides a platform for only one user at a time. • They are popularly associated with Desk Top operating system which run on standalone systems where no user accounts are required. • Example: DOS

37. Multi-User Systems: • Provides regulated access for a number of users by maintaining a database of known users. • Refers to computer systems that support two or more simultaneous users. • Another term for multi-user is time sharing. • Ex: All mainframes and  are multi-user systems. • Example: Unix

38. University Questions • What are system calls and its types • Difference between monolithic and micro kernel • What is kernel and how do u design it? • Short notes on monitor • What is shared memory? Difference between shared memory and message passing • What are the various system calls for process management • Services provided by OS