Operating System Overview

1. Operating System Overview Chapter 2

2. Operating System Definition: • A program that controls the execution of application programs • An interface between applications and hardware

3. Operating System Objectives • Convenience • Makes the computer more convenient to use • Efficiency • Allows computer system resources to be used in an efficient manner • Ability to evolve • Permit effective development, testing, and introduction of new system functions without interfering with service

4. OS as a User/Computer Interface • Hardware and software used in providing applications to a user can be viewed in a layered or hierarchical fashion as shown in figure below:

5. OS as a User/Computer Interface.. • OS in 3rd layer masks the details of hardware from the programmer with a convenient interface for using the system. • OS act as a mediator,making it easier for the programmer and for application programs to access and use those facilities and services.

6. OS as a User/Computer Interface.. • OS typically provides services in the following areas: • Program development • Editors and debuggers • Program execution • Access to I/O devices • Controlled access to files • System access

7. OS as a User/Computer Interface.. • Error detection and response • Errors occur while computer run: • internal and external hardware errors • Internal and external hardware errors • Software errors • OS cannot grant request of application • OS provide response that clear the error condition with least impact to running applications

8. OS as a User/Computer Interface.. • Response: • Ending program that cause error • Retrying the operation • Reporting error to application

9. OS as a User/Computer Interface.. • Accounting • collect statistics • monitor performance • used to anticipate future enhancements • used for billing users

10. OS as Resource Manager • A computer is a set of resources for the movement, storage and processing of data and for the control of these functions. • OS is responsible for managing the resources. • Functions same way as ordinary computer software • It is program that is executed • The differences are OS directs the processor in the use of the other system resources and in the timing of its execution of other programs.

11. OS as Resource Manager.. • Operating system relinquishes control of the processor to execute other programs and then resumes control long enough to prepare the processor to do the next piece of work. • Figure 2.2 shows the main resources that are managed by the operating system. • A portion of the OS is stored in main memory. • This include the kernel or nucleus which contains the most frequently used functions in the OS and other portions of OS that currently in use.

13. Ease of Evolution of an OS • Major OS will evolved over the time because: • Hardware upgrades and new types of hardware • New services • Fixes • Fix a faults

14. Evolution of Operating Systems • Serial Processing • No operating system/ interact directly with computer hardware. • Machines run from a console with display lights and toggle switches, input device, and printer • This mode of operation could be termed serial processing because users have access to the computer in series.

15. Evolution of Operating Systems.. • How it works? • Programs in machine code were loaded via the input device (e.g.. Card Reader) • If there is an error (indicating by the lights) halt the program. • Then the programmer will proceed to examine processor registers and main memory to determine the cause of error. • If the program proceeds to normal completion, the output will be printed.

16. Evolution of Operating Systems.. • 2 main problems: • Scheduling • Used a hardcopy sign-up sheet to reserve machine time. • Problem arise when user sign up more that the time needed or can’t complete their job with the time reserve. • Setup time • Many steps involve in running a single program/job • If error occurred, user had to go back to the beginning setup step. • Much of the time is spending on setting up program.

17. Evolution of Operating Systems.. • Over the time, various system software tools were developed to attempt to make serial processing more efficient. • Libraries of common functions, linkers, loaders, debuggers and I/O driver routines.

18. Evolution of Operating Systems .. • Simple Batch Systems • Early machine was so expensive so it is important to maximize machine utilization  min the idle time. • To improve utilization, the concept of a batch OS was developed. • Central idea behind the simple batch-processing scheme was the used of software known as the monitor.

19. Evolution of Operating Systems.. • With monitor user have no longer direct access to the machine. • Monitors: • Software that controls the running programs • Batch jobs together • Program branches back to monitor when finished • Resident monitor is in main memory and available for execution

21. Evolution of Operating Systems.. • How monitor works: • Monitor reads jobs one at a time from the input device • Monitor places a job in the user program area • A monitor instruction branches to the start of the user program • Execution of user program continues until • End of program occurs • Error occurs • This will cause the CPU to fetch its next instruction from monitor.

22. Evolution of Operating Systems.. • With each job, instructions are included in a primitive form of job control language (JCL) • Special type of programming language used to provides instruction to the monitor • what compiler to use • what data to use

23. Evolution of Operating Systems.. • Example of job format using FORTRAN language: $JOB $FTN … FORTRAN Program … $LOAD $RUN … DATA … $END

24. Evolution of Operating Systems.. • With JCL: • Each read instruction (in user program) causes one line of input to be read • Causes input routine (OS) to be invoke • Checks for not reading JCL line • Skip to the next JCL line at completion of user program

25. Evolution of Operating Systems.. • In summary, the monitor or batch OS: • Simply a computer program • Relies on the ability of the processor to fetch instructions from the various portions of the main memory to alternately seize and relinquish control. • Relies also on available hardware to effectively alternate execution from various parts of memory

26. Evolution of Operating Systems.. • Desirable hardware features for batch OS: • Memory protection • do not allow the memory area containing the monitor to be altered by users programs • If it happen, processor should detect an error and transfer control to the monitor. • Monitor will abort the job, print out error message and load the next job.

27. Evolution of Operating Systems.. • Timer • prevents a job from monopolizing the system • Timer is set at the beginning of each job. • An interrupt occurs when time is expired • Privileged instruction • Can be executed only by the monitor • An interrupt occurs if a program tries these instructions. • E.g. I/O instructions. • Interrupts • Provides flexibility for relinquishing control to and regaining control from user programs

28. Evolution of Operating Systems.. • Uniprogramming: • Processor must wait for I/O instruction to complete before preceding

29. Evolution of Operating Systems.. • Multiprogramming: • When one job needs to wait for I/O, the processor can switch to the other job

30. Evolution of Operating Systems..

32. Evolution of Operating Systems.. • Example: JOB1 JOB2 JOB3 Type of job Heavy compute Heavy I/O Heavy I/O Duration 5 min. 15 min. 10 min. Memory required 50K 100 K 80 K Need disk? No No Yes Need terminal No Yes No Need printer? No No Yes

33. Evolution of Operating Systems.. • Effects of multiprogramming: Uniprogramming Multiprogramming Processor use 22% 43% Memory use 30% 67% Disk use 33% 67% Printer use 33% 67% Elapsed time 30 min. 15 min. Throughput rate 6 jobs/hr 12 jobs/hr Mean response time 18 min. 10 min.

34. Evolution of Operating Systems.. • Time Sharing • Desirable to provide a mode in which the user interacts directly with the computer, i.e. transaction processing • Batch multiprogramming does not support interactions with users so time sharing was developed. • Time sharing extends multiprogramming to handle multiple interactive jobs.

35. Evolution of Operating Systems.. • It is called time sharing because the processor’s time is shared among multiple users. • Multiple users simultaneously access the system through the terminals, with OS interleaved the execution of user program in a short burst of computation.

36. Evolution of Operating Systems.. • Both batch OS and time sharing use multiprogramming. • The differences between batch OS and time sharing is shown below.

38. Major Achievements • OS is the most complex pieces of software ever developed. • Five major theoretical advances in the development of OS: • Process • Memory Management • Information protection and security • Scheduling and resource management • System structure

39. Major Achievements – Process • Introduced to obtain a systematic way of monitoring and controlling program execution. • Definitions of process: • A program in execution • An instance of a program running on a computer • The entity that can be assigned to and executed on a processor • A unit of activity characterized by a single sequential thread of execution, a current state, and an associated set of system resources

40. Major Achievements – Process.. • Difficulties with Designing System Software • Improper synchronization • ensure a process waiting for an I/O device receives the signal • Failed mutual exclusion • Must permit only one program at a time to perform a transaction on a portion of data. • Nondeterminate program operation • program should only depend on input to it, not relying on common memory areas

41. Major Achievements – Process.. • Deadlocks • Two or more programs wait endlessly after each other to perform an operation. • Consists of three components • An executable program • Associated data needed by the program • Execution context of the program • All information the CPU needs to execute the process • All information the operating system needs to manage the process

42. Major Achievements – Process..

43. Major Achievements – Memory Management • Five principle storage management responsibilities: • Process isolation • Automatic allocation and management • Support for modular programming • Protection and access control • Long-term storage • The key contribution is virtual memory and file system facilities.

44. Major Achievements – Memory Management.. • Virtual memory: • It allows programs to address memory from a logical point of view without regard to the amount that is physically available • While a program is running only a portion of the program and data is kept in (real) memory • Other portions are kept in blocks on disk • the user has access to a memory space that is larger than real memory

45. Major Achievements – Memory Management.. • All memory references made by a program are to virtual memory which can be either • a linear address space • a collection of segments (variable-length blocks) • The hardware (mapper) must map virtual memory address to real memory address • If a reference is made to a virtual address not in memory, then • (1) a portion of the content of real memory is swapped out to disk • (2) the desired block of data is swapped in

46. Major Achievements – Memory Management.. • File System: • Implements long-term store (often on disk) • Information stored in named objects called files • a convenient unit of access and protection for OS • Files (and portions) may be copied into virtual memory for manipulation by programs

47. Major Achievements – Memory Management.. • Paging: • Allows process to be comprised of a number of fixed-size blocks, called pages • Virtual address is a page number and an offset within the page • Each page may be located any where in main memory • Real address or physical address in main memory

49. Major Achievements – Memory Management..

50. Major Achievements – Information Protection and Security • Protection of information is needed since the growth in the use of time-sharing systems and computer network. • Work in security and protection as it relates to OS can be roughly grouped into four categories. • Availability • Protecting the system against interruption