Operating Systems

1. Operating Systems Biswas Architecture

2. Text Book: 1. Operating System Concepts - Galvin References: 1. OS Book - William Stallings 2. Operating Systems - Crowley 3. UNIX Network Programming - Stevans

3. Content Process Management 1. Process 2. Threads 3. Scheduling 4. Synchronization 5. Deadlocks Memory Management 1. Overview 2. Virtual Memory Storage Management 1. File System Interface 2. File System Implementation I/O systems 1. I/O Systems 2. Mass-storage Structure Protection and Security 1. Protection 2. Security

4. Introduction What is an operating system? �Code� that: sits between programs & hardware sits between different programs sits between different users What does it do? Provides an orderly and controlled allocation of the processors, memories and I/O devices among the various programs competing for them.

5. Introduction An operating system makes computers simple. Resources: Allocation: finite resources and competing demands Protection: ensures some degree of safety and security Reclamation: voluntary at run time, implied at termination, involuntary and cooperative Virtualization: illusion of infinite, private resources Services: Abstraction 3. Convenience 2. Simplification 4. Standardization

6. Components of a Computer System

7. History of Operating Systems First generation 1945 � 1955 vacuum tubes, plug boards (no OS) Second generation 1955 � 1965 transistors, batch systems Third generation 1965 � 1980 ICs and multiprogramming Fourth generation 1980 � present personal computers, hand-held devices, sensors

8. History of Operating Systems

9. Computer Startup bootstrap program is loaded at power-up Typically stored in ROM or EPROM, generally known as firmware Initializes all aspects of system Loads operating system kernel and starts execution

10. Computer Startup

11. Operating System Architectures Today�s operating systems tend to be complex Provide many services Support variety of hardware and software Operating system architectures help manage this complexity Organize operating system components Specify privilege with which each component executes

12. Monolithic Architecture

13. Monolithic Architecture Monolithic operating system Every component contained in kernel Any component can directly communicate with any other Tend to be highly efficient Disadvantage is difficulty determining source of subtle errors

14. Monolithic Architecture Examples: Linux UNIX (BSD, Solaris) MS-DOS MAC-OS till 8.6

15. Layered Architecture

16. Layered Architecture Layered approach to operating systems Tries to improve on monolithic kernel designs Groups components that perform similar functions into layers Each layer communicates only with layers immediately above and below it Processes� requests might pass through many layers before completion System throughput can be less than monolithic kernels Additional methods must be invoked to pass data and control

17. Microkernel Architecture

18. Microkernel Architecture Microkernel operating system architecture Provides only small number of services Attempt to keep kernel small and scalable High degree of modularity Extensible, portable and scalable Increased level of intermodule communication Can degrade system performance

19. Microkernel Architecture Examples: MACH NT EROS MINIX

20. Computer System Organization Computer-system operation One or more CPUs, device controllers connect through common bus providing access to shared memory Concurrent execution of CPUs and devices competing for memory cycles

21. Computer System Operation I/O devices and the CPU can execute concurrently. Each device controller is in charge of a particular device type. Each device controller has a local buffer. CPU moves data from/to main memory to/from local buffers I/O is from the device to local buffer of controller. Device controller informs CPU that it has finished its operation by causing an interrupt.

22. Interrupts Suspends the normal sequence of execution

23. Simple Interrupt Processing

24. Multiple Interrupts

25. Multiple Interrupts

30. Direct Memory Access (DMA) DMA improves data transfer between memory and I/O devices Devices and controllers transfer data to and from main memory directly Processor is free to execute software instructions DMA channel uses an I/O controller to manage data transfer Notifies processor when I/O operation is complete Improves performance in systems that perform large numbers of I/O operations (e.g., mainframes and servers)

31. Direct Memory Access (DMA)

32. Memory-Storage Hierarchy

33. Operating System Structure Multiprogramming needed for efficiency Single user cannot keep CPU and I/O devices busy at all times Multiprogramming organizes jobs (code and data) so CPU always has one to execute A subset of total jobs in system is kept in memory One job selected and run via job scheduling When it has to wait (for I/O for example), OS switches to another job

34. Operating System Structure Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing Response time should be < 1 second Each user has at least one program executing in memory ?process If several jobs ready to run at the same time ? CPU scheduling If processes don�t fit in memory, swapping moves them in and out to run Virtual memory allows execution of processes not completely in memory

35. Operating System Structure Real-time systems rigid time requirements are placed, often used as a control device for a dedicated application. Two flavors: Hard real-time system: guarantees task to be completed in time. No virtual memory. Hence time sharing is not possible. Soft real-time system: critical real-time task gets priority over other tasks, delays are bounded.

36. Operating-System Operations Dual-mode operation allows OS to protect itself and other system components User mode and kernel mode Mode bit provided by hardware provides ability to distinguish when system is running user code or kernel code some instructions designated as privileged, only executable in kernel mode system call changes mode to kernel, return from call resets it to user

37. Transition from User to Kernel Mode Timer to prevent infinite loop / process hogging resources Set interrupt after specific period Operating system decrements counter When counter zero generate an interrupt Set up before scheduling process to regain control or terminate program that exceeds allotted time

38. End of Session 2