1. OS Fall ’ 02 Introduction Operating Systems Fall 2002

2. OS Fall ’ 02 What is Operating System?  It is a program!  It is the first piece of software to run after boot  It coordinates the execution of all other software User programs  It provides various common services needed by users and applications

3. OS Fall ’ 02 Today ’ s plan  Operating system functionality  Hardware support for the operating system  Course overview, bibliography, administrative questions

4. OS Fall ’ 02 The Operating System controls the machine User Application Operating System Hardware OS Kernel Hard ware gcc gdb emacs vi date grep xterm netscape diff

5. OS Fall ’ 02 A better picture Hardware Operating System Privileged instructions System calls Machine instructions Application One Hardware One Operating System Many applications

6. OS Fall ’ 02 A typical scenario 1.OS executes and schedules an application to run 2. Application runs CPU executes app ’ s instructions OS is not involved 3.The system clock interrupts the CPU Clock interrupt handler is executed The handler is the OS function

7. OS Fall ’ 02 A typical scenario (continued) 4. In handler: OS chooses another application to run Context switch 5. The chosen app. runs directly on the hardware 6. The app. performs a system call to read from a file

8. OS Fall ’ 02 A typical scenario (continued) 7.The sys. call causes a trap into the OS  OS sets up the things for I/O and puts the application to sleep  OS schedules another application to run 8.The third application runs Note: At any given time only one program is running: OS or a user application

9. OS Fall ’ 02 The running application state diagram Application code runs OS runs Sleep Ready To run Interrupt/ System call Wait for I/O completion I/O completed Schedule Resume execution of the app. code

10. OS Fall ’ 02 A question  The operating system gets an input, performs a computation, produces an output, and quits Yes or no? The answer: No  The operating system is a reactive program

11. OS Fall ’ 02 The OS is a reactive program  It is idly waiting for events  When an event happens, the OS reacts It handles the event Schedules another application to run  The event handling must take as little time as possible  Event types Interrupts and system calls

12. OS Fall ’ 02 The OS performs Resource Management  Resources for user programs CPU, main memory, disk space  OS internal resources Disk space for paging memory (swap space) Entries in system tables  Process table, open file table Statically allocated

13. OS Fall ’ 02 CPU management  How to share one CPU among many processes  Time slicing: Each process is run for a short while and then preempted  Scheduling: The decision about which application to run next

14. OS Fall ’ 02 Memory management  Programs need main memory frames to store their code, data and stack  The total amount of memory used by currently running programs usually exceed the available main memory  Solution: paging Temporarily unused pages are stored on disk (swapped out) When they are needed again, they are brought back into the memory (swapped in)

15. OS Fall ’ 02 The OS supports abstractions  Creates an illusion that each application got the whole machine to run on In reality: an application can be preempted, wait for I/O, have its pages being swapped out, etc …  A tree-like file system organization Disk controllers can only write/read blocks

16. OS Fall ’ 02 Hardware support for OS  Support for executing certain instructions in a protected mode  Support for interrupts  Support for handling interrupts  Support for system calls  Support for other services

17. OS Fall ’ 02 CPU execution modes  CPU has (at least) 2 execution modes: User mode Kernel mode  Supervisor mode, privileged mode, monitor mode, system mode  The execution mode is indicated by a bit in the processor status word (PSW)  Some CPU instructions are available only when executing in the kernel mode

18. OS Fall ’ 02 Kernel Mode  OS kernel is a collection of functions responsible for the most basic services OS kernel executes in the kernel mode  Privileged instructions: To load/store special CPU registers To map memory pages to the address space of a specific process Instructions to set the interrupt priority level Instructions to activate I/O devices

19. OS Fall ’ 02 Protecting Kernel mode  Is it possible for the user program to cause the CPU to enter kernel mode?  Yes: This must be possible (system call)  The problem: how to prevent the user program from executing privileged instructions?  Solution: change the program counter (PC) to point to the OS code upon switch

20. OS Fall ’ 02 Handling interrupts  Interrupts cause the CPU to enter kernel mode  The address of the kernel function to execute is loaded from the interrupt vector  The interrupt vector address and the interrupt numbering is a part of the hardware specification Handlers are registered during the boot

21. OS Fall ’ 02 Interrupt types (I)  Asynchronous interrupts are generated by external devices at unpredictable times Clock interrupt is the basis for time slicing:  Update the system time, preempt/schedule processes I/O device interrupt  Informs the OS about completion of a requested I/O

22. OS Fall ’ 02 Interrupt types (II)  Internal (synchronous) interrupts are generated synchronously by CPU as a result of an exceptional condition An error condition: the application is trying to perform an illegal operation:  E.g., division by 0, issuing a privileged instr., …  The handler typically kills the application A temporary problem:  E.g., the requested page is not in the memory  Handling: bring the page into the memory

23. OS Fall ’ 02 System calls  Used to request a service from the OS A collection of the system calls is the OS API Packaged as a library  Typical system calls Open/read/write/close the file Get the current time Create a new process Request more memory

24. OS Fall ’ 02 Handling system calls  An application executes a special trap (syscall) instruction Causes the CPU to enter kernel mode and set PC to a special system entry point (gate routine)  The gate routine address is typically stored in a predefined interrupt vector entry Intel architecture: int[80] A single entry serves all system calls (why?)

25. OS Fall ’ 02 An example open( “ /tmp/foo ” ): store the system call number and the parameters in a predefined kernel memory location; trap(); retrieve the response from a predefined kernel memory location; return the response to the calling application; trap(): PC:=&int[80]; // transfer control to the gate routine Gate routine: switch(sys_call_num) { case OPEN: … }

26. OS Fall ’ 02 Other hardware support  Translating virtual address into a physical address Assist in supporting the virtual memory abstraction  Support for “ used ” bits for memory pages Helps to determine which pages can be swapped out when needed

27. OS Fall ’ 02 What we are going to study  Performance evaluation (brief)  Process handling Process concept, scheduling, concurrency control  Memory management Paging, virtual memory  File system

28. OS Fall ’ 02 Advanced topics  Distributed systems Communication, networking, middleware  Other possible topics: Reliable distributed systems Real-time systems Parallel systems Modern storage architectures

29. OS Fall ’ 02 Bibliography  Notes by Dror Feitelson Will be published weekly  Operating System Concepts, by A. Silberschatz, P. Galvin, G. Gagne  Operating Systems Internals and Design Principles, by W. Stallings  See the notes for more references

30. OS Fall ’ 02 Next:  Performance evaluation