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Sec6.2.1 operating systems


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Courtesy: Sir Nabeel Sabir's Share Folder

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  • 1. OperatingSystemsCS208
  • 2. What is Operating System? It is a program. It is the first piece of software to run after the system boots. It coordinates the execution of all other software. It provides various common services needed by users and applications.
  • 3. Operating System Definition Short definition: a program that manages a computer’s resources and acts as an intermediary between a user and those resources Computer User Application Operating system Application User Hardware Application
  • 4. Operating System (OS) Functions  For the User: Provides a user interface to let the user access hardware and software resources.  For the System: Manages all the system tasks and resources to provide security, and fair, efficient use.
  • 5. Hardware Kernel Operating Shell System Applications
  • 6. Operating System Goals  Execute user programs and make solving user problems easier.  Make the computer system convenient to use.  Use the computer hardware in an efficient manner.
  • 7. Parts of an Operating System  User Interface (Shell)  Menu  Command Language  Graphical User Interface (GUI)  System Security  Login control  Process Management  CPU Scheduler
  • 8. Parts of an Operating System  Memory Management  Swapper/Pager  Temporarily unused pages are stored on disk (swapped out)  When they are needed again, they are brought back into the memory (swapped in)  Garbage Collection  Resource Management  File Management  Device Management
  • 9. Operating System Terminology  Kernel  Software component that controls the hardware directly, and implements the core privileged OS functions.  Process  An executing program.
  • 10. Time Sharing  Allows multiple processes to run on the same computer, seemingly at the same time.  CPU is multiplexed among several processes that are kept in memory (the CPU is allocated to a process only if the process is in memory). A process is swapped in and out of memory to the disk as needed.  OS must manage process scheduling and provide memory protection to keep one program from crashing the system or corrupting other programs.
  • 11. CPU Time Slicingfor Time Sharing  Only ONE process can be running on the CPU at a time.  Each process is allocated a “slice” of time in the CPU.  When the time runs out, the process is interrupted, and another process is loaded into the CPU.  The act of giving each process a small slice of time to run is called time slicing.
  • 12. CPU Time SlicingCPU Main Memory Allocate CPU to Process 1 Process 1 Allocate CPU Current to Process 2 Process 2 Process Process 3 Allocate CPU to Process 3Repeat until all processes have completed.
  • 13. CPU execution modes  CPUs supports (at least) 2 execution modes:  User mode  The code of the user programs  Kernel (supervisor, privileged, monitor, system) mode  The code of OS  The execution mode is indicated by a bit in the processor status word (PSW) (a register in the CPU)
  • 14. Protecting Kernel mode OS code executes in the Kernel mode  Called via interrupts and system calls Only the OS code is allowed to be executed in the Kernel mode The user code must never be executed in the Kernel mode  The program counter (PC) is only set to point to the OS code when the CPU goes to the Kernel mode
  • 15. Interrupts  An interrupt is the way by which the hardware informs the OS of special conditions that require OS attention  Interrupts cause the CPU not to execute the next instruction  Instead, the control is passed to OS
  • 16. System Calls  A System Call is used by a process to request a service from the OS  Typical system calls  Open/read/write/close the file  Get the current time  Create a new process  Request more memory
  • 17. A typical scenario 1. OS executes and chooses (schedules) an process to run 2. Process runs • CPU executes process’ instructions • OS is not involved 3. The system clock interrupts the CPU • Clock interrupt handler is executed • The handler is an OS function
  • 18. A typical scenario (continued) 4. In the interrupt handler: Process’ time has expired. The OS chooses another process to run 5. Process runs • CPU executes process’ instructions • OS is not involved 5. The process performs a system call to read from a file.
  • 19. A typical scenario (continued) 7. The system call causes a trap into the OS  OS sets up the I/O  OS schedules another application to run 7. The third application runs…. NOTE: At any given time only ONE program can be running: Either the OS or a user process
  • 20. Parallel Systems Multi-processor systems with more than one CPU in close communication. Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory. Advantages of parallel system:  Economical  Increased throughput  Increased reliability
  • 21. Parallel Systems  Symmetric multiprocessing (SMP)  Each processor runs an identical copy of the operating system.  Many processes can run at once without performance deterioration.  Most modern operating systems support SMP  Asymmetric multiprocessing  Each processor is assigned a specific task; master processor schedules and allocates work to slave processors.  More common in extremely large systems
  • 22. Distributed Systems  Distribute the computation among several physical computers.  Each has its own CPU, local memory, stable storage, I/O paths connecting to the environment  Interconnections  Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines.  Systems cooperate to maintain shared state and coordinate global information
  • 23. Distributed Systems Advantages of distributed systems.  Inherent distribution  Speedup - improved performance due to load sharing  Fault tolerance/Reliability  Resource Sharing  Scalability  Flexibility
  • 24. Distributed Systems Network Operating System  provides file sharing  provides communication scheme  runs independently from other computers on the network Distributed Operating System  less autonomy between computers  gives the impression there is a single operating system controlling the network.