Operating Systems

1. Operating Systems CS208

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. Computer Application User Operating system User Hardware Application Operating System Definition • Short definition: • a program that manages a computer’s resources and acts as an intermediary between a user and those resources 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. Operating 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 Slicing for 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. Allocate CPU to Process 1 Allocate CPU to Process 2 Allocate CPU to Process 3 CPU Time Slicing CPU Main Memory Process 1 Current Process Process 2 Process 3 Repeat 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. 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

18. 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

19. 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

20. Distributed Systems • Advantages of distributed systems. • Inherent distribution • Speedup - improved performance due to load sharing • Fault tolerance/Reliability • Resource Sharing • Scalability • Flexibility

21. 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.