Operating systems
- 4. Components of OS • Shell Vs Kernel • Components of kernel • File manager • Device drivers • Memory manager • Scheduler • Dispatcher
- 5. File Manager • File manager maintains records of all the files stored in mass storage, including where each file is located, which users are allowed to access the various files, and which portions of mass storage are available for new files or extensions to existing files • Any access to a file by other software units is obtained at the discretion of the file manager • Folder, directory, sub-directory, directory path
- 6. Device Drivers • Device drivers which are the software units that communicate with the controllers (or at times, directly with peripheral devices) to carry out operations on the peripheral devices attached to the machine • Each device driver is uniquely designed for its particular type of device • Control Panel System Device Manager
- 7. Memory Manager • The memory manager coordinates the use of the computer’s main memory for various programs • Single task vs multitasking • Memory areas no longer occupied • Scenario : the total main memory space required exceeds the space actually available in the computer. • Solution: Paging, Virtual Memory
- 8. Scheduler • Program vs Process • The activity of executing a program under the control of the operating system is known as a Process • Associated with a process is the current status of the activity, called the Process state • To keep track of all the processes, the Scheduler maintains a block of information in main memory called the Process table • Process states : Running, Runnable in Memory (Ready) , Runnable Swapped, Sleeping in memory (Waiting), Sleeping swapped • Threads
- 9. Scheduler • The scheduler maintains a record of the processes present in the computer system, introduces new processes to this pool, and removes completed processes from the pool. Thus when a user requests the execution of an application, it is the scheduler that adds the execution of that application to the pool of current processes • Process table
- 10. Dispatcher • The dispatcher is the component of the kernel that oversees the execution of the scheduled processes by multiprogramming • Time slice • Process switch (context switch) • Interrupt
- 11. Scheduling Algorithms • First In First Out (FIFO) • Shortest Job First (SJF) • Shortest Remaining Job First (SRJF) • Round Robin (RR) • Priority based • Multilevel queues • Multilevel feedback queues
- 12. Semaphores • Print example with set and clear state • Issue with Flag system • Disable interrupt instruction and enable interrupt instruction • Test and set • A properly implemented flag, as just described, is called a semaphore • Critical region, mutual exclusion
- 13. Deadlocks
- 14. Deadlock conditions 1. mutual exclusion, i.e. the resources involved can only be accessed by one process at a time 2. hold and wait, a process can keep access to a resource while it waits for another resource to be freed 3. no pre-emption, i.e. the operating system will not force a process to release a resource once it has obtained access 4. circular wait, i.e. there is a circular chain of (two or more) processes that wait for each other
- 15. Deadlock Solutions • Deadlock detection and correction • Deadlock avoidance • Spooling