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Operating System.ppt
- 1. Operating System Structure 1.Multi programming and multitasking are 2 types 2. Simple structure 3. Monolithic structure 4. Layered Structure 5. Micro kernels 6. Modules Ms.C.Nithiya Computer Science and Engineering KIT-Kalaignarkarunanidhi Institute Of Technology
- 2. Simple Structure --MS-DOS Very old and it has Microsoft Disk os Base is hardware, device drivers has access to hardware. Hardware can be accessed by all the above so the interfaces are not well separated. Application program can directly read and write to hardware. When user program fails causes entire system to crash. Not well protected, well structured and not good structure. Ms DOS was written on Intel 8088 does not provide dual mode(security and protection) or hardware protection.
- 3. MONOLITHIC STRUCTURE Usedby earlier UNIX OS Kernel and system programs grouped, below system call and above hardware is kernel. Everything packed in one level, too many functions to maintain. If there is a change need to modify entire kernel(debug). System difficult to maintain and implement.
- 4. Layered Approach Zerois hardware, Layer 1 to n is user interface. Broken down function into several layers Easy to implement and debug. One layer has problem, if Scheduling layer gives problem, look at that layer and debug. Which is top and which layer below is main criteria. Backing store is performed (disk , space used by VM) should be below memory management layer. Not very efficient compared to other structures.
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- 6. Microkernel System Structure Micro kernel is present instead of entire kernel system. Remove all non essential components from kernel and implement as system and user level programs. Micro kernel provide core functionalities of kernel above to which device drivers, file services, process service, virtual memory are placed under user level. Micro kernel provide communication between client program and these services . Client request for any service micro kernel gives access through message passing, runs in user mode. As function runs in User mode entire system does not get crashed. Performance decrease as there is system overhead while passing message all time.
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- 8. Modules Object orientedtechniques to create kernel. Other function should be loaded to the kernel either at boot time or run time Kernel section has defined and protected interfaces , protect from things don’t need to access. More flexible than a layered system. They are loaded dynamically directly into kernel.
- 9. CPU Scheduling Assigntime for doing a task Switching CPU among process OS can make computer productive. Introduce CPU scheduling – Multi programmed Which algorithm work better? Difference of algorithm need to be known In single processor one process run at time. Any other must wait and rescheduled.
- 10. CPU SCHEDULING Aprocess is executed until it must wait completion of I/o operation. CPU sets idle when processor inside CPU uses I/o operation and sits inside CPU unit. CPU does no useful work so productivity gets reduced. CPU and I/o Burst cycles – Process execution has CPU execution and I/o execution.
- 11. CPU Scheduling Whenprocess is under CPU execution for some time it is CPU Burst CPU waits for I/o operation to complete is i/o burst Process execution– CPU burst I/o Burst CPU Burst….. At final CPU burst system request to terminate execution.
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- 13. PRE-EMPTIVE AND NON-PREEMPTIVE SCHEDULING Ways in which scheduling takes place. CPU SCHEDULER – When CPU is idle , os select One of processes in ready queue , in a process p1,p2,p3,p4 …p3 is selected from set to place in ready queue to run. Selection of process is done by Short-term scheduler(ready queue to CPU through i/o operation. DISPATCHER– CPU control is given to selected process.
- 14. PRE-EMPTIVE AND NON-PREEMPTIVESCHEDULING Time taken for dispatcher to stop one process and start another running is Dispatch Latency CPU scheduling decisions takes place under 4 categories. Process state switches from Running (Already using CPU)to Waiting state (I/o operation) Interrupt occurs, Running ( Process hault, goes back to ready to get CPU to complete) to Ready state
- 15. PRE-EMPTIVE AND NON-PREEMPTIVE SCHEDULING Waiting state(has any i/o request) to ready state(continue its execution) when i/o operation occur. When process terminates CPU assigns to another process. Based on CPU Allocation, 1 & 4 considered as no choice (CPU should be assigned ) 2 & 3 there is a choice process gets hault, CPU assigned to new process or old one . Scheduling under 1 &4 is Non preemptive. Pre emptive , CPU can be taken away from process before it completes its execution . When higher priority process enters switching from non preemptive to pre emptive occurs.
- 16. SCHEDULING CRITERIA CPUutilization, Through put, Turn around time , waiting time, Response time are 5 criteria need to be known before scheduling. CPU utilization – We need to keep the CPU as busy as possible. CPU should not be idle. Through put – One measure of work is calc as number of processes that are completed per time unit Turn around time – Sum of the periods spent waiting to get into memory, waiting in ready queue, executing on CPU and doing I/o. From submission to completion. Waiting time – Sum of the period spent waiting in ready queue. It does not depend on scheduling process but the length of process is taken into account .
- 17. SCHEDULING CRITERIA Responsetime – Time from the submission of request until the first response is produced. Time it takes to start responding not the time it takes to output the response. Turn around time is generally limited by the speed of output time in fast system. So we go for response time. Video output – first it start processing and takes time to give output.