Operating Systems Questions

A shell is an interactive user interface to an operating system services that allows an user to enter commands as character strings or through a graphical user interface. The shell converts them to system calls to the OS or forks off a process to execute the command. System call results and other information from the OS are presented to the user through an interactive interface. Commonly used shells are sh,csh,ks etc.

Distributed systems work in a network. They can share the network resources,communicate with each other.

1. Mutual exclusion (where at least one resource is non-sharable)

2. Hold and wait (where a process holds one resource and waits for other resource)

3. No preemption (where the resources can’t be preempted)

4. Circular wait (where p[i] is waiting for p[j] to release a resource. i= 1,2,…n

j=if (i!=n) then i+1

else 1 )

It is the transfer of sufficient amount of the state of process from one machine to the target machine.

Scheduling is the activity of the deciding when process will receive the resources they request.

FCS ---> FCSFS stands for First Come First Served. In FCFS the job that has been waiting the longest is served next.

Round Robin Scheduling--->Round Robin scheduling is a scheduling method where each process gets a small quantity of time to run and then it is preempted and the next process gets to run. This is called time-sharing and gives the effect of all the processes running at the same time

Shortest Job First ---> The Shortest job First scheduling algorithm is a nonpreemptive scheduling algorithm that chooses the job that will execute the shortest amount of time.

Priority Scheduling--->Priority scheduling is a scheduling method where at all times the highest priority process is assigned the resource.

It does not have the features like

1. ability to support multiple processors


2. virtual storage


3. source level debugging

Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. CPU scheduling decisions may take place when a process: 1.Switches from running to waiting state. 2.Switches from running to ready state. 3.Switches from waiting to ready. 4.Terminates. Scheduling under 1 and 4 is non-preemptive. All other scheduling is preemptive.

• Program execution


• I/O operations


• File system manipulation


• Communication


• Error detection


• Resource allocation


• Accounting


• Protection