Here are some questions of operating systems which are generally asked in practical exams and vivas or in operating system endsem and midsem paper. Also these questions covers the basic of o.s.,so are useful for better understanding the concept of operating system.

1.       Explain the concept of Reentrancy.

It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record. Generally, the activation record is kept on the stack.

Note: A reentrant procedure can be interrupted and called by an interrupting program, and still execute correctly on returning to the procedure.

2.       List the Coffman's conditions that lead to a deadlock.

       Mutual Exclusion: Only one process may use a critical resource at a time.

       Hold & Wait: A process may be allocated some resources while waiting for others.

       No Pre-emption: No resource can be forcible removed from a process holding it.

       Circular Wait: A closed chain of processes exist such that each process holds at least one resource needed by another process in the chain.


3.   When is a system in safe state?

The set of dispatchable processes is in a safe state if there exists at least one temporal order in which all processes can be run to completion without resulting in a deadlock.


4.   What is busy waiting?

The repeated execution of a loop of code while waiting for an event to occur is called busy-waiting. The CPU is not engaged in any real productive activity during this period, and the process does not progress toward completion.


5.   What are local and global page replacements?

Local replacement means that an incoming page is brought in only to the relevant process' address space. Global replacement policy allows any page frame from any process to be replaced. The latter is applicable to variable partitions model only.