1. 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.

2. 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.

3. Define latency, transfer and seek time with respect to disk I/O.

Seek time is the time required to move the disk arm to the required track. Rotational delay or latency is the time it takes for the beginning of the required sector to reach the head. Sum of seek time (if any) and latency is the access time. Time taken to actually transfer a span of data is transfer time.

4. How are the wait/signal operations for monitor different from those for semaphores?

If a process in a monitor signal and no task is waiting on the condition variable, the signal is lost. So this allows easier program design. Whereas in semaphores, every operation affects the value of the semaphore, so the wait and signal operations should be perfectly balanced in the program.

5. What are the four layers that Windows NT have in order to achieve independence?

•  Hardware abstraction layer
•  Kernel
•  Subsystems
•  System Services

6. What is process spawning?

When the OS at the explicit request of another process creates a process, this action is called process spawning.

7. What is FtDisk?

It is a fault tolerance disk driver for Windows NT.

8. What are the possible threads a thread can have?

•  Ready
•  Standby
•  Running
•  Waiting
•  Transition
•  Terminated

9. What level of security does Windows NT meets?

C2 level security

10. In the context of memory management, what are placement and replacement algorithms?

Placement algorithms determine where in available real-memory to load a program. Common methods are first-fit, next-fit, best-fit. Replacement algorithms are used when memory is full, and one process (or part of a process) needs to be swapped out to accommodate a new program. The replacement algorithm determines which are the partitions to be swapped out.

11. What is process migration?

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

12. What is cycle stealing?

We encounter cycle stealing in the context of Direct Memory Access (DMA). Either the DMA controller can use the data bus when the CPU does not need it, or it may force the CPU to temporarily suspend operation. The latter technique is called cycle stealing. Note that cycle stealing can be done only at specific break points in an instruction cycle.

13. Explain the popular multiprocessor thread-scheduling strategies.

Load Sharing : Processes are not assigned to a particular processor. A global queue of threads is maintained. Each processor, when idle, selects a thread from this queue. Note that load balancing refers to a scheme where work is allocated to processors on a more permanent basis.

Gang Scheduling : A set of related threads is scheduled to run on a set of processors at the same time, on a 1-to-1 basis. Closely related threads / processes may be scheduled this way to reduce synchronization blocking, and minimize process switching. Group scheduling predated this strategy.

Dedicated processor assignment : Provides implicit scheduling defined by assignment of threads to processors. For the duration of program execution, each program is allocated a set of processors equal in number to the number of threads in the program. Processors are chosen from the available pool.

Dynamic scheduling : The number of thread in a program can be altered during the course of execution.

14. When does the condition 'rendezvous' arise?

In message passing, it is the condition in which, both, the sender and receiver are blocked until the message is delivered.

15. What are the stipulations of C2 level security?

C2 level security provides for:

•  Discretionary Access Control
•  Identification and Authentication
•  Auditing
•  Resource reuse