Sample questions on OS

1. When a process requests for I/O, how many process switches take place?

 Answer:

Two. In the first switch, the process to be switched is taken out and the scheduler starts executing. Then the next process is brought to execution. So there are two process switches.

2. Consider a system consisting of four resources of the same type that are shared by three processes, each of which needs at most two resources. Show that the system is deadlock free.

 Answer:

Suppose the system is deadlocked. This implies that each process is holding one resource and is waiting for one more. Since there are three processes and four resources, one process must be able to obtain two resources. This process requires no more resources and, therefore it will return its resources when done.

3. Explain the sequence of events that happens when a page-fault occurs.

Answer:

When the operating system cannot load the desired page into memory, a page-fault occurs. First, the memory reference is checked for validity. In the case of an invalid request, the program will be terminated. If the request was valid, a free frame is located. A disk operation is then scheduled to read the page into the frame just found, update the page table, restart the instruction that was interrupted because of the page fault, and use the page accordingly.

4. Explain the usefulness of a modify bit.

Answer:

A modify bit is associated with each page frame. If a frame is modified (i.e. written), the modify bit is then set. The modify bit is useful when a page is selected for replacement. If the bit is not set (the page was not modified), the page does not need to be written to disk. If the modify bit is set, the page needs to be written to disk when selected for replacement.

5. If you were creating an operating system to handle files, what would be the six basic file operations that you should implement?

Answer:

 The six basic file operations include: creating a file, writing a file, reading a file, repositioning within a file, deleting a file, and truncating a file. These operations comprise the minimal set of required file operations.