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Operating Systems Interview Questions - Part 3


Describe the Buddy system of memory allocation.

Free memory is maintained in linked lists, each of equal sized blocks. Any such block is of size 2^k. When some memory is required by a process, the block size of next higher order is chosen, and broken into two. Note that the two such pieces differ in address only in their kth bit. Such pieces are called buddies. When any used block is freed, the OS checks to see if its buddy is also free. If so, it is rejoined, and put into the original free-block linked-list.

What is time-stamping ?

It is a technique proposed by Lamport, used to order events in a distributed system without the use of clocks. This scheme is intended to order events consisting of the transmission of messages. Each system 'i' in the network maintains a counter Ci. Every time a system transmits a message, it increments its counter by 1 and attaches the time-stamp Ti to the message. When a message is received, the receiving system 'j' sets its counter Cj to 1 more than the maximum of its current value and the incoming timestamp Ti. At each site, the ordering of messages is determined by the following rules: For messages x from site i and y from site j, x precedes y if one of the following conditions holds....(a) if TiHow 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.

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.

In loading programs into memory, what is the difference between load-time dynamic linking and run-time dynamic linking ?

For load-time dynamic linking: Load module to be loaded is read into memory. Any reference to a target external module causes that module to be loaded and the references are updated to a relative address from the start base address of the application module. With run-time dynamic loading: Some of the linking is postponed until actual reference during execution. Then the correct module is loaded and linked.

What are demand- and pre-paging ?

With demand paging, a page is brought into memory only when a location on that page is actually referenced during execution. With pre-paging, pages other than the one demanded by a page fault are brought in. The selection of such pages is done based on common access patterns, especially for secondary memory devices.

Paging a memory management function, while multiprogramming a processor management function, are the two interdependent ?

Yes.

What is page cannibalizing ?

Page swapping or page replacements are called page cannibalizing.

What has triggered the need for multitasking in PCs ?

  • Increased speed and memory capacity of microprocessors together with the support fir virtual memory and
  • Growth of client server computing

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

  • Hardware abstraction layer
  • Kernel
  • Subsystems
  • System Services.

    Operating Systems Interview Questions - Part 4 >>>




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