Operating System

51.Define logical address and physical address. 

An address generated by the CPU is referred as logical address. An address seen by the memory unit that is the one loaded into the memory address register of the memory is commonly referred to as physical address.

52.What is logical address space and physical address space? 

The set of all logical addresses generated by a program is called a logical address space; the set of all physical addresses corresponding to these logical addresses is a physical address space.

53.What is the main function of the memory-management unit? 

The runtime mapping from virtual to physical addresses is done by a hardware device called a memory management unit (MMU).

54.Define dynamic loading. 

To obtain better memory-space utilization dynamic loading is used. With dynamic loading, a routine is not loaded until it is called. All routines are kept on disk in a relocatable load format. The main program is loaded into memory and executed. If the routine needs another routine, the calling routine checks whether the routine has been loaded. If not, the relocatable linking loader is called to load the desired program into

memory.

55.Define dynamic linking. 

Dynamic linking is similar to dynamic loading, rather that loading being postponed until execution time, linking is postponed. This feature is usually used with system libraries, such as language subroutine libraries. A stub is included in the image for each library-routine reference. The stub is a small piece of code that indicates how to locate the appropriate memory-resident library routine, or how to load the library if the routine is

not already present.

56.What are overlays?

To enable a process to be larger than the amount of memory allocated to it, overlays are used. The idea of overlays is to keep in memory only those instructions and data that are needed at a given time. When other instructions are needed, they are loaded into space occupied previously by instructions that are no longer needed.

57.Define swapping. 

A process needs to be in memory to be executed. However a process can be swapped temporarily out of memory to a backing tore and then brought back into memory for continued execution. This process is called swapping.

58.What are the common strategies to select a free hole from a set of available holes? 

The most common strategies are

a. First fit

b. Best fit

c. Worst fit

59.What do you mean by best fit? 

Best fit allocates the smallest hole that is big enough. he entire list has to be searched, unless it is sorted by size. his strategy produces the smallest leftover hole.

60.What do you mean by first fit? 

First fit allocates the first hole that is big enough. earching can either start at the beginning of the set of holes orwhere the previous first-fit search ended. Searching can be stopped as soon as a free hole that is big enough is found.