Difference between Paging and Segmentation

Characteristics	Paging	Segmentation
Definition	Paging breaks physical memory Into fixed-sized blocks called frames and logical memory into blocks of the same size called pages. When a process is to be  executed, its pages are loaded into any available memory frames from the backing store. The backing store is divided into fixed sized blocks that are of the same size as the memory frames.	Segmentation is the division of computer’s primary memory into segments or sections. In segmentation, a reference to a memory location includes a value that identifies a segment and an offset within that segment.
Length	All pages are divided into equal length.	The size of a memory segment is generally not fixed and may be as small as a single byte.

Demand Paging

Whenever a program is executed it is required to be loaded into main memory. This loading can be done by two methods:

1) By loading the entire program into main memory. But Problem with this technique is that if user wants to select limited options, whole program will be loaded regardless of user selection.

2) Another Approach is to load only those pages which are currently needed. “The Technique of loading in which only those pages are loaded into main memory which is currently needed is called Demand Paging”. The pages which are not required are not loaded into main memory.

This technique has the advantage that less memory is consumed and more processes can be loaded at the same time which utilizes more resources. Demand paging is very similar to swapping. The whole program resides in secondary storage and when process execution is required, it is swapped into main memory and in spite of loading whole program only currently required pages ware swapped.

To perform this task swapper is used which is called pager. It does not swap any program until it is required in the main memory. Additional hardware is also required to check that which pages are in memory and which pages are on disk.