PAGING

  • External Fragmentation is avoided by using paging technique.

  • Paging is the memory management scheme in which the physical memory allocated to a process is non contiguous.

  • In this approach, Physical Memory is divided into fixed size blocks called frames.

  • The Logical Memory is divided into fixed size blocks called pages.

  • The size of a page is same as that of a frame.

  • Whenever a process is to be executed, its pages are placed or loaded into any available memory frames. It means a page is mapped into a frame.

  • Individual pages and frames are recognized by unique numbers as page number and frame number respectively.

  • Thus, a page number forms a part of logical address and a frame number forms a part of a physical address.

  • Logical address space of a process can be non-contiguous and a process is allocated physical memory whenever the free memory frame is available. Operating system keeps track of all free frames.

  • Address generated by CPU is divided into:

    • Page number (p): Page number is used as an index into a page table which contains base address of each page in physical memory.

    • Page offset (d): page offset is combined with base address to define the physical memory address.

Advantages of Paging:

  • It does not suffer from external fragmentation.

  • It achieves a higher degree of multiprogramming as compared to contiguous storage allocation methods.

  • It also allows the sharing of common code among the processes.

Disadvantages of Paging:

  • The disadvantages of paging are extra resource consumption, memory overhead for storing page tables.

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