Paging

Paging: 

  • The Technique of  mapping  cpu generated logical address to physical address is called as paging
  • Paging is implemented in the hardware level.
  • Paging is a memory management scheme that  eliminates the need for Contigous allocation of physical memory
  • It permits the physical address space  of a process to be non contigous. 
  • Paging is simple paging and multilevel paging

Important Points

  1. Logical address space (L.A.S)or virtual address space (V.A.S) is represented in  the form of words or bytes . Ex 512kB or 512W
  2. Logical Address (LA) or virtual Address(VA) is represented in the form of bits .Ex19bits
  3. Physical address space (P.A.S) is represented in the form of words or bytes .Ex 32kW or 32kB
  4. Physical address (P.A) is represented in the form of bits .Ex 17bits
  5.   L.A.S is divided in to equal size pages  

No. Of Pages(N)= L.A.S / page Size​  

6.P.A.S is divided in to equal size frames

No. Of Frames(f)= P.A.S / frame sige 

7. Page size =Frame size(always)

8.some of the pages of L.A.S is brought in to the P.A.S 

9.Page table =Tanslation Table and Frame number =Translation Bits

10.P=No.of bits required to represents  pages of L.A.S or page number 

11.No. of Bits required to represents Page size or word number of the page or Page offset 

12.F=No. of bits required to represent the frames of P.A.S or frame Number.

13.No. of bits required to represent  the frame size or word number of the frame

or frame offset

Page table overview

   

Important points for paging

  1. Whenever  the process is created ,paging will be applied on the process and page table will be created and base address of the page table will be stored in PCB(process control block).
  2. Paging with respect to every process and every process will have its own page table .
  3. Page table of processes will be stored in the main memory.
  4. There is no  external fragmentation  in the paging .
  5.  the internal fragmentation in the last page and internal  fragmentation is considered as (P/2),where 'P' is page size 
  6. Maintaining the page table is considered as overhead for the system 

Note :windows follows the page size of 4KB.

Note :The page table entry definitly contains frame number but some times along with the frame number it may contains additional bits .

 

 

 

Valid/Invalid or present /absent bit: This bit is used to identify whether the page is available or not in main memory. 

 '0' indicates page not present in M.M

'1' indicates page present in M.M

Dirty bit or modified bit: This bit is used to identify whether the page is modified  or not

'0' not modified

'1' modified

Reference bit: This bit is used to identified that how many times  the  page is reffered while execution

Page protection or permission bits:  This bits  are used for protection and security from the unauthorised access

Caching : This bit is used to enables or disable caching of the page .

 

Q. Consider a system with logical address=32 bits and physical address space=4KB and memory is byte addressable. Then what is the approx size of page table in Bytes?

 

Answer

Sol.

Logical address = 32 bits

Page size = 4KB, so page is recognized with 12 bits

no.  of pages = 2logical address / page size

                       = 232 /212

                      =220 pages

page size = frame size

Physical address space = 64MB, so it is recognized with 26bits

no. of frames = physical address space / page size

                        = 226/212

                        =214 frames, so it is recognized with 14 bits

Page table size = no. of pages * no. of bits required to recognize a frame

                            = (220 * 14) , in bits

                            = (220 * 14) /8 , in Bytes

                            =1835008 Bytes

                    

                            

 

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