Recents in Beach

Paging

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·   Paging is a memory management scheme that permits the physical address space of a process to be non contiguous.

 Divide physical memory into fixed-sized blocks called frames (size is power of 2, for example 512 bytes).

· Divide logical memory into blocks of 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.

·  Every address generated by the CPU is divided into two parts:

page number (p) and

 page offset (d).

The page number is used as an index into a page table. The page table contains the base address of each page in physical memory. This base address is combined with the page offset to define the physical memory address that is sent to the memory unit.



            The paging model of memory is shown in below figure. The page size is defined by the hardware. 
The size of a page is typically of a power of 2, varying between 512 bytes and 16 MB per page, depending on the computer architecture. The selection of a power of 2 as a page size makes the translation of a logical address into a page number and page offset particularly easy. 
If the size of logical address is 2m, and a page size is 2n addressing units, then the high order m-n bits of a logical address designate the page number, and the n low order bits designate the page offset 



·  Keep track of all free frames.

· To run a program of size n pages, need to find n free frames and load program.

· Set up a page table to translate logical to physical addresses.

· Internal fragmentation may occur.





Example

               Let us consider the main memory size 16 Kb and Frame size is 1 KB therefore the main memory will be divided into the collection of 16 frames of 1 KB each.
There are 4 processes in the system that is P1, P2, P3 and P4 of 4 KB each. Each process is divided into pages of 1 KB each so that one page can be stored in one frame.
Initially, all the frames are empty therefore pages of the processes will get stored in the contiguous way.
Frames, pages and the mapping between the two is shown in the image below.




                  Let us consider that, P2 and P4 are moved to waiting state after some time. Now, 8 frames become empty and therefore other pages can be loaded in that empty place. The process P5 of size 8 KB (8 pages) is waiting inside the ready queue.
Given the fact that, we have 8 non contiguous frames available in the memory and paging provides the flexibility of storing the process at the different places. Therefore, we can load the pages of process P5 in the place of P2 and P4.





Memory Management Unit : 

                The purpose of Memory Management Unit (MMU) is to convert the logical address into the physical address. The logical address is the address generated by the CPU for every page while the physical address is the actual address of the frame where each page will be stored.
When a page is to be accessed by the CPU by using the logical address, the operating system needs to obtain the physical address to access that page physically.
The logical address has two parts.
  1. Page Number
  2. Offset
Memory management unit of OS needs to convert the page number to the frame number.

Example

Considering the above image, let's say that the CPU demands 10th word of 4th page of process P3. Since the page number 4 of process P1 gets stored at frame number 9 therefore the 10th word of 9th frame will be returned as the physical address.

What is Paging Protection?

The paging process should be protected by using the concept of insertion of an additional bit called Valid/Invalid bit. 
Paging Memory protection in paging is achieved by associating protection bits with each page. 
These bits are associated with each page table entry and specify protection on the corresponding page.

Advantages of Paging

  • Easy to use memory management algorithm
  • No need for external Fragmentation
  • Swapping is easy between equal-sized pages and page frames.

Disadvantages of Paging


  • May cause Internal fragmentation
  • Complex memory management algorithm
  • Page tables consume additonal memory.
  • Multi-level paging may lead to memory reference overhead.


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  1. What is an Operating System ?
  2. Discuss the structure off OS ?
  3. Explain type of OS?
  4. Explain Function of OS?
  5. Explain OS Services ?
  6. What do mean by system call ?List different type ofsystem call available ?

  1. what is process ? and Characteristics ?
  2. What is different process state? explain the same in details?
  3. write short note on user level and kernal level threads?
  4. explain what is thread and its type ?
  5. explain scheduler ? (short term,medium term,and long term)
  6. state and explain scheduling criteria ?
  7. Explain scheduling algorithm ? [ FCFS,SJF,PRIORITY,ROUND ROBINE.]    

  1. What is process synchronization ? explain critical section problem and race condition ?
  2. what is Race Condition ?
  3. what is critical section problem?
  4. explain classical problem of synchronization?
  5. explain bounded - buffer problem?
  6. explain reader - writer problem ?
  7. explain Dining Philosophers Problem ?
  8. explain semaphores ? its type ?

  1.  What is deadlock ?
  2. What are the 4 condition to produce deadlock ?
  3. explain methods of handling deadlock ?
  4. explain in detail deadlock prevention ?
  5. write short note on deadlock avoidance ?
  6. explain deadlock detection ? 
  7. explain Banker algorithm with example ? 

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