Operating systems date 26 3 14 by b harishma 11cs30009
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Operating Systems Date:26/3/14 By B Harishma 11CS30009. Paging. Logical Address  Address generated by the CPU Divide logical memory into blocks of same size called PAGES . Physical Address  Address seen by the memory unit Physical address of a process can be non contiguous.

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Operating Systems Date:26/3/14 By B Harishma 11CS30009

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Operating systems date 26 3 14 by b harishma 11cs30009

Operating Systems

Date:26/3/14

By

B Harishma

11CS30009


Paging

Paging

  • Logical Address  Address generated by the CPU

    Divide logical memory into blocks of same size called PAGES.

  • Physical Address  Address seen by the memory unit

    Physical address of a process can be non contiguous.

    Divide physical memory into fixed size blocks called FRAMES.

    “Page table is used to translate logical address to physical address”


Operating systems date 26 3 14 by b harishma 11cs30009

  • Size of frame: power of 2 with a range between 512 bytes to 16 Mbytes.

  • To run a program of size N pages, one need to have N free frames and the program is then loaded.

0

1

2

0

3

1

4

2

3

5

Physical

Memory

Logical Memory

6

Page Table

Task is to perform mapping.

7


Operating systems date 26 3 14 by b harishma 11cs30009

MMU

CPU

RAM

LA

PA

Address Translation scheme:

  • Logical address space= 2mPage Size = 2n

  • Then the high-order bits of logical address designates the page number and the n low-order bits designates the page offset.

    Page number Page offset

    m-n n

    p is an index into the page table contains base address of each page in physical memory.

    d is the displacement within the page and when combined with the physical memory address that is sent to the memory unit.

p d


Operating systems date 26 3 14 by b harishma 11cs30009

Physical address

Logical address

CPU

f000....0000

f

Paging Hardware:

p d

f d

f111....1111

f

p

Page Table

Physical Memory


Paging example

Paging Example

External Fragmentation: Holes are scattered in memory.

Paging scheme does not suffer from external fragmentation.

Reason : There is no need of contiguous memory.

But, it does suffer from --------------------INTERNAL FRAGMENTATION

Reason : Pages and frames are basic units and cannot be divided further.


Operating systems date 26 3 14 by b harishma 11cs30009

Small page size  low page sizesize of page table increases  I/O operations from disk increases initially slow

User’s View: Process Contained in single contiguous memory space.

By implementation, process can only access its memory--- PROTECTION.


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