Virtual Memory

1
Virtual Memory

• BY
• JEMINI ISLAM

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What is Virtual Memory

• Virtual memory is a memory management system
  that gives a computer the appearance of having
  more main memory than it really has.

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Reasons for using virtual memory

• To free programmers from the need to carry out
  storage allocation and to permit efficient
  sharing of memory space among different users.
• To make programs independent of the configuration
  and the capacity of the memory systems used
  during their execution.
• To achieve the high access rates and low cost per
  bit that is possible with a memory hierarchy.

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2 Main Advantages of Virtual Memory

• 1. Main memory is used more efficiently
• 2.Programs that are bigger than
• main memory can still be executed.

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Implementation of Virtual Memory System

• Paging - Fixed-size blocks
• Segmentation- Variable size blocks
• Paged Segmentation Combination of two systems.

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Paging

• Paging systems use fixed-length blocks called
  pages and assign them to fixed regions of
  physical memory called page frames.

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Page Table
Page Table
Frame Number
Page Number
4
6
2
5
0
1
2
3
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Pages

• Example Each page has the same length, typically
  a power of 2.

Code
Data


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Advantage of Paging

• The main advantage of paging is that memory
  allocation is greatly simplified, since an
  incoming pages can be assigned to any available
  page frame which is also known as fully
  associative.

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Memory Mapping and Page Table

• Memory Mapping With virtual memory, the CPU
  produces virtual addresses that are translated by
  combination of hardware and software to physical
  addresses, which access main memory. This is
  called memory mapping or address translation.
• Page Table Translation between virtual addresses
  and physical addresses is done via a page table.

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The Mapping of a Virtual Addressto a Physical
Address Via a PageTable
Virtual address
Main memory
Virtual page number Page offset
Page table
Physical Address
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Techniques for Fast Address Translation

• To reduce address translation time, computers use
  a cache dedicated to these address translation,
  which is called a translation look-aside buffer
  or TLB.

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Structure of a Virtual Memory System








Physical block Address
TLB

Memory Map




Virtual Block Address

Logical (virtual) Address
Displacement

Physical Address
Main Memory
Secondary Memory
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Representative Organization of a Page Table
Page Page Presence
Change Access Address Frame
bit P bit C
rights
A 0
1 0
R, X
C D6C7T9
0
R, W, X E
24 1
1
R, W, X F 16
1 0
R
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Selecting Page Size

• Choosing a page size is a question of balancing
  forces that favor a larger page size versus those
  favoring a smaller size.
• They both have some advantages and disadvantages
  

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Advantage of Choosing Large Page Size

• The size of the page table is inversely
  proportional to the page size Memory
• can therefore be saved by making the pages
  bigger
• Transferring larger pages to or from secondary
  storage, possibly over a network, is more
  efficient than transferring smaller pages.

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Advantage Of Choosing Small Page Size

• The main reason for choosing smaller page is
  conserving storage. A small page size will result
  in less wasted storage when a contiguous region
  of virtual memory is not equal in size to a
  multiple of the page size. The term for this
  unused memory in a page is internal fragmentation.

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Page Replacement Policies

• 3 major page replacement policies
• FIFO
• LRU
• OPT

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FIFO

• FIFO(first in first out) With FIFO, the oldest
  page in memory is selected for replacement
• Example
• 2 3 2 1 5 2 4 5 3 2 5
  2

2 2 2 2 5 5 5 5 3 3 3 3
3 3 3 3 2 2 2 2 2 5 5
1 1 1 4 4 4 4 4 2
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LRU (least recently used)

• LRU Policy- a policy that selects the page that
  has gone unused for the longest period of time.
• 2 3 2 1 5 2 4 5 3
  2 5 2

2 3 2 1 5 2 4 5 3 2 5 2
2 3 2 1 5 2 4 5 3 2 5
3 2 1 5 2 4 5 3 3
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OPT(optimal replacement policy)

• In this policy we will replace the memory with
  the page address which we will not use in the
  near future.

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Examples of OPT

• 2 3 2 1 5 2 4 5 3 2 5 2

2 2 2 2 2 2 4 4 4 2 2 2
3 3 3 3 3 3 3 3 3 3 3
1 5 5 5 5 5 5 5 5
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Segmentation

• Segment A segment is a set of logically related
  contiguous words generated by a compiler or a
  programmer.
• Segmentation A memory management technique that
  allocates main memory by segments is called
  segmentation.

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Size of Segments

• Variable size blocks are called segments.
• Segments size varies.
• The largest segment supported on any machine
  ranges from 216 bytes up to 232 bytes

Data
Code
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Segment Table

• The physical addresses assigned to the segments
  are maintained in a memory map called a segment
  table.

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Segment-Table Entry

• Segment-Table
• 0 1 2 3 8 18 33
  47

Tag
Physical address S
Presence Bit P
Segment size Z
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External Fragmentation

• External Fragmentation It happens because of
  unused pieces of main memory.

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Paged Segments

• Combination of paging and segmentation.
• This is done by dividing each segment into pages.
• A word then has a logical address with 3
  components-
• Segment Address
• Page Address
• Line Address

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Advantage of Paged Segments

• It eliminates the need to store the segment in a
  contiguous region of main memory.

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References

• Dos Reis, Anthony J. Assembly Language And
  Computer Architecture Using C And Java.
• Hennessy, John and Patterson, David. Computer
  Organization and Design. San Mateo,California
  Morgan Kaufmann, 1994.
• Hennessy, John and Patterson, David. Computer
  Architecture A Quantitative Approach.
  Sanfrancisco Morgan Kaufmann, 1996.
• Hennessy, John and Patterson, David. Computer
  Architecture A Quantitative Approach Morgan
  Kaufmann, 1990.
• Hayes, John. Computer Architecture And
  Organization. McGraw-Hill Book Company, 1988.