Virtual Memory BY JEMINI ISLAM

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.

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 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 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. To achieve the high access rates and low cost per bit that is possible with a memory hierarchy.

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.

Implementation of Virtual Memory System: Paging: - Fixed-size blocks Segmentation:- Variable size blocks Paged Segmentation: Combination of two systems.

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

Page Table: Frame Number Page Number Page Table

Pages: Example: Each page has the same length, typically a power of 2. CodeData

Advantage of Paging: fully associative. 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.

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.

Virtual page number Page offset Page table Main memory The Mapping of a Virtual Address to a Physical Address Via a Page Table Virtual address Physical Address

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.

Structure of a Virtual Memory System : TLB Memory Map Virtual Block Address Logical (virtual) Address Physical Address Displacement Physical block Address Main Memory Secondary Memory

Representative Organization of a Page Table : Page Page Presence Change Access Address Frame bit P bit C rights A R, X C D6C7T9 0 R, W, X E R, W, X F R

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 :

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.

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.

Page Replacement Policies: 3 major page replacement policies:  FIFO  LRU  OPT

FIFO FIFO(first in first out) : With FIFO, the oldest page in memory is selected for replacement Example: *2*22555 *5* *2*

LRU: (least recently used) LRU Policy- a policy that selects the page that has gone unused for the longest period of time *2*215 *2*24 *5*532 *5*5 *2*

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.

Examples of OPT : *2*222 *2* *2* *3* *5*555 *5*55

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.

Size of Segments :  Variable size blocks are called segments.  Segments size varies.  The largest segment supported on any machine ranges from 2^16 bytes up to 2^32 bytes Code Data

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

Segment-Table Entry Segment-Table Tag Presence Bit P Segment size Z Physical address S

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

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

Advantage of Paged Segments: It eliminates the need to store the segment in a contiguous region of main memory.

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, Hennessy, John and Patterson, David. Computer Architecture A Quantitative Approach. Sanfrancisco : Morgan Kaufmann, Hennessy, John and Patterson, David. Computer Architecture A Quantitative Approach : Morgan Kaufmann, Hayes, John. Computer Architecture And Organization. McGraw-Hill Book Company, 1988.