virtual memorry

Virtual memory: 

Virtual memory Team members:- Anadi Vats (100101030) Kumar S iddarth Bansal (100101114) Mansi Mahajan (100101126) Jishnu V. Nair(100101100)

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M E M O R Y H I E R A R C H y

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Main memory: Refers to physical memory that is internal to the computer . The computer can manipulate only data that is in main memory . amount of main memory on a computer is crucial. computers often have too little main memory Auxiliary memory: Devices that provide backup storage are called auxiliary memory. Most common devices used are magnetic disks and magnetic tapes Bits are recorded by write heads and read by read head Associative memory: Memory unit accessed by content is called associative memory or content addressable memory. This memory is accessed simultaneously and in parallel on the basis of data content rather than by specific address or location. Cache memory: Cache memory is random access memory (RAM) that a computer microprocessor can access more quickly than it can access regular RAM. T Y P E S O F M E M O R Y

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A VIRTUAL MEMORY SYSTEM PROVIDES A MECHNANISM FOR TRANSLATING PROGRAM-GENERATED ADDRESSES INTO CORRECT MAIN MEMORY. THE TRANSLATION OR MAPPING IS HANDLED AUTOMATICALLY BY THE HARDWARE BY MEANS OF A MAPPING TABLE. V I R T U A L M E M O R Y

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AN ADDRESS USED BY A PROGRAMMER WILL BE CALLED A VIRTUAL ADDRESS, AND THE SET OF SUCH ADDRESSES IS CALLED ADDRESS SPACE. AN ADDRESS IN MAIN MEMORY IS CALLED A LOCATION OR PHYSICAL ADDRESS,AND THE SET OF SUCH ADDRESSES IS CALLED THE MEMORY SPACE. A M D E D M R O E R S Y S & S P A C E

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MEMORY MAPPING TABLE MEMORY TABLE BUFFER REGISTER MAIN MEMORY ADDRESS REGISTER (15 bits) MAIN MEMORY MAIN MEMORY BUFFER REGISTER VIRTUAL ADDRESS REGISTER (20 bits) M E M O R Y T A B L E F O R M A P P I N G

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THE ADDRESS SPACE AND THE MEMORY SPACE ARE DIVIDED INTO GROUPS OF FIXED SIZE. THE PHYSICAL MEMORY IS BROKEN DOWN INTO GROUPS OF EQUAL SIZE CALLED BLOCKS. THE ADDRESS SPACE IS BROKEN INTO GROUPS OF EQUAL SIZE CALLED PAGES. THE PAGE AND BLOCK ARE SPLIT INTO GROUPS OF 1K WORDS. A U D S D I R N E G S S p A M G A E P S P I N G

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1 0 1 0 1 0 1 0 1 0 0 1 1 0 11 1 00 1 0 0 01 1 10 1 0 01 1 01 0101010011 Presence bit Block 0 Block 1 Block2 Block3 MBR Main memory Page no. Line number Main memory Address register Memory page table Table address Virtual address 000 001 010 011 100 101 110 111

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THE CONTENT OF THE WORD IN THE MEMORY PAGE TABLE AT THE PAGE NUMBER ADDRESS IS READ OUT INTO THE MEMORY TABLE BUFFER REGISTER. IF THE PRESENCE BIT IS 1, THE BLOCK NUMBER THUS READ IS TRANSFERD TO THE 2 HIGH ORDER BITS OF THE MAIN MEMORY ADDRESS REGISTER. THE LINE NUMBER FROM THE VIRTUAL ADDRESS IS TRANSFERRED INTO THE 10 LOW ORDER BITS OF THE MEMORY ADDRESS REGISTER. THE READ SIGNAL TO MAIN MEMORY TRANSFER THE CONTENT OF THE WORD TO THE MAIN MEMORY BUFFER REGISTER READY TO BE USED BY CPU. IF THE PRESENCE BIT IS ZERO, IT SIGNIFIES THAT THE CONTENT OF THE WORD REFERNCED BY THE VIRTUAL ADDRESS DOES NOT RESIDE IN MAIN MEMORY.

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A technique used by virtual memory operating system to ensure that the data you need is available as quickly as possible. The operating system copies a page into the memory whenever a program requires a particular page from our storage device. It copies another page back into the disk in place of the page just removed. P A G I N G

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Page tables are used to translate the virtual addresses seen by the application into physical addresses seen by the hardware to process instructions. Such hardware that handle this specific translation are often referred to as the memory management unit. P A G E T A B L E S

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Goal: Want lowest page-fault rate Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string In all our examples, the reference string is 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 P A A L G G E O R R I E T P H L M A C E M E N T

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F I F O

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LRU replacement associates with each page the time of that page’s last use When a page must be replaced, LRU chooses the page that has not been used for the longest period of time L R U

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L R U

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The major problem is how to implement LRU replacement: Counter: whenever a reference to a page is made, the content of the clock register are copied to the time-of-use filed in the page table entry for the page. We replace the page with the smallest time value Stack: Whenever a page is referenced, it is removed from the stack and put on the top. In this way, the most recently used page is always at the top of the stack L R U

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When paging is used, a problem called thrashing can occur, in which the computer spends an unsuitable amount of time swapping pages to and fro from the backing store hence slowing down the useful work. T H R A S H I N G