Computer architecture virtual memory
- 3. History virtual memory was developed in approximately 1959 – 1962, at the University of Manchester for the Atlas Computer, completed in 1962. In 1961, Burroughs released the B5000, the first commercial computer with virtual memory.
- 4. Virtual Memory OS and hardware produce illusion of a disk as fast as main memory Virtual memory as an alternate set of memory addresses. Programs use these virtual addresses rather than real addresses to store instructions and data. When the program is actually executed, the virtual addresses are converted into real memory addresses. Process runs when not all pages are loaded in memory Only keep referenced pages in main memory Keep unreferenced pages on slower, cheaper backing store (disk) Bring pages from disk to memory when necessary. Virtual memory is imaginary memory: it gives you the illusion of a memory arrangement that’s not physically there. The simple tactic of breaking a process up into pages led to the development of important concept : Virtual memory
- 5. Virtual Memory ……(cont’d) The basic idea of Virtual Memory is to keep only those parts of the program currently in use in the memory and the rest on the disk drive. For example: a 16M program can run on a 4M machine by carefully choosing which 4M to keep in memory at each instant, with pieces of the program being swapped between disk and memory as needed. It can work in a Multiprogramming system, with bits and pieces of many programs brought to the memory at once by the Operating System. While a program is waiting for part of itself to be brought in, it is waiting for I/O and cannot run, so the CPU can be given to another process, the same way as for any other multiprogramming system.
- 6. Advantages You can run more applications at once. Allows you to fit many large programs into a relatively small RAM. You don't have to buy more memory RAM VM supports Swapping. Common data or code may be shared to save memory. Process need not be in memory as a whole , Only part of a program needs to be loaded into memory. Process may even be larger than all of physical memory. Data / code can be read from disk as needed. Code can be placed anywhere in physical memory without relocation. More processes can be maintained in Main Memory which increases effective use of CPU. Don't need to break program into fragments to accommodate memory limitations
- 7. Why is it needed…. Before the development of the virtual memory technique, programmers in the 1940s and 1950s had to manage directly two-level storage such as main memory or ram and secondary memory in the form of hard disks or earlier, magnetic drums. Enlarge the address space, the set of addresses a program can utilize. Virtual memory might contain twice as many addresses as main memory.
- 8. How does it work… To facilitate copying virtual memory into real memory, the operating system divides virtual memory into pages, each of which contains a fixed number of addresses. Each page is stored on a disk until it is needed. When the page is needed, the operating system copies it from disk to main memory, translating the virtual addresses into real addresses.
- 9. Paging • Extends partition idea by dividing up both memory and processes into equal size pieces • Split memory into equal sized, small chunks – called page frames • Split programs (processes) into equal sized small chunks - called pages • Allocate the required number page frames to a process • Operating System maintains list of free frames • A process does not require contiguous page frames • Each process has a list of frames that it is using, called a page table, stored in the PCB • Paging Use page table to keep track
- 10. Allocation of Free Frames
- 11. Logical and Physical Addresses -Paging
- 12. Demand paging • Fetch Policy Determines when a page should be brought into main memory. One of common policies is Demand paging • Refines paging by demand paging - each page of a process is brought in only when needed, that is, on demand • Demand paging – Do not require all pages of a process in memory – Bring in pages as required – Less I/O needed – Less memory needed – Faster response – More users „
- 13. Page fault If a page is needed which is not in memory, a page fault is triggered, which requires an I/O operation • Page fault – Required page is not in memory – Operating System must swap in required page – May need to swap out a page to make space
- 14. Page fault……(cont’d) An interrupt to the software raised by the hardware when a program accesses a page that is not mapped in physical memory. when a program accesses a memory location in its memory and the page corresponding to that memory is not loaded when a program accesses a memory location in its memory and the program does not have privileges to access the page corresponding to that memory.
- 15. Problems faced with Virtual Memory implementation: Thrashing Possibly large page table Solution: Use multilevel page tables. Use TLB (Translation Lookaside Buffer) or some ti mes called ‘Associative Memory’. Page fault rates can be reduced by using a Wo rking Set Model’ approach or a ‘Prepaging’ app roach. Increase RAM!
- 16. Thrashing Pages currently in memory may have to be replaced. In some situations this can lead to thrashing, where the processor spends too much time swapping the same pages in and out Thrashing Swapping out a piece of a process just before that piece is needed The processor spends most of its time swapping pieces rather than executing user instructions Too many processes in too little memory Operating System spends all its time swapping Little or no real work is done Disk light is on all the time Solutions Good page replacement algorithms Reduce number of processes running Fit more memory
- 17. Page Table Structure Each process has a list of frames that it is using, called a page table, stored in the PCB Page Table Structure is a Basic mechanism for reading a word from memory involves using a page table to translate : • a virtual address - page number and offset into • a physical address - frame number and offset Page tables may be very large • they cannot be stored in registers • they are often stored in virtual memory (so are subject to paging!) • sometimes a page directory is used to organize many pages of page tables ,Pentium uses such a two-level structure • sometimes an inverted page table structure is used to map a virtual address to a real address using a hash on the page number of the virtual address AS/400 and PowerPC use this idea.
- 18. 18 Page Table Structure Page tables are variable in length (depends on process size) then must be in main memory instead of registers A single register holds the starting physical address of the page table of the currently running process
- 19. Page Tables Memory-resident page table (physical page or disk address) Physical Memory Disk Storage (swap file or regular file system file) Valid 1 1 1 1 1 1 1 0 0 0 Virtual Page Number
- 20. Inverted Page Table Structure
- 21. Translation Lookaside Buffer )TLB) • Each virtual memory reference can causes two physical memory access One to fetch the appropriate page table entry One to fetch the desired data • To overcome this problem a high-speed cache is set up for page table entries – Called a Translation Lookaside Buffer (TLB) • TLB is a special cache, just for page table entries • Contains page table entries that have been most recently used
- 22. Translation Lookaside Buffer • Given a virtual address, processor examines the TLB • If page table entry is present (TLB hit), the frame number is retrieved and the real address is formed • If page table entry is not found in the TLB (TLB miss), the page number is used to index the process page table
- 23. Translation Lookaside Buffer • First checks if page is already in main memory – If not in main memory a page fault is issued • The TLB is updated to include the new page entry
- 24. TLB Operation
- 25. TLB and Cache Operation
- 26. Segmentation • Another way in which addressable memory can be subdivided, known as Segmentation • May be unequal, dynamic size • Paging is not (usually) visible to the programmer • Segmentation is visible to the programmer • Usually different segments allocated to program and data • May be a number of program and data segments
- 27. Advantages of Segmentation • Simplifies handling of growing data structures • Allows programs to be altered and recompiled independently, without re-linking and re-loading • Lends itself to sharing data among processes • Lends itself to protection • Some systems combine segmentation with paging
- 28. Combined Paging and Segmentation • Paging is transparent to the programmer • Segmentation is visible to the programmer • Each segment is broken into fixed-size pages.