VM is hardware implementation and assisted by OS’s Memory Management Task. Definition: Virtual memory is the feature of an operating system (OS). The virtual memory technique allows users to use more memory for a program than the real memory of a computer. There is a possibility that some of the pages may have contents less than the page size, as we have in our printed books. In the Paging Mechanism, Page Frames of fixed size are allotted. So, ideally, the page table should be situated within the MMU. A segment... Paging. The programs are also considered to be split into pages. This is again similar to the misses that we have already discussed with respect to cache memory. Textbook Reading •In the online textbook, read •Appendix A busses •Chapter 8 I/O controllers •Chapter 7 external storage. Since each page consists of 211 = 2K words, the high order nine bits of the virtual address will specify one of the 512 pages and the low-order 11 bits give the offset within the page. Virtual Memory Lecture Slides By 2. Computer architecture virtual memory 1. Since loading a page from auxiliary memory to main memory is basically an I/O operation, the operating system assigns this task to the I/O processor. Instead of processing each instruction sequentially, a parallel processing system provides concurrent data processing to increase the execution time.. However, there is only one real '0' address in Main Memory. A Segment is a logically related contiguous allocation of words in MM. In such cases, Dynamic Address Translation is used. The mapping is used during address translation. The page table entry contains the physical page frame address, if the page is available in main memory. We will discuss some more differences with the help of comparison chart shown below. In the example above, we considered a virtual address of 20 bits. Learn new and interesting things. They overlap the cache access with the TLB access. Many are downloadable. Page fault will be generated only if it is a miss in the Page Table too but not otherwise. virtual address of 20 bits. a virtual address, the MMU looks in the TLB for the referenced page. Generally, a Segment size coincides with the natural size of the program/data. Along with this address information, the page table entry also provides information about the privilege level associated with the page and the access rights of the page. The LRU policy is more difficult to implement but has been more attractive on the assumption that the least recently used page is a better candidate for removal than the least recently loaded page as in FIFO. Segment/Page access rights are checked to verify any access violation. In this scenario, what is the hierarchy of verification of tables for address translation and data service to the CPU? 4. Virtual memory also permits a program’s memory to be physically noncontiguous , so that every portion can be allocated wherever space is available. That is, the high order bits of the virtual address are used to look in the TLB while the low order bits are used as index into the cache. On the other hand hardware manages the cache memory. (Remember your single file may be stored in different sectors of the disk, which you may observe while doing defrag). In this case, as we discussed for caches, a replacement has to be done. Drawback of Virtual memory: So far we have assumed that the page tables are stored in memory. All of us are aware of the fact that our program needs to be available in main memory for the processor to execute it. Flexibility - portions of a program can be placed anywhere in Main Memory without relocation, Storage efficiency -retain only the most important portions of the program in memory, Concurrent I/O -execute other processes while loading/dumping page. Finally, we shall have a word on the types of misses that can occur in a hierarchical memory system. Since, the page table information is used by the MMU, which does the virtual to physical address translation, for every read and write access, every memory access by a program can take at least twice as long: one memory access to obtain the physical address and a second access to get the data. There is no need for the whole program code or data to be present in Physical memory and neither the data or program need to be present in contiguous locations of Physical Main Memory. Note that, even though they are contiguous pages in the virtual space, they are not so in the physical space. Rest of the views are transparent to the user. The physical memory is broken down into groups of equal size called page frames and the logical memory is divided into pages of the same size. Virtual Memory provides an illusion of unlimited memory being available to the Processes/ Programmers. Virtual memory is a concept implemented using hardware and software. Storage management - allocation/deallocation either by Segmentation or Paging mechanisms. It should be noted that it is always a write back policy that is adopted, because of the long access times associated with the disk access. If the page table entry for this page is found in the TLB, the physical address is obtained immediately. Virtual memory, apart from overcoming the main memory size limitation, allows sharing of main memory among processes. The binary addresses that the processor issues for either instructions or data are called virtual or logical addresses. The FIFO replacement policy has the advantage of being easy to implement. This mapping is necessary to be maintained in a Page Table. Computer Architecture:Introduction 2. This separation provides large virtual memory for programmers when only small physical memory is available. During address translation, few more activities happen as listed below but are not shown in figures ( 19.4 and 19.7), for simplicity of understanding. When the operating system changes the contents of page tables, it must simultaneously invalidate the corresponding entries in the TLB. An essential requirement is that the contents of the TLB be coherent with the contents of page tables in the memory. Static Translation – Few simpler programs are loaded once and may be executed many times. While the size of cache memory is less than the virtual memory. Figure 30.2 shows how four different pages A, B, C and D are mapped. Creative Commons Attribution-NonCommercial 4.0 International License, M – indicates whether the page has been written (dirty), R – indicates whether the page has been referenced (useful for replacement), Protection bits – indicate what operations are allowed on this page, Page Frame Number says where in memory is the page. The program is executed from main memory until it attempts to reference a page that is still in auxiliary memory. In case, the free space/Page frame is unavailable, Page Replacement algorithm plays the role to identify the candidate Segment/Page Frame. Virtual Memory (VM) Concept is similar to the Concept of Cache Memory. Figure 19.5 explains how two program’s pages are fitted in Page Frames in MM. Segmentation. Having discussed the various individual Address translation options, it is to be understood that in a Multilevel Hierarchical Memory all the functional structures coexist. The flow is as shown below. In a computer with 2 p words per page, p bits are used to specify an offset and the remaining high-order bits of the virtual address specify the page number. Virtual Memory. Thus, the virtual memory model provides decoupling of addresses used by the program (virtual) and the memory addresses (physical). T he a ddre s s e s a ... the Atlas computer This process is done temporarily and is designed to work as a combination of RAM and space on the hard disk. Virtual Memory 3. To summarize, we have looked at the need for the concept of virtual memory. The Change bit indicates that the segment/page in main memory is not a true copy of that in Disk; if this segment/page is a candidate for replacement, it is to be written onto the disk before replacement. Virtual memory acts as a cache between main memory and secondary memory. Since these fragments are inside the allotted Page Frame, it is called Internal Fragmentation. In this the system may have two or more ALU's and should be able to execute two or more instructions at the same time.