CSC2/452 Computer Organization Virtual Memory Sreepathi Pai URCS October 30, 2019. Outline Administrivia Recap Memory Virtualization x86-64 Implementation of Virtual Memory Kernel virtual memory Memory mapped region for shared libraries Run-time heap (created by malloc) User stack (created at runtime) 0 Memory invisible to user code system divides virtual memory into segments of varying lengths and moves any segments not in use from the computer's virtual memory space to its hard drive. Similar to page tables, segment tables track whether the computer is storing the segment in memory or a physical address.Segmentation differs from paging because it divides memory into Virtual Memory is a storage allocation scheme in which secondary memory can be addressed as though it were part of the main memory. The addresses a program may use to reference memory are distinguished from the addresses the memory system uses to identify physical storage sites, and program-generated addresses are translated automatically to the corresponding machine addresses. Computer Organization and Networks (INB.06000UF, INB.07001UF) Winter 2021/2022 Stefan Mangard, iaik.tugraz.at Chapter 12: Virtual Memory. Note on Material •Virtual memory system brings the page into a physical frame and adjusts the mapping →this is called demand paging Virtual Memory (VM) Concept is similar to the Concept of Cache Memory. While Cache solves the speed up requirements in memory access by CPU, Virtual Memory solves the Main Memory (MM) Capacity requirements with a mapping association to Secondary Memory i.e Hard Disk. Both Cache and Virtual Memory are based on the Principle of Locality of Reference. Virtual Memory -Objectives 1. Allow program to be written without memory constraints • program can exceed the size of the main memory 2. Many Programs sharing DRAM Memory so that context switches can occur 3. 9.2 Memory Organization • Memory can be organized in different ways - One process uses entire memory space - Each process gets its own partition in memory • Dynamically or statically allocated • Trend: Application memory requirements tend to increase over time to fill main memory capacities A programmer or user perceives a much larger memory that is allocated on the disk. This memory is referred to as virtual memory. The program enjoys a huge virtual memory space to develop his or her program or software. The execution of a program is the job of operating system and the underlying hardware. Usually virtual memory reduces also the time to start a program, as not all code and data has to be present in the memory to start; after the minimum amount has been brought into main memory, the program may start. 10.1 Some Definitions The basic concepts of a memory hierarchy apply for the virtual memory Memory organization refers to the collection and associations of memorized items and their recall ( Kirimura, 1999 ), and implicitly serves as a strategy for facilitating memory itself. With memory In the recent era of computing, applications an operating system cannot survive without efficient memory management, especially if an application has to be under Surve load for an undefined long time. Operating System Concepts 9.3 Silberschatz, Galvin and Gagne ©2005 Background Virtual memory - separation of user logical memory from physical memory. zOnly part of the program needs to be in memory for execution. zLogical address space can therefore be much larger than ph