Operating system 36 virtual memory
- 1. Operating System 36 Virtual Memory Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
- 2. Virtual Memory • Code needs to be in memory to execute, but entire program rarely used – Error code, unusual routines, large data structures • Entire program code not needed at same time • Consider ability to execute partially-loaded program – Program no longer constrained by limits of physical memory – Each program takes less memory while running -> more programs run at the same time • Increased CPU utilization and throughput with no increase in response time or turnaround time – Less I/O needed to load or swap programs into memory -> each user program runs faster
- 3. Virtual Memory • Virtual memory is a technique that permits the execution of processes that may not be completely resident in the memory. • The main advantage of this scheme is that programs can be larger than physical memory. • Further, it views main memory into an extremely large, uniform array of storage, separating logical memory as viewed by the user from physical memory. • It, as a matter of fact augments the main memory with secondary storage devices (like hard disks). • This technique frees programmers from concern over memory storage limitations.
- 4. Background (Cont.) • Virtual memory – separation of user logical memory from physical memory – Only part of the program needs to be in memory for execution – Logical address space can therefore be much larger than physical address space – Allows address spaces to be shared by several processes – Allows for more efficient process creation – More programs running concurrently – Less I/O needed to load or swap processes
- 5. Background (Cont.) • Virtual address space – logical view of how process is stored in memory – Usually start at address 0, contiguous addresses until end of space – Meanwhile, physical memory organized in page frames – MMU must map logical to physical • Virtual memory can be implemented via: – Demand paging – Demand segmentation
- 6. Virtual Memory That is Larger Than Physical Memory
- 7. Virtual-address Space Usually design logical address space for stack to start at Max logical address and grow “down” while heap grows “up” Maximizes address space use Unused address space between the two is hole No physical memory needed until heap or stack grows to a given new page Enables sparse address spaces with holes left for growth, dynamically linked libraries, etc System libraries shared via mapping into virtual address space Shared memory by mapping pages read- write into virtual address space Pages can be shared during fork(), speeding process creation
- 8. Shared Library Using Virtual Memory
- 9. Implementation • Virtual memory makes the task of programming much easier, because the programmer no longer needs to worry about the amount of physical memory available, or about what code can be placed in overlays, but can concentrate instead on the problem to be programmed. • On systems, which support virtual memory, overlays have virtually disappeared, therefore. • Virtual memory is commonly implemented by Demand Paging. • It can also be implemented in a segmentation system. • Several systems provide a paged segmentation scheme, where segments are broken into pages. • Thus, the user view is segmentation, but the operating system can implement this view with demand paging.
- 10. Assignment • Explain the concept and usage of virtual Memory