Virtual Memory
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This document discusses virtual memory and page faults. It describes how virtual memory allows logical addresses to be larger than physical memory by paging portions of programs in and out of RAM. When a program accesses a page not currently in memory, a page fault occurs. The OS then selects a free frame, swaps the needed page in from disk, updates address mappings, and restarts the faulting instruction. This allows more programs to run concurrently using less I/O than if entire programs had to be swapped at once.
Virtual Memory
- 1. B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar Operating System KCS – 401 Virtual Memory Dr. Pankaj Kumar Associate Professor – CSE SRMGPC Lucknow
- 2. Background B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar 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 B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar 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
- 4. Virtual Memory B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar 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 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
- 5. Virtual Memory B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar 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 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
- 6. Page Fault B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar If there is a reference to a page, first reference to that page will trap to operating system: page fault 1.Operating system looks at another table to decide: Invalid reference abort Just not in memory 2.Find free frame 3.Swap page into frame via scheduled disk operation 4.Reset tables to indicate page now in memory Set validation bit = v 5.Restart the instruction that caused the page fault