Virtual memory
- 1. Virtual Memory Amirkabir University of Technology Negar Neda
- 2. What is Virtual Memory? Virtual Memory is an address mapping Source:Amirhosein Peyberah • What is Virtual Memory? • Virtual Memory is an address mapping ● Maps virtual address space to physical address space – Maps virtual addresses to physical RAM – Maps virtual addresses to hardware devices o Separation of user logical memory from physical memory o Only part of the program needs to be in memory for execution. o Virtual Memory is an address mapping maps virtual address space to physical address space – Maps virtual addresses to physical RAM – Maps virtual addresses to hardware devices
- 3. Image source: Virtual Memory (Amirhosein Pyeberah(
- 4. Virtual Memory Features • Each process has its own virtual address space – Process invisible to each other – Process cannot access another process memory • MMU checks protection bits on memory access – Pages can be protected from being written or being executed or even being read – System can distinguish different protection level
- 5. Address space • Physical addresses – Addresses as used by the hardware - DMA, peripherals • Virtual addresses – Addresses as used by software • MMU must map logical to physical.
- 7. Memory-Management Unit (MMU) • The Memory Management Unit (MMU) is a hardware component which manages virtual address mappings • Sits between the CPU core and memory • The MMU operates on basic units of memory called pages • Transparently handles all memory accesses from Load/Store instructions – Maps accesses using virtual addresses to system RAM – Handles permissions – Generates an exception (page fault) on an invalid access
- 8. Virtual Memory Sharing • Pages of different processes mapped to one single frame on physical memory – Allows sharing of code – Frame may be used to several processes until one writes to it Image source: Virtual Memory (Amirhosein Pyeberah(
- 9. Paging
- 10. What is paging Image source: google.com
- 11. Demand Paging • Rather than swapping the entire process into memory, we use a lazy swapper. • I A lazy swapper never swaps a page into memory unless that page will be needed. Image source: Virtual Memory (Amirhosein Pyeberah(
- 12. Demand Paging • Trap to the OS. • Save the user registers and process state. • Determine that the interrupt was a page fault. • Check that the page reference was legal and determine the location of the page on the disk.
- 13. Demand Paging (cont’d) • Issue a read from the disk to a free frame: – Wait in a queue for this device until the read request is serviced. – Wait for the device seek and/or latency time. – Begin the transfer of the page to a free frame. • Issue a read from the disk to a free frame: – Wait in a queue for this device until the read request is serviced. – Wait for the device seek and/or latency time. – Begin the transfer of the page to a free frame.
- 14. Page Fault
- 15. Page Fault • A page fault is a CPU exception, generated when software attempts to use an invalid virtual address. There are two cases: – The virtual address is not mapped for the process requesting it. – The processes has insufficient Image source: Virtual Memory (Amirhosein Pyeberah(
- 16. Handling Page Fault • I Check an internal table for the process to determine whether the reference was a valid or an invalid memory access. – I If the reference was invalid, we terminate the process. – If it was valid but we have not yet brought in that page, we now page it in.
- 18. Translation Lookaside Buffer (TLB) • Each virtual memory reference can cause two physical memory access – One to fetch the page table – One to fetch the data • To overcome this problem a high-speed cache is set up for page table entries called TLB • TLB is part of the MMU consulted by the MMU when the CPU access a virtual address. – Contains page table entries that have been most recently used – Functions like a cache
- 19. Translation Look aside Buffer • First checks if page is already in main memory – If not a page fault is issued • The TLB is updated to include the new page entry • The TLB holds the entries of the mapping – Virtual address – Physical address – permissions
- 20. Negar Neda Thank you ! Any Questions ?