Virtual Memory Vivek Seshadri 15740/18740 Computer Architecture Readings • Jacob & Mudge, “Virtual Memory in Contemporary Microprocessors,” IEEE Micro, 1998 • Hennessy and Patterson, Appendix C.4C.5
Download ReportTranscript Virtual Memory Vivek Seshadri 15740/18740 Computer Architecture Readings • Jacob & Mudge, “Virtual Memory in Contemporary Microprocessors,” IEEE Micro, 1998 • Hennessy and Patterson, Appendix C.4C.5
Virtual Memory Vivek Seshadri 15740/18740 Computer Architecture Readings • Jacob & Mudge, “Virtual Memory in Contemporary Microprocessors,” IEEE Micro, 1998 • Hennessy and Patterson, Appendix C.4C.5 Why Virtual Memory? X Load X CPU Store Y Y Main Memory Why Virtual Memory? • • • • Small available physical memory Contiguous address space Portability Protection – Code & data – OS data and application data – One application’s data from another • Sharing Segmentation • Segment: Base address + Size + Permissions • Segments are re-locatable – Can be copied from one location to another • Each process is associated with a list of segments it can access Virtual Memory All problems in computer science can be solved by another level of indirection - David Wheeler For an application to make progress, it is sufficient that the next instruction and the data required to execute the next instruction are present in main memory. Virtual Memory X Load X CPU Store Y Y Main Memory Virtual Address Space Virtual Memory Solves It All! • Small available physical memory – Large virtual address space (VAS)! • Contiguous address space – Data should be contiguous only in VAS • Protection – Different process -> Different VAS • Sharing – Map VAS to same location in physical memory Implementation • Pages – Large (to reduce mapping overhead) • Page tables – One entry per virtual page – Hierarchical (to reduce size of page tables) • Inverted page tables • Speeding up translations – Translation Look-aside Buffers (TLB) Physical Memory Management • Allocating pages • Replacing pages (page fault) – Swap space in disk • Replacement policy – LRU? Virtual Memory & Caching Virtually-Indexed Virtually-Tagged Caches Virtual Page Number Cacheline Tag Offset Index Two processes can have the same virtual address Solution - 1 Physically-Indexed Physically-tagged Cache Virtual Page Number Offset PhysicalPage Number Offset Cacheline Tag Index Solution - 2 Virtually-Indexed Physically-tagged Cache Virtual Page Number Offset Physical Page Number Offset Cacheline Tag Index Synonyms Two virtual pages can map to the same physical page Virtual Page Number Cacheline Tag Offset Index