The document discusses virtual memory and paging. It describes why virtual memory is used, how it maps virtual to physical addresses, and its key features like protection, sharing, and swapping. Paging is discussed as a common virtual memory implementation where memory is divided into equal-sized pages and pages are mapped to equal-sized frames in physical memory using page tables. Page faults and translation look-up buffers are described to improve performance of virtual memory systems.

1. Principles of Virtual Memory Virtual Memory, Paging, Segmentation

16. 2.1 What i s Paging? Page 7 Page 5 Page 4 Page 3 Page 2 Page 1 Page 6 Virtual memory (divided into equal size pages) Page 0 0x00 Page Table (one per process, one entry per page maintained by OS) Page 7 Page 5 Page 4 Page 3 Page 2 Page 1 Page 6 Page 0 v v v Frame 0 Frame 1 Frame 3 Frame 0 Frame 1 Frame 2 Frame 3 Physical memory (divided into equal size page frames) 0x00 v v v

17. 2.2 Paging: Implementation Typical Page Table Entry other Page Frame # execute x write w read r r w x valid v r w x v referenced re v re modified m re m shared s m s caching disabled c s c super - page su c su process id pid su pid guard data (extended) guard gd g pid g gd g gd other

18. 2.2 Paging: Implementation Singlelevel Page Tables Problem: Page tables can get very large , e.g. 32 bit address space, 4KB pages  2^20 entries per process  4MB at 4B per entry 64 bit  16777216 GB page table!!!! one entry per page one table per process 0x14 0x 2 Virtual address Page # Offset Physical address 0x14 Offset 0x8 Frame # Page Table Base Register (PTBR) Page Table 0x8 ... 0x0 * L 0x1 * L 0x2 * L L : size of entry

19. 2.2 Paging: Implementation Multilevel Page Tables not all need be present saves memory table size can be restricted to one page Offset Page #1 Page #2 Page #3 Page Directory Page Middle Directory Page Table Page Frame # Offset Offset Frame # Oversized Super -P age v=0

21. Steps in Handling a Page Fault

31. 2. 6 Summary: Conversion o f a Virtual Address Virtual address Physical address OS Hard ware no protection fault reference legal? copy on write? HDD I/O complete: interrupt memory full? update page table process into ready state copy page process into blocking state yes page fault no exception to process no TLB page table miss page in mem? hit access rights? yes update TLB yes exception to process no swap out a page yes HDD I/O read req. no yes bring in page from HDD ! process into blocking state

38. Virtual-address Space

39. Shared Library Using Virtual Memory