40 demand paging
- 1. Storage Management • Demand Paging 1
- 2. Recap In the last class, you have learnt: Virtual Memory • Advantages of virtual memory 2
- 3. HOME PREVIOUS TOPIC NEXT PREVIOUS QUESTION PAPERS FOR OS CPP TUTORIALS 3
- 4. Objective On completion of this class you will be able to know • Demand Paging 4
- 5. Demand Paging • Demand paging system is similar to a paging system with swapping • A lazy swapper swaps a page into memory only when it is needed • A process is a sequence of pages rather than one large contiguous address space • In demand paging instead of swapper the term pager is used 5
- 6. Demand Paging • Bring a page into memory only when it is needed – Less I/O needed – Less memory needed – Faster response – More users • Page is needed ⇒ reference to it – invalid reference ⇒ abort – not-in-memory ⇒ bring to memory 6
- 7. Demand Paging •When a page is referenced, and that page isn’t in memory, • then get the page from disk and re-execute the statement •Here’s migration between memory and disk Program A Program B Physical Disk Memory Fig 1 7
- 8. Transfer of a Paged Memory to Contiguous Disk Space Fig 2 8
- 9. Valid-Invalid Bit • With each page table entry a valid–invalid bit is associated (1 ⇒ in-memory, 0 ⇒ not-in-memory) • Initially valid–invalid but is set to 0 on all entries • Example of a page table snapshot Page table • During address translation, if valid– Frame # valid-invalid bit 1 invalid bit in page table entry is 1 0 ⇒ page fault 1 1 0 0 0 Fig 3 9
- 10. Page Table When Some Pages Are Not in Main Memory Fig 4 10
- 11. Demand Paging Steps in handling a page fault • The process has touched a page not currently in memory • Check an internal table for the target process to determine if the reference was valid ( do this in hardware ) • If it was valid, but page isn't resident, then try to get it from secondary storage • Find a free frame; a page of physical memory not currently in use (May need to free up a page ) 11
- 12. Demand Paging • Schedule a disk operation to read the desired page into the newly allocated frame • When memory is filled, modify the page table to show the page is now resident • Restart the instruction that failed • Do these steps using fig 5 which you can see on the next slide 12
- 13. Demand Paging Fig 5 13
- 14. Frequently Asked Questions • What is demand paging ? • What is page fault ? 14
- 15. Summary In this class, you have learnt • Demand paging is used to get a page into memory only when it is needed 15
- 16. Quiz 1. Demand paging system is similar to a paging system with a) Segmentation b) Swapping c) None 2. In demand paging instead of swapper the term __________is used a) Page offset b) Hole c) Pager d) None 16
- 17. Other subject materials • Web designing • Micro processors • C++ tutorials • java home