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Lecture 32 Syed Mansoor Sarwar

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1 Lecture 32 Syed Mansoor Sarwar
Operating Systems Lecture 32 Syed Mansoor Sarwar

2 © Copyright Virtual University of Pakistan
Agenda for Today Review of previous lecture MVT Paging Addressing and logical to physical address translation Page table implementation Recap of lecture 1 December 2018 © Copyright Virtual University of Pakistan

3 © Copyright Virtual University of Pakistan
Review of Lecture 31 Dynamic linking Overlays Swapping Contiguous storage allocation MFT 1 December 2018 © Copyright Virtual University of Pakistan

4 © Copyright Virtual University of Pakistan
MVT Multiprogramming with variable tasks (MFT) Both the number and size of partitions change with time A process still resides in one region but can be of any size up to the size of the user space and can change with time. 1 December 2018 © Copyright Virtual University of Pakistan

5 © Copyright Virtual University of Pakistan
MVT Jobs can move from one partition to another Dynamic address binding No internal fragmentation Introduces external fragmentation 1 December 2018 © Copyright Virtual University of Pakistan

6 © Copyright Virtual University of Pakistan
MVT 50K 200K 1 December 2018 © Copyright Virtual University of Pakistan

7 External Fragmentation
External Fragmentation refers to the state of memory space when total amount of unused memory space exists to satisfy a request but this memory space is not contiguous. 1 December 2018 © Copyright Virtual University of Pakistan

8 External Fragmentation
Compact main memory (shuffle processes around to put free space in one contiguous area) Slows down execution of currently running processes 1 December 2018 © Copyright Virtual University of Pakistan

9 © Copyright Virtual University of Pakistan
Paging Divide physical memory into fixed-sized blocks, called frames Divide logical memory into blocks of the same size, called pages Size of a page is a power of 2 Typical page sizes: 1K – 16K 1 December 2018 © Copyright Virtual University of Pakistan

10 © Copyright Virtual University of Pakistan
Paging Keep track of all free frames. To run a program of size n pages, find n free frames and load program pages into these frames. Keep track of a program’s pages in the main memory by using the page table. 1 December 2018 © Copyright Virtual University of Pakistan

11 Paging Process Address Space Physical Address Space 1 2 1 2 … 3 4 5 6
1 2 … 9 10 15 1 2 3 4 5 6 7 Process Address Space Physical Address Space 1 December 2018 © Copyright Virtual University of Pakistan

12 Paging Process Address Space Physical Address Space Page Page Table 1
1 2 … 9 10 15 1 2 3 4 5 6 7 Page 3 9 11 … 1 1 2 3 … 7 Page Table Process Address Space Physical Address Space 1 December 2018 © Copyright Virtual University of Pakistan

13 Paging Process Address Space Physical Address Space Page Table 1 2 3 …
1 2 … 9 10 15 1 2 3 4 5 6 7 9 … 1 2 3 … 7 Page Table Process Address Space Physical Address Space 1 December 2018 © Copyright Virtual University of Pakistan

14 © Copyright Virtual University of Pakistan
Paging Logical address space of a process can be noncontiguous; a process page is allocated a frame whenever the latter is available. Set up a page table to translate logical addresses to physical addresses. Internal fragmentation 1 December 2018 © Copyright Virtual University of Pakistan

15 © Copyright Virtual University of Pakistan
Addressing in Paging Logical address: (p, d) where, p is page number and d is offset within the page p is used to index the process page table; page table entry contains the frame number, f, where page p is loaded 1 December 2018 © Copyright Virtual University of Pakistan

16 © Copyright Virtual University of Pakistan
Addressing in Paging Physical address of the location referenced by (p,d) is computed by appending d at the end of f, as shown below: f d 1 December 2018 © Copyright Virtual University of Pakistan

17 © Copyright Virtual University of Pakistan
Address Translation f 1 December 2018 © Copyright Virtual University of Pakistan

18 © Copyright Virtual University of Pakistan
Addressing Example 1 December 2018 © Copyright Virtual University of Pakistan

19 © Copyright Virtual University of Pakistan
Paging Example Page size = 4 bytes Process address space = 4 pages Physical address space = 8 frames Logical address: (1,3) = 0111 Physical address: (6,3) = 11011 Page Frame 1 0111 2 3 11011 1 December 2018 © Copyright Virtual University of Pakistan

20 © Copyright Virtual University of Pakistan
Addressing in Paging Logical address space of 16 pages of 1024 words each, mapped into a physical memory of 32 frames. Logical address size? Physical address size? Number of bits for p, f, and d? 1 December 2018 © Copyright Virtual University of Pakistan

21 © Copyright Virtual University of Pakistan
Recap of Lecture MVT Paging Logical and physical addresses Address translation 1 December 2018 © Copyright Virtual University of Pakistan

22 © Copyright Virtual University of Pakistan
Operating Systems Lecture 32 Syed Mansoor Sarwar 1 December 2018 © Copyright Virtual University of Pakistan

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