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Lecture 36 Syed Mansoor Sarwar

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1 Lecture 36 Syed Mansoor Sarwar
Operating Systems Lecture 36 Syed Mansoor Sarwar

2 © Copyright Virtual University of Pakistan
Agenda for Today Review of previous lecture Paged segmentation Examples of paged segmentation: MULTICS under GE 345 and OS/2, Windows, and Linux under Intel CPUs 28 December 2018 © Copyright Virtual University of Pakistan

3 © Copyright Virtual University of Pakistan
Review of Lecture 35 Sharing in paging Segmentation Logical to physical address translation Hardware support needed Protection and sharing Issues with segmentation 28 December 2018 © Copyright Virtual University of Pakistan

4 © Copyright Virtual University of Pakistan
Shared Pages 28 December 2018 © Copyright Virtual University of Pakistan

5 © Copyright Virtual University of Pakistan
Segmentation CPU 28 December 2018 © Copyright Virtual University of Pakistan

6 © Copyright Virtual University of Pakistan
Example 28 December 2018 © Copyright Virtual University of Pakistan

7 © Copyright Virtual University of Pakistan
Sharing of Segments 28 December 2018 © Copyright Virtual University of Pakistan

8 © Copyright Virtual University of Pakistan
Protection Associate valid/invalid bit with each segment table entry to indicate if the referenced segment is part of the process address space or not Read, write, and execute bits to define legal operations on a segment 28 December 2018 © Copyright Virtual University of Pakistan

9 © Copyright Virtual University of Pakistan
Paged Segmentation Divide every segment in a process into fixed size pages Need for a page table per segment CPU’s memory management unit must support both segmentation and paging 28 December 2018 © Copyright Virtual University of Pakistan

10 © Copyright Virtual University of Pakistan
Paged Segmentation 1 2 3 4 5 logical memory physical memory 28 December 2018 © Copyright Virtual University of Pakistan

11 © Copyright Virtual University of Pakistan
Paged Segmentation 1 2 3 . 10 1 1 2 3 3 4 1 126 3 10 5 page table 2 logical memory physical memory 28 December 2018 © Copyright Virtual University of Pakistan

12 © Copyright Virtual University of Pakistan
Paged Segmentation Logical address is still <s,d>, with s used to index the segment table Each segment table entry consist of the tuple <segment-length, page-table-base> The logical address is legal if d < segment-length 28 December 2018 © Copyright Virtual University of Pakistan

13 © Copyright Virtual University of Pakistan
Paged Segmentation Segment offset, d, is partitioned into two parts: p and d’, where p is used to index the page table associated with segment, s, and d’ is used as offset within a page 28 December 2018 © Copyright Virtual University of Pakistan

14 Paged Segmentation p indexes the page table to retrieve frame, f, and physical address (f,d’) is formed s d index segment table p d’ index page table offset within the page p 28 December 2018 © Copyright Virtual University of Pakistan

15 © Copyright Virtual University of Pakistan
Paged Segmentation 28 December 2018 © Copyright Virtual University of Pakistan

16 © Copyright Virtual University of Pakistan
MULTICS Example GE 345 processor Logical address = 34 bits Page size = 1 KB s is 18 bits and d is 16 bits Size of p and d’, largest segment size, and max. number of segments per process? 28 December 2018 © Copyright Virtual University of Pakistan

17 © Copyright Virtual University of Pakistan
MULTICS Example Largest segment = 2d bytes = 216 bytes Maximum number of pages per segment = 216 / 1 K = 64 |p| = log2 64 bits = 6 bits |d’| = log2 1 K = 10 bits Maximum number of segments per process = 2s = 218 28 December 2018 © Copyright Virtual University of Pakistan

18 © Copyright Virtual University of Pakistan
MULTICS Example s d 18 bits p d’ 6 bits 10 bits 28 December 2018 © Copyright Virtual University of Pakistan

19 © Copyright Virtual University of Pakistan
MULTICS Example Consider a process with its segment 15 having 5096 bytes. The process generates a logical address (15,3921). Is it a legal address? How many pages does the segment have? What page does the logical address refer to? 28 December 2018 © Copyright Virtual University of Pakistan

20 © Copyright Virtual University of Pakistan
MULTICS Example Is it a legal address? Yes How many pages does the segment have? ceiling[5096/1024]= 5 What page does the logical address refers to? ceiling[3921/1024]= 4 (i.e., page number 3) 28 December 2018 © Copyright Virtual University of Pakistan

21 © Copyright Virtual University of Pakistan
MULTICS Example What are the value of d’ and the physical address if page number 3 (i.e., the fourth page) is in frame 12? d’ = 3921 – 3*1K = 849 Physical address = 12*1K = 13137 28 December 2018 © Copyright Virtual University of Pakistan

22 © Copyright Virtual University of Pakistan
MULTICS Example 15 3921 s d 3 849 p d’ 1 2 3 4 12 page table for segment 15 28 December 2018 © Copyright Virtual University of Pakistan

23 © Copyright Virtual University of Pakistan
MULTICS Example 3921 12 13137 28 December 2018 © Copyright Virtual University of Pakistan

24 © Copyright Virtual University of Pakistan
Intel Example IBM OS/2, Microsoft Windows, and Linux Paged segmentation with two-level paging Logical address = 48 bits 16-bit selector and 32-bit offset Page size = 4 KB 28 December 2018 © Copyright Virtual University of Pakistan

25 © Copyright Virtual University of Pakistan
Intel Example 4-byte page table entry 32-entry TLB, covering 32*4K (128 KB) memory … TLB Reach 28 December 2018 © Copyright Virtual University of Pakistan

26 Intel 80386 Example 16-bit Selector 32-bit Offset s g p
13-bit Segment # s g p 2-bit field for specifying the privilege level 1-bit field to specify GDT or LDT 28 December 2018 © Copyright Virtual University of Pakistan

27 © Copyright Virtual University of Pakistan
Intel Example Real Mode 20-bit physical address is obtained by shifting left the Selector value by four bits and adding to it the 16-bit effective address 28 December 2018 © Copyright Virtual University of Pakistan

28 © Copyright Virtual University of Pakistan
Recap of Lecture Paged segmentation Examples of paged segmentation: MULTICS under GE 345 and OS/2, Windows, and Linux under Intel CPUs 28 December 2018 © Copyright Virtual University of Pakistan

29 © Copyright Virtual University of Pakistan
Operating Systems Lecture 36 Syed Mansoor Sarwar 28 December 2018 © Copyright Virtual University of Pakistan

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