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Segmentation and Paging in Operating Systems: Examples and Concepts

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Understand the concepts of segmentation and paging in OS with examples from MULTICS and Intel 80386, including logical and physical address mapping, page tables, legality, and more.

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Segmentation and Paging in Operating Systems: Examples and Concepts

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  1. OperatingSystems

  2. Shared Pages

  3. Segmentation CPU

  4. Example

  5. Sharing of Segments

  6. 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

  7. 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

  8. Paged Segmentation 1 2 3 4 5 logical memory physical memory

  9. Paged Segmentation 0 1 2 3 . . . 10 126127 1 1 0 2 3 3 4 1 126 3 10 5 page table 2 logical memory physical memory

  10. 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

  11. 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

  12. Paged Segmentation • pindexes 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

  13. Paged Segmentation

  14. 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?

  15. 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

  16. MULTICS Example s d 18 bits p d’ 6 bits 10 bits

  17. 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?

  18. 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)

  19. 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 + 849 = 13137

  20. MULTICS Example 15 3921 s d 3 849 p d’ 0 1 2 3 4 12 page table for segment 15

  21. MULTICS Example 15 3921 3921 3 849 12 12 849 13137

  22. Intel 80386 Example • IBM OS/2, Microsoft Windows, and Linux • Paged segmentationwithtwo-level paging • Logical address = 48 bits • 16-bit selector and 32-bit offset • Page size = 4 KB

  23. Intel 80386 Example • 4-byte page table entry • 32-entry TLB, covering 32*4K (128 KB) memory … TLB Reach

  24. Intel 80386 Example 16-bit Selector 32-bit Offset 13-bit Segment # s g p 2-bit field for specifying the privilege level 1-bit field to specify GDT or LDT

  25. Intel 80386 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

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