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Operating Systems

This lecture explores the concepts of segmentation and paging in operating systems, focusing on their differences, advantages, and implementation. Segmentation provides a logical view of memory, eliminating internal fragmentation and enabling the user to manage memory in segments like code, data, and stack. Paging addresses fixed memory sizes, ensuring efficient allocation. The discussion also covers Pentium's architecture, combining both techniques for effective memory management. We delve into the hardware requirements and the roles of descriptor tables in managing segmented memory efficiently.

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Operating Systems

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Presentation Transcript

  1. Operating Systems Prof. Navneet Goyal Department of Computer Science & Information Systems BITS, Pilani

  2. Topics for Today • Segmentation • Segmentation vs. paging • Pentium Segmentation

  3. Paging • Suffers from internal fragmentation • NO user view • Page size fixed • In paging, user specifies only a single address • Single address is partitioned by HW into page no. and offset

  4. Segmentation: Introduction Requirements • Eliminate internal fragmentation • Should provide user view • User view program as data, code, stack, etc. • Partition (segment) size could be anything • Partitions (segments) could be placed anywhere in the memory

  5. Segmentation: Introduction For implementing, we need: • For each segment we need segment description consisting of: • Base address (starting address) of segment • Length of segment • Protection attributes • Segment description is stored in segment table • LAS is a collection of segments • Logical address should comprise of segment number & offset

  6. Segmentation • All segments of a programs do not have to be of the same length • There is a maximum segment length • Since segments are not equal, segmentation is similar to dynamic partitioning

  7. Segmentation • Memory-management scheme that supports user view of memory • A program is a collection of segments. A segment is a logical unit such as: main program, procedure, function, method, object, local variables, global variables symbol table, arrays

  8. User’s View of a Program

  9. 1 4 2 3 Logical View of Segmentation 1 2 3 4 physical memory space user space

  10. Segmentation Architecture • Logical address consists of a two tuple: <segment-number, offset>, • Segment table – maps two-dimensional physical addresses; each table entry has: • base – contains the starting physical address where the segments reside in memory • limit – specifies the length of the segment • Segment-table base register (STBR) points to the segment table’s location in memory • Segment-table length register (STLR) indicates number of segments used by a program; segment number s is legal if s < STLR

  11. Segmentation Hardware Base d

  12. Example of Segmentation

  13. Pentium Segmentation • X86 Processor has Several segment registers • CS (code segment) • DS (Data Segment) • SS (Stack segment ) • ES (Extra segment) • GS & FS segment • Pentium allows a segment to be as large as 4 GB • Selector is 16 bit • Offset is 32 bit

  14. Pentium Segmentation • LDT (Local Descriptor Table): one per process • GDT (Global Descriptor Table): one for the system • Pentium can support • pure segmentation or • (2) segmentation and paging

  15. Pentium Segmentation

  16. Pentium Segmentation

  17. Memory Management Registers • Processor provides 4 memory management registers : • GDTR Global descriptor table register • LDTR Local Descriptor Table register • IDTR Interrupt descriptor Table register • TR Task register • These registers specify the location of data structure which control segmented memory management

  18. Logical Address to linear address Translation

  19. Protection Ring

  20. Address Translation Steps • OS finds a place for LDT - It finds it at 0000500H • OS finds how much memory is needed by the segment. Finds free memory where it can be placed • Create LDT at 0000500H • Create entry in GDT corresponding to LDT • Scheduler finds the GDT entry and accesses it • Sets LDTR • Scheduler transfers the control of accessing the memory to processor

  21. Combined Paging & Segmentation • Paging is transparent to the programmer • Paging eliminates external fragmentation • Pieces are to be moved in and out – paging helps! • Segmentation is visible to the programmer • Segmentation allows for growing data structures, modularity, and support for sharing and protection • Each segment is broken into fixed-size pages

  22. Combined Paging & Segmentation • Why we need to combine paging & segmentation? • We want the best of both worlds!! • What about overheads in terms of HW support? • Is it worth?

  23. Combined Paging & Segmentation • Why we need to combine paging & segmentation? • We want the best of both worlds!! • What about overheads in terms of HW support? • Is it worth?

  24. Combined Paging & Segmentation • Segmentation and Paging can be used together. • Programmer is aware of segments. • Gains all the protection and sharing benefits. • Within each segment, paging is used. • Avoids external memory fragmentation • Uses memory efficiently. • Intel x86 processors since the 386 combine both.

  25. Combined Paging & Segmentation • User process • Segments • Pages

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