1. Lecture 35 Syed Mansoor Sarwar
Operating Systems
Lecture 35
Syed Mansoor Sarwar

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Agenda for Today
Review of previous lecture
Sharing in paging
Segmentation
Logical to physical address translation
Hardware support needed
Protection and sharing
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Review of Lecture 34
Performance of paging
Protection in paging
Structure of the page table
Multi-level paging
Hashed page tables
Inverted page table
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Shared Pages
Reentrant (read-only) code pages of a process address space can be shared
Shared code appears in the same logical address space of all processes
Multiple invocations of pico or gcc
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Shared Pages
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Segmentation
A memory management scheme that supports programmer’s view of memory.
A segment is a logical unit such as: main program, procedure, function, method, object, global variables, stack, symbol table
A program is a collection of segments
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Segmentation
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Segmentation 1 2 3 4 5
logical memory
physical memory
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Segmentation 1 1 4 2 3 1 4 2 5 3 4 2 3 5
logical memory
physical memory
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Segmentation 1 1 4 2 3 4 1 2 5 3 4 2 3
segment table 5
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logical memory
physical memory
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Segmentation
Logical address consists of a two tuple:
<segment-number, offset>
Segment table – maps two-dimensional logical addresses to physical addresses
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Segmentation
Each segment table entry has:
base – contains the starting physical address where the segments reside in memory.
limit – specifies the length of the segment.
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Segmentation
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
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Segmentation
CPU
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15. Segmentation Architecture
Relocation
Dynamic
By segment table
Sharing
Shared segments
Same segment number
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16. Segmentation Architecture
Dynamic Storage Allocation
First fit
Best fit
Worst fit
External fragmentation
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17. Segmentation Architecture
Protection: Bits are associated for this purpose with each entry in segment table:
Validation bit = 0  illegal segment
Read, write, execute bits
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Example
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Address Translation
Logical and Physical Addresses
(2, 399) – PA: = 4699
(4, 0) – PA: = 4700
(4, 1000)  trap
(3, 1300)  trap
(6, 297)  trap
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Sharing of Segments
Sharing at the segment level and not at the fixed-size page level
Sharing at the code or data level
Segment table of multiple processes point to the same segment
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Sharing of Segments
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22. Issues with Segmentation
External Fragmentation
Total memory space exists to satisfy a space allocation request for a segment, but memory space is not contiguous.
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23. Issues with Segmentation
Reduce external fragmentation by compaction
Shuffle segments to place free memory together in one block.
Compaction is possible only if relocation is dynamic, and is done at execution time.
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24. Issues with Segmentation
I/O problem
Latch job in memory while it is involved in I/O.
Do I/O only into OS buffers
Very large segments  page program segments—paged segmentation
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Recap of Lecture
Sharing in paging
Segmentation
Logical to physical address translation
Hardware support needed
Protection and sharing
Issues with segmentation
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Operating Systems
Lecture 35
Syed Mansoor Sarwar
29 April 2019
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