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Operating Systems Lecture 35 Segmentation Read Ch. 9.5 - 9.6

Operating Systems Lecture 35 Segmentation Read Ch. 9.5 - 9.6. Operator Overrides. To use some of the list functions, such as display( ), search( ) and valueInsert( ) or valueRemove( ), you need to override the output and comparison operators for your Process struct. Here's how:

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Operating Systems Lecture 35 Segmentation Read Ch. 9.5 - 9.6

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  1. Operating SystemsLecture 35 Segmentation Read Ch. 9.5 - 9.6 Operating System Concepts

  2. Operator Overrides To use some of the list functions, such as display( ), search( ) and valueInsert( ) or valueRemove( ), you need to override the output and comparison operators for your Process struct. Here's how: ostream& operator << (ostream& outs, const Process &myProcess) { outs << "Process ID: " << myProcess.pid << " Number of bursts: " << myProcess.numBursts << endl; return outs; } bool operator < (const Process proc1, const Process proc2) { return proc1.pid < proc2.pid; //This sorts by pid. You could choose } //something else, e.g. burstTime bool operator == (const Process proc1, const Process proc2) { return proc1.pid == proc2.pid; } bool operator != (const Process proc1, const Process proc2) { return proc1.pid != proc2.pid; } Operating System Concepts

  3. User's view of physical memory • Physical memory is a linear array of memory addresses. • With paging, the user's view is a contiguous storage space that is linear. The process is not actually stored contiguously in memory. • Users don't usually think of memory in terms of a linear array. • Users generally think in terms of a collection of items (segments) that are stored in memory with no particular order among segments. • Example: A C++ program may consist of: • A main function • A set of functions or classes • data structures • etc. • We refer to each of these by name. • We don't care where each piece is stored relative to another. Operating System Concepts

  4. User’s View of a Program Operating System Concepts

  5. Segmentation • Segmentation is a memory-management scheme that supports user view of memory. • The logical address space is a collection of segments. • A segment is a logical unit such as: main program, function, method, object, local variables, global variables, stack, symbol table, arrays • Each segment has a name and a length. For simplicity, the segments are given numbers. • The logical address specifies the segment number and the offset: <segment-number, offset> Operating System Concepts

  6. Logical View of Segmentation 1 4 2 3 1 2 3 4 user space physical memory space Operating System Concepts

  7. Segmentation Architecture • Logical address consists of two numbers: <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. • The limit register is used to check for valid memory references. • 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. Operating System Concepts

  8. Segmentation Hardware Diagram of segmentation hardware: We will draw this in class. Operating System Concepts

  9. Segmentation Hardware Operating System Concepts

  10. Example • Suppose a program is divided into 5 segments: • Segment 0: Subroutine • Segment 1: sqrt function • Segment 2: main program • Segment 3: stack • Segment 4: symbol table • The segment table is as follows: Segment Limit Base 0 1000 1400 1 400 6300 2 400 4300 3 1100 3200 4 1000 4700 Operating System Concepts

  11. Example of Segmentation We will draw a diagram in class. Question: Where does segment 1, offset 43 map in physical memory? Where does segment 2, offset 567 map? Operating System Concepts

  12. Example of Segmentation Operating System Concepts

  13. Protection with Segmentation • Protection. With each entry in segment table associate: • validation bit = 0  illegal segment • read/write/execute privileges • Protection bits associated with segments. • This makes more sense than with paging, because the entire segment is likely to have the same read/write/execute privileges. • For example, instructions for non-self-modifying code may be read-only or execute only. These will belong to a single segment and will not be grouped with other types of data. • In the paging system, a page may be half filled with instructions and half filled with data. Operating System Concepts

  14. Sharing of Segments • Each process has a segment table associated with it. • Segments can be shared when entries in the segment tables of two different processes point to the same physical location. • Sharing occurs at the segment level. Any information can be shared if it is defined as a segment. • Full programs can be shared (even with multiple segments). For example, text editors. • Parts of programs can be shared (e.g. functions from function libraries). Operating System Concepts

  15. Diagram of Segment Sharing Diagram of a text editor shared between 2 processes. We will draw this in class. Operating System Concepts

  16. Sharing of Segments Operating System Concepts

  17. Self Reference in Shared Code • What happens if a shared segment refers to itself? • If it refers to itself by segment number, then all processes using it must have the same segment number in their segment tables. • If it refers to itself indirectly, e.g. specifying an offset from the current program counter, then the processes can have different segment numbers for it in their segment tables. Operating System Concepts

  18. Fragmentation • Segments are of variable length. • Memory allocation is a dynamic process that uses a best fit or first fit algorithm. • Is there internal fragmentation in a segmentation system? • Is there external fragmentation? Operating System Concepts

  19. Segmentation vs. Paging • Paging and Segmentation each have different advantages and disadvantages. • What are advantages and disadvantages of paging? • No external fragmentation (advantage) • There is internal fragmentation (disadvantage) • Units of code and data are broken up into separate pages (disadvantage) • What are advantages and disadvantages of segmentation? • No internal fragmentation (advantage) • There is external fragmentation (disadvantage) • Keeps blocks of code or data as single units (advantage. • Can get advantages of both systems by combining them. Operating System Concepts

  20. Segmentation with Paging – MULTICS • The MULTICS system solved problems of external fragmentation and lengthy search times by paging the segments. • Solution differs from pure segmentation in that the segment-table entry contains not the base address of the segment, but rather the base address of a page table for this segment. • The offset for the segment is translated to a page number and an offset for that page. Operating System Concepts

  21. MULTICS Address Translation Scheme We will draw this diagram in class. Operating System Concepts

  22. MULTICS Address Translation Scheme Operating System Concepts

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