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Chapter 3

Chapter 3. Memory Management II In this chapter we continue our study of memory management techniques. I. Paged Memory Allocation. this technique still requires the entire program to load into memory, but, not into contiguous memory.

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Chapter 3

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  1. Chapter 3 • Memory Management II • In this chapter we continue our study of memory management techniques.

  2. I. Paged Memory Allocation • this technique still requires the entire program to load into memory, • but, not into contiguous memory. • secondary storage and primary memory are divided into equal size units • these are called page frames , for example, 512 bytes per page frame

  3. Paged Memory Allocation, continued • The Jobs to be executed are divided into the same sized units called pages • the operating system loads pages of a job into free page frames in memory

  4. Paged Memory Allocation, continued • to keep track of what's where the operating system uses tables: • The PMT= Page Memory Table lists ]ocation of each page of a job • i.e. each job has a PMT which tells what page frames its pages occupy

  5. Paged Memory Allocation, continued • The Job Table lists each job and location of its Page Memory Table • The Memory Map Table lists every memory page frame and its status • i.e. free or busy.

  6. How the OS locates a page: • To find any particular location within a program: • the operating system must use an address reference to compute a page number and offset

  7. For Example, consider this instruction Load x, 0214 • (This is an instuction which says load the contents of register X into memory location 0214.)

  8. Suppose one page = 100 bytes 214 div100 = 2 214 mod 100 = 14 • The operating system then consults the Page Memory Table to see which physical page frame page 2 is loaded at.

  9. Thus, the desired location is at location 14 relative to the beginning of that page. • Suppose, for example, page 2 is loaded at page frame 5 in memory. 5 x 100 + 14 will be the location

  10. Internal fragmentation is still a problem but only on the last page of a job. • tasks are often subdivided at physical subdivisions which don't correspond to any logical program subdivisions.

  11. II. Demand Paging • A newer variation of paging is “demand paging” which no longer requires all pages of a job to be loaded into memory at execution. • Demand Paging facilitated development of Virtual Memory

  12. Jobs are still divided into equal sized pages, • consequently, one logical module of a job may often cover one or more pages.

  13. Demand Paging continued • In Demand Paging the operating system keeps same tables except that three new fields are added to PMT: • a "page in memory” Boolean field • a "contents modified” Boolean field

  14. Demand Paging continued • a “referenced recently” Boolean field or • this may be a time-stamp field if a Boolean field is set to "t" when page is accessed - • Look at figure 3.5, p54

  15. As a job processes ……... • if a location is referenced the operating system needs to determine its page # • and it must check the Page Memory Table to see if that page is resident, • i.e., loaded in some page frame.

  16. If the page is resident then, OK, otherwise the needed page must be loaded into memory. • This implies "swapping” the page into an availab]e page frame.

  17. If none are available it must swap one out using some algorithm to decide which. • This is known as a "page fault" and • dealing with it is the responsibility of the Page Interrupt Handler.

  18. As the page swap is done the operating system must update its tables, • i.e.. Update the PMT for both affected pages and Memory Map Table.

  19. Thrashing • Frequent page swapping is called " Thrashing" . • This is a very poor use of processor's Time. • See figure 3.6 Pg. 56

  20. III. Page Replacement Policies • Page Replacement Policies have been studied extensively. Several in common use: • Longest Time Resident is a "first in first out" (FIFO) policy in which • resident pages are tracked by a queue arranged by residency time.

  21. Least Recently Used is another FIFO po]icy in which • pages are tracked by a queue arranged by the time a page was last accessed.

  22. Most Recently Used • a LIFO policy which, as its name suggests, • results in the operating system swapping the page most recently accessed,

  23. Least Frequently Used • a policy which is often very effective. • The queue is arranged by frequency of use • See Tables 3.3, 3.4, 3.5 Pages 58 - 61.

  24. IV. The Working Set Concept • As a job processes often it uses a certain set of its pages most often. • If there is enough room in memory the OS tries to keep this "working set" of pages resident • thus page faults are minimal and swapping for this job is greatly reduced.

  25. V. Segmented Memory Allocation • This method is somewhat like paging, but the units are not all of same size. • This method is based on idea that a structured program consists of (semi) independent modules which may be of varying sizes.

  26. Memory is not divided into pages as before, rather, • The operating system allocates memory in various segment sizes dynamically, as needed. • Like demand paging, jobs are not required to load totally or into contiguous memory.

  27. As a job is compiled or assembled code segments are created and numbered sequentially. • The operating system still keeps a Job Table and Memory Map table, but • It now keeps a segment table...

  28. The Segment Table for each job contains • segment numbers, • segment sizes, • memory load address, and • fields for "segment recently referenced" and "access rights", • (see figures p65).

  29. each segment is indivisible but • not all are loaded at once or in contiguous memory.

  30. As with the Dynamic Partition method • External Fragmentation can be a problem so periodic Compaction is needed. • There is no internal fragmentation since segment size is dynamic.

  31. VI. Segmented/Demand Paging • This method combines the ideas of the last two. • It tries to reduce external fragmentation by subdividing segments into pages. • In this methcd jobs are divided into segments but • pages and page frames also exist.

  32. When a segment is loaded all of its pages load into memory • but not necessarily contiguously.

  33. the operating system uses Four Tables to manage all of this information: • a Job Table, • a Memory Map Table, • a Segment Map Table for each job and • a Page Map Table for each segment. (pages of a segment are not necessarily contiguous) • See figure 3.11, page 68.

  34. This method manages memory efficiently, but it has a lot of overhead. • It eliminates external fragmentation and most internal fragmentation.

  35. VI. Virtual Memory Management • Invented by IBM, • this technique evolved from the technique of Demand Paging

  36. Virtual Memory Management gives users the impression that their jobs are loading totally and • that memory is virtually unlimited.

  37. Before Virtual Memory programmers often worked hard to squeeze code into precious memory • Programmers developed an overlay technique in which some sections of code would replace others.

  38. Virtual Memory works very well in multitasking environments. • It takes advantage of speed disparity between processor and users. • Entire jobs are sometimes swapped out between execution slices! • See pp. 71, 72 for other adventages.

  39. End of Chapter 3

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