MEMORY MANAGEMENT: Real Storage. Unit IV

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1 MEMORY MANAGEMENT: Real Storage Unit IV

2 OUTLINE Storage Organization Storage Management Storage Hierarchy Storage Management Strategies Storage Placement Strategies Segmentation Paging & Demand Paging

3 Memory Types Relatively Expensive Memory Main Types Relatively Small High Performance Cheap Secondary Large Capacity Slow Extremely Fast Cache Stores frequently used data Small Size

4 Memory Hierarchy

5 MEMORY MANAGEMENT An art of Managing memory Memory Mgmt Storage Orgn Storage Mgmt

6 STORAGE ORGANIZATION view of main storage Single user/multiple user in main memory Fixed partition/ variable partition Contiguous block/ non-contiguous block

7 Storage Management: Real Storage Strategy to adopt for optimal performance When to place the program in main memory? where to place the program?

8 MANAGEMENT STRATEGIES Fetch MS Placement Decides which data to remove from main memory to make more space Replacement Demand Decides which piece of data to load next Anticipatory First Fit Worst Fit Best Fit Decides where in main memory to place incoming data FIFO LRU LFU NUR OPT

9 Storage Organization: Contiguous Vs Non-Contiguous Contiguous : Single Block of storage is allocated. Advantage: Easy to manage Disadvantage : Program may not get placed even if chunks are available. Non-Contiguous : Divide the program in segments. Segments may not be placed adjacently. Advantage: Unused blocks are used optimally. Disadvantage : Difficult to manage.

10 Single-User Contiguous Allocation Main Memory at a user s disposal Programs are limited to the size of memory. To run pgms larger than main memory : Overlays 0 a b c OS User Code Overlay

11 OVERLAYS 0 a b c OS Portion of Code that must stay in main memory Overlay (1) (2) (3) Program divided into logical sections, Only place currently active section in memory b b b Initialization Phase (1) Processing Phase (2) O/P Phase (3)

12 Overlays Drawbacks of Overlays: Think about it and Discuss.

13 Protection in a Single-User Environment Operating system must not be damaged by programs System cannot function if operating system overwritten Solution : Boundary register Contains address where program s memory space begins Any memory accesses outside boundary are denied Can only be set by privileged commands Applications can access OS memory to execute OS procedures using system calls, which places the system in executive mode

14 Protection in a Single-User Environment

15 Fixed Partition Multiprogramming I/O requests can tie up a processor for long periods Solution : Multiprogramming : Mulltiple pprograms reside in the memory at the same time Partitions remained fixed i.e. Each active process receives a fixed-size block of memory Processor rapidly switches between each process Multiple boundary registers protect against damage Intensive computations: CPU in use I/O, CPU WAIT USE CPU USE CPU USE CPU Processor utilization on a single-user system

16 FPM : Absolute Translation Fixed Partitions of equal size Separate job Qu for each partition Jobs of Q1 runs in P1 only else it waits until the partition is free. Operating System Job Qu for P 1 Job Qu for P 2 Job Qu for P 3 Partition 1 Partition 2 Partition 3 Job Qu for P 4 Partition 4 Storage Wastage If the requested partition was full, code could not load Later resolved by relocating translation

17 Storage Wastage in FPM (AT)

18 FPM : Relocatable Translation Fixed Partitions of equal size No Separate job Qu for each partition Common Job Qu for all Partitions Operating System Partition 1 Partition 2 Partition 3 Partition 4 A Job may be placed in any available partion in which it fits Storage Wastage eliminated to an extent

19 Fixed-Partition Multiprogramming : Protection Can be implemented by boundary registers, called base and limit (also called low and high)

20 FPM : DRAWBACKS Internal fragmentation Process does not take up entire partition, wasting memory

21 Variable Partition Multiprogramming System designers found fixed partitions too restrictive Internal fragmentation Potential for processes to be too big to fit anywhere Variable partitions designed as replacement

22 Variable Partition Multiprogramming Occupy the space as required No Fixed Boundaries Allocated the next available adjacent space. No wastage OS A 15K Unused Unused D 14K C 10K B 20K A 15K OS A 15K B 20K Free OS A 15K B 20K C 10K Free

23 Variable-Partition Characteristics Jobs placed where they fit No space wasted initially Internal fragmentation impossible Partitions are exactly the size they need to be External fragmentation can occur when processes removed Leave holes too small for new processes Eventually no holes large enough for new processes

24 External Fragmentation

25 Solution of External Fragmentation: Coalescing/ Compaction/Burping/GC) Free Storage List may be maintained by entering the location of new/additional hole Merge the new hole with existing holes (Coalescing/ Compaction/Burping/GC) OS A 15K B 20K C 10K Free Finish OS HOLE 15K B 20K C 10K Free OS B 20K HOLE 10K+15K+ Free: Compaction

26 Coalescing Vs Compaction Coalescing Combine adjacent free blocks into one large block Often not enough to reclaim significant amount of memory Compaction Sometimes called garbage collection (not to be confused with GC in object-oriented languages) Rearranges memory into a single contiguous block free space and a single contiguous block of occupied space Makes all free space available Significant overhead

27 Coalescing : Example

28 Compaction/Garbage Collection OS OS OS A 15K B 20K C 10K Free Finish HOLE 15K B 20K C 10K Free B 20K HOLE 10K+15K+ Free: Compaction

29 Memory Compaction

30 MEMORY MANAGEMENT: Virtual Storage

31 OUTLINE Virtual Storage: Basic Concepts MultiLevel Storage Organization Block Mapping Paging Segmentation Paging/Segmentation

32 Virtual Storage Disassociating the addresses referenced in a running process from the addresses available in primary storage Virtual Addresses (VA): Referenced by running programs Real Addresses (RA): Available in primary storage Virtual Address Space (V) : Range of VA s that may be referred by running program. Real Address Space (R) : Range of RA s that may be referred by running program. V >> R

33 Dynamic Address Translation Mapping of VA s to RA s Contiguous in V, Non-Contiguous in R: Artificial Contiguity NonConti Contiguous Virtual Storage Address Mapping Mechanism Real Storage

34 Block Mapping V >> R, byte-by-byte mapping is voluminous. Group information into blocks. Block s initial address need to be mapped. Blocks of same size : Pages & Mapping Technique: Paging Blocks of Different Sizes: Segments & Mapping Technique : Segmentation Dynamic Mapping

35 Virtual Address Translation With Base Address a a 1 + b BTOR Block Mapping Block Map Table b 1 Block Number b Virtual Address V =(b,d) Displacement d 2 b = a + b 3 b b + 4 Real Address R= b + d

36 Pure Paging Fixed Equal sized blocks: Pages 0 p-1 p 2p-1 2p 3p-1 4p 4p-1 Real Storage Page Frame 0 Page Frame Frame 1 p Page Frame 3 d Size p Size p Size p Size p p Page Number p Displacement d Virtual Address V =(p,d) Page no. p in which item resides at displacement d from base address p Page Mapping Mechanism p = p * page size RA=p +d

37 Page Mapping Mechanism Page Residence Bit Secondary Storage Address (if the page is not in real storage) Page frame no. if the page is in real storage r s p r= 0 if page is not in real storage r=1 if page is in real storage Page Map Table Entry

38 Paging by Pure Direct Mapping Base Address b b 1 p + b p PTOR Page Map Table 1 Page Number p Virtual Address V =(p,d) Displacement d 2 p = p + b 3 b p + 4 Real Address R= p + d

39 Paging by Pure Associative Mapping 1 Page Number p Displacement d Virtual Address V =(p,d) Associative Map p p p Real Address r= p + d

40 Paging by Combined Associative/Direct Mapping Base Address a b Performed only if no match in associative map 1 + b p p PTOR Page Map Table 1 Page Number p p p 2 Virtual Address V =(p,d) Displacement d Performed only if match in associative map 2 p = p + b Partial Associative Map (most active pages) p + b 3 4 Real Address R= p + d

41 Segmentation Block size is not fixed: SEGMENTS Segmentation : A program occupies separate segments/ blocks of real storage. The contents of the segment are given contiguous storage locations but one segment may not be adjacent to other. Major Limitation: one user segment may destroy another user segment. To protect individual segments: Storage Protection Keys.

42 Storage Protection of Segments OS A A B B C A C 2 CPU Storage Protection Key Storage Protection Key w.r.t user B: 2 User B Segments can refer to other segments with storage protection key 2. SPK are strictly under the control of OS. Manipulated by special privileged

43 Pure Segmentation Segment Number s Displacement d Virtual Address V =(s,d) A Process may run only if the current segment is in primary memory. An incoming segment may be placed in any available area in primary storage.

44 Dynamic Address Translation under Segmentation Base Address b b 1 + b s STOR Page Map Table s Segment Number s Virtual Address V =(s,d) s 2 s = s + b Displacement 4 d Real Address R= s + d

45 Address Control in Segmentation Type of Access Abrevn Explanation Remarks Read R Read the Segment Copy the segment Write W Write/Modify the segment Destroy the segment Execute E Execute/ Run the code Normally Denied Append A May add info at End May not modify already existing information.

Chapter 9 Real Memory Organization and Management

Chapter 9 Real Memory Organization and Management Outline 9.1 Introduction 9.2 Memory Organization 9.3 Memory Management 9.4 Memory Hierarchy 9.5 Memory Management Strategies 9.6 Contiguous vs. Noncontiguous