Chapter07_ds.ppt
- 2. 2 Agenda • Discuss the principal requirements for memory management. • Understand the reason for memory partitioning and explain the various techniques that are used. • Understand and explain the concept of paging. • Understand and explain the concept of segmentation. • Assess the relative advantages of paging and segmentation. • Summarize key security issues related to memory management. • Describe the concepts of loading and linking.
- 3. 3 Memory Management • What??? – Uniprogramming System Vs Multiprogramming System • Subdividing user part of memory to accommodate multiple processes dymamically. • Why??? • Memory needs to be allocated to ensure a reasonable supply of ready processes to consume available processor time
- 4. 4 Memory Management Requirements • Purpose???? – The requirements that memory management is intended to satisfy • Relocation – Programmer does not know where the program will be placed in memory when it is executed – While the program is executing, it may be swapped to disk and returned to main memory at a different location (relocated) – Memory references must be translated in the code to actual physical memory address
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- 6. 6 Memory Management Requirements • Protection – Processes should not be able to reference memory locations in another process without permission – Impossible to check absolute addresses at compile time – Must be checked at run time – Memory protection requirement must be satisfied by the processor (hardware) rather than the operating system (software) • Operating system cannot anticipate all of the memory references a program will make
- 7. 7 • Sharing – Allow several processes to access the same portion of memory – To allow each process access to the same copy of the program rather than have their own separate copy Memory Management Requirements
- 8. 8 • Logical Organization – main memory in a computer system is organized as a linear 1D address space consisting of a sequence of bytes or words. – Programs are written in modules – Modules can be written and compiled independently – Different degrees of protection given to modules (read-only, execute-only) – Share modules among processes Memory Management Requirements
- 9. 9 • Physical Organization – Memory available for a program plus its data may be insufficient • Overlaying allows various modules to be assigned the same region of memory – Programmer does not know how much space will be available – The task of moving information between the two levels of memory should be a system responsibility Memory Management Requirements
- 10. Essence of memory management. – The task of moving information between the two levels of memory should be a system responsibility Not Programmer’s 10 Memory Management Requirements
- 11. Principle responsibility of memory management – To bring processes into main memory for execution by the processor to ensure the sufficient no. of ready processes. 11 Memory Management Requirements
- 12. 12 Memory Partitioning: Fixed Partitioning • Equal-size partitions – Any process whose size is less than or equal to the partition size can be loaded into an available partition – If all partitions are full, the operating system can swap a process out of a partition – A program may not fit in a partition. The programmer must design the program with overlays
- 13. 13 Fixed Partitioning • Main memory use is inefficient. Any program, no matter how small, occupies an entire partition. This is called internal fragmentation. • A program may not fit in a partition. It become programmers headache to write the program in overlayed form.
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- 15. 15 Placement Algorithm with Partitions • Equal-size partitions – Because all partitions are of equal size, it does not matter which partition is used • Unequal-size partitions – Can assign each process to the smallest partition within which it will fit – Queue for each partition – Processes are assigned in such a way as to minimize wasted memory within a partition
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- 17. 17 Dynamic Partitioning • Partitions are of variable length and number • Process is allocated exactly as much memory as required • Eventually get holes in the memory. This is called external fragmentation • Must use compaction to shift processes so they are contiguous and all free memory is in one block
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- 19. 19 Dynamic Partitioning Placement Algorithm • Operating system must decide which free block to allocate to a process • Best-fit algorithm – Chooses the block that is closest in size to the request – Worst performer overall – Since smallest block is found for process, the smallest amount of fragmentation is left – Memory compaction must be done more often
- 20. 20 Dynamic Partitioning Placement Algorithm • First-fit algorithm – Scans memory form the beginning and chooses the first available block that is large enough – Fastest – May have many process loaded in the front end of memory that must be searched over when trying to find a free block
- 21. 21 Dynamic Partitioning Placement Algorithm • Next-fit – Scans memory from the location of the last placement – More often allocate a block of memory at the end of memory where the largest block is found – The largest block of memory is broken up into smaller blocks – Compaction is required to obtain a large block at the end of memory
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- 23. 23 Buddy System • Both fixed and dynamic partitioning schemes have drawbacks. – A fixed partitioning scheme limits the number of active processes – May use space inefficiently if there is a poor match between available partition sizes and process sizes. – A dynamic partitioning scheme is more complex to maintain – Also includes the overhead of compaction.
- 24. 24 Buddy System • Memory blocks are available of size 2K words, L<= K<= U • 2L smallest size block that is allocated • 2U largest size block that is allocated; generally 2U is the size of the entire memory available for allocation. • Initially, • Entire space available is treated as a single block of 2U • If a request of size s such that 2U-1 < s <= 2U, entire block is allocated – Otherwise block is split into two equal buddies – Process continues until smallest block greater than or equal to s is generated.
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- 27. 27 Buddy Systems • Application of Buddy systems: – In parallel systems as an efficient means of allocation and release for parallel programs. • A modified form of the buddy system is used for UNIX kernel memory allocation
- 28. 28 What next???? • Ways of dealing with the shortcomings of partitioning let it be any – Fixed • Equal sized • Unequal sized – Dynamic – Buddy
- 29. 29 What next???? • A process may occupy different partitions during the course of its life. • When a process image is first created, it is loaded into some partition in main memory. • The process may be swapped out; • When it is subsequently swapped back in, it may be assigned to a different partition than the last time.
- 30. 30 Relocation • When program loaded into memory the actual (absolute) memory locations are determined. • A process may occupy different partitions which means different absolute memory locations during execution (from swapping) • Compaction will also cause a program to occupy a different partition which means different absolute memory locations.
- 31. 31 Addresses • A distinction is made among several types of addresses. • Logical – Reference to a memory location independent of the current assignment of data to memory – Translation must be made to the physical address • Relative – Address expressed as a location relative to some known point (Type of logical address) • Physical – The absolute address or actual location in main memory
- 32. 32 Registers Used during Execution • Base register – Starting address for the process • Bounds register – Ending location of the process • These values are set when the process is loaded/updated or when the process is swapped in.
- 33. 33 Typically, all of the memory references in the loaded process are relative to the origin of the program
- 34. 34 Process of Relocation • The value of the base register is added to a relative address to produce an absolute address. • The resulting address is compared with the value in the bounds register. • If the address is not within bounds, an interrupt is generated to the operating system
- 35. 35 Small exercise • Consider a fixed partitioning scheme with equal-size partitions of 2^16 bytes and a total main memory size of 2^24 bytes. A process table is maintained that includes a pointer to a partition for each resident process. How many bits are required for the pointer?
- 36. 36 Paging: • Partition memory into small equal fixed-size chunks and divide each process into the same size chunks • The chunks of a process are called pages and chunks of memory are called frames • Operating system maintains a page table for each process – Contains the frame location for each page in the process – Memory address consist of a page number and offset within the page
- 37. 37 Assignment of Process Pages to Free Frames
- 38. 38 Assignment of Process Pages to Free Frames
- 39. 39 Page Tables for Example
- 40. 40 Segmentation • All segments of all programs do not have to be of the same length • There is a maximum segment length • Addressing consist of two parts - a segment number and an offset • Since segments are not equal, segmentation is similar to dynamic partitioning
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- 45. Let us do : A 1-Mbyte block of memory is allocated using the buddy system. a. Show the results of the following sequence in a figure Request 70; Request 35; Request 80; Return A; Request 60; Return B; Return D; Return C. b. Show the binary tree representation following Return B. 45
- 46. Let us do : (For chapter 8) Consider a simple paging system with the following parameters: 2^32 bytes of physical memory; page size of 2^10 bytes; 2^16 pages of logical address space. a. How many bits are in a logical address? b. How many bytes in a frame? c. How many bits in the physical address specify the frame? d. How many entries in the page table? e. How many bits in each page table entry? 46
- 47. • Physical: 232 • Page Size: 210 • Number of Pages: 216 • How many bits in a logical address? That's the page address bits plus the number of pages bits. The upper portion of an address is the page number (16 bits), and the lower portion is the offset within that address (10 bits), so the whole address size is 26 bits (1026 bytes). 47
- 48. • Physical: 232 • Page Size: 210 • Number of Pages: 216How many bytes are in a frame? A frame is where a page can be mapped into memory, so a frame has to be the same size as a page - 210 bytes. • How many bits are in the physical address specifying the frame? Well, you have 32 bits of physical address, and a frame is 210 big, so that leaves 22 of the bits (32 - 10) for the frame's base address. 48
- 49. • Physical: 232 • Page Size: 210 • How many entries in the page table? The page table is the full list of pages, whether mapped or unmapped - so there are 216 entries in the page table, since there are 216 pages. 49
- 50. • Physical: 232 • Page Size: 210 • How many bits in each page table entry? Assume each page table entry contains a valid/invalid bit. If each page maps to an entry in the page table, and that table is a list of the addresses at which the pages are mapped in physical memory, then each address in the table must be the size of answer 3 (22 bits) plus one bit for the valid/invalid bit, so 23 bits. 50