Chapter 7: Main Memory

1. Chapter 7: Main Memory CS 170, Fall 2011

2. Program Execution & Memory Management • Program execution • Swapping • Contiguous Memory Allocation • Paging • Structure of the Page Table • Segmentation

3. Multistep Processing of a User Program

4. Binding of Instructions and Data to Memory • happen at three different stages • Compile time: If memory location known a priori, absolute codecan be generated. • Load time: Must generate relocatable code if memory location is not known at compile time • Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another.

5. Logical vs. Physical Address Space • Logical address– generated by the CPU; also referred to as virtual address • Physical address– address seen by the memory unit • Logical and physical addresses are the same in compile-time and load-time address-binding schemes; • logical (virtual) and physical addresses differ in execution-time address-binding scheme

6. Memory-Management Unit (MMU) • Hardware device that maps virtual to physical address • In MMU scheme, the value in the relocation register is added to every address generated by a user process at the time it is sent to memory

7. Base and Limit Registers • A pair of baseandlimitregisters define the logical address space

8. Hardware Support for Relocation and Limit Registers

9. Swapping • A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution • Backing store– fast disk large enough to accommodate copies of memory images for user processes • Roll out, roll in– swapping variant used for priority-based scheduling algorithms; lower-priority process is swapped out so higher-priority process can be loaded and executed

10. Schematic View of Swapping

11. Contiguous Memory Allocation • Main memory has two partitions: • Resident operating system, usually held in low memory • User processes then held in high memory • Hardware support and protection • Base register contains value of smallest physical address • Limit register contains range of logical addresses – each logical address must be less than the limit register • MMU maps logical address dynamically OS process 5 process 8 process 2

12. Contiguous Allocation (Cont.) • Multiple-partition allocation • Hole – block of available memory; holes of various size are scattered throughout memory • When a process arrives, it is allocated memory from a hole large enough to accommodate it • Operating system maintains information about:a) allocated partitions b) free partitions (hole) OS OS OS OS process 5 process 5 process 5 process 5 process 9 process 9 process 8 process 10 process 2 process 2 process 2 process 2

13. Example: Allocate 16K block 8K 12K 22K Last allocated block (14K) Which free block to allocate for a new request of 16K block? 18K 8K 6K Allocated block Free block 14K 36K Before

14. Dynamic Memory Allocation Methods How to satisfy a request of size n from a list of free holes • First-fit: Allocate the first hole that is big enough • Fast • Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size • Produces the smallest leftover hole • Slow, leading more external fragmentation • Worst-fit: Allocate the largest hole; must also search entire list • Produces the largest leftover hole • Slow

15. Example: Allocate 16K block 8K 8K 12K First Fit 12K 22K 6K Last allocated block (14K) Best Fit 18K 2K 8K 8K 6K 6K Allocated block Free block 14K 14K Worst Fit 36K 20K Before After

16. Fragmentation Issues • External Fragmentation– total memory space exists to satisfy a request, but it is not contiguous Case 1 Case 2

17. Methods to reduce external fragmentation • Compaction • Shuffle memory contents to place all free memory together in one large block • Compaction is possible only if relocation is dynamic, and is done at execution time • I/O problem • Latch job in memory while it is involved in I/O • Do I/O only into OS buffers • Another strategy: paging

18. Example of Relocation for Compaction

19. Paging: Equal-size memory partitioning • Divide physical memory into fixed-sized blocks called frames (size is power of 2, between 512 bytes and 8,192 bytes) • Divide logical memory into blocks of same size called pages or logical pages • To run a program of size n pages, need to find n free frames and load program

20. Paging Model of Logical and Physical Memory A page table is needed to translate logical to physical addresses

21. Management of Free Space After allocation Before allocation

22. process 1 Fragmentation Issues • Remove/reduce external fragmentation, but lead to Internal Fragmentation – allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used 8K 8K 8 K allocated for Process 1 8 M Unused 8K 8K

23. Address Translation Scheme • Address generated by CPU is divided into: • Page number (p)– used as an index into a pagetable which contains base address of each page in physical memory • Page offset (d)– combined with base address to define the physical memory address that is sent to the memory unit page offset page number p d m - n n

24. Paging Hardware

25. Hardware Support of Page Table • Page table is kept in main memory • Page-table base register (PTBR)points to the page table • Page-table length register (PRLR)indicates size of the page table • Every data/instruction access requires two memory accesses. • One for the page table and one for the data/instruction. • Slowness due to two memory access can be addressed by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers (TLBs)

26. Associative Memory for TLB • Associative memory – parallel search Address translation (p, d) • If p is in associative register, get frame # out • Otherwise get frame # from page table in memory Page # Frame #

27. Paging Hardware With TLB

28. Example: Performance Characteristics of TLB • Typical TLB • Size: 8 - 4,096 entries • Access time: 0.5 - 1 clock cycle • Miss penalty: 10 - 100 clock cycles • Miss rate: 0.01 - 10% • If a TLB hit takes 1 clock cycle, a miss takes 30 clock cycles, and the miss rate is 1%, • the effective memory cycle rate for page mapping: 1*0.99 + (1+30)X0.01=1.30 • Total memory access = page mapping + memory access = 31.30 clock cycles

29. Memory Protection • Memory protection implemented by associating protection bit with each frame • Valid-invalidbit attached to each entry in the page table: • “valid” indicates that the associated page is in the process’ logical address space, and is thus a legal page • “invalid” indicates that the page is not in the process’ logical address space

30. Valid (v) or Invalid (i) Bit In A Page Table

31. Shared Pages through Paging • Shared code • One copy of read-only code shared among processes (i.e., text editors, compilers, window systems). • Shared code must appear in same location in the logical address space of all processes • Private code and data • Each process keeps a separate copy of the code and data • The pages for the private code and data can appear anywhere in the logical address space

32. Shared Pages Example

33. Structure of the Page Table • How is a page table implemented? • Hierarchical Paging • Hashed Page Tables • Inverted Page Tables

34. One-Level Page Table • What is the maximum size of logical space? Mapping of pages

35. Hierarchical Page Tables • Motivating example: • 32 -bit address space with 4KB per page. • Page table would contain 2^32/ 2^12= 1 million entries. • 4 bytes per entry • Need a 4MB page table with contiguous space. • Is there 4MB contiguous space for each process? • Can we divide this page table into multiple page tables hierarchically?

36. Two-Level Page-Table Scheme

37. Two-Level Paging Example • A logical address (on 32-bit machine with 4K page size) is divided into: • a page number consisting of 20 bits • a page offset consisting of 12 bits • The page number is further divided into: • a 10-bit page number • a 10-bit page offset where pi is an index into the outer page table, and p2 is the displacement within the page of the outer page table page number page offset p2 pi d 10 10 12

38. Address-Translation Scheme

39. Three-level paging in a 64-bit address space

40. Linear Address in Linux Broken into four parts:

41. Hashed Page Tables • Common in address spaces > 32 bits • Use hash table to limit the cost of search • to one — or at most a few — page-table entries • One hash table per process • This page table contains a chain of elements hashing to the same location • Use this hash table to find the physical page of each logical page • If a match is found, the corresponding physical frame is extracted

42. Hashed Page Table

43. Inverted Page Table • One hash table for all processes • One entry for each real page of memory • Entry consists of the virtual address of the page stored in that real memory location, with information about the process that owns that page • Decreases memory needed to store each page table, but increases time needed to search the table when a page reference occurs

44. Inverted Page Table Architecture

45. Segmentation • Memory-management scheme that supports user semantic view of memory • A program is a collection of segments • A segment is a logical unit such as: main program procedure function method object common block stack symbol table arrays

46. Example of Segmentation

47. Segmentation Hardware