Main Memory. CISC3595, Spring 2015 X. Zhang Fordham University

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1 Main Memory CISC3595, Spring 2015 X. Zhang Fordham University 1

2 Memory Management! Background!! Contiguous Memory Allocation!! Paging!! Structure of the Page Table!! Segmentation!! Example: The Intel Pentium 2

3 Background! Main memory and registers are the only storages that CPU can access directly!! Register access: one CPU clock (or less)!! Main memory access: many CPU cycles!! Cache sits between main memory and CPU registers!! Memory is cheap today, and getting cheaper!! But applications are demanding more and more memory, there is never enough! 3

4 Review: Logic View of a Computer 4 Main memory:! a large array of words or bytes. Each words has its own address.

5 The need for memory management! Memory Management, involves swapping blocks of data from secondary storage!! Bring processes into main memory for execution by processor!! Protection of memory required to ensure correct operation!! Memory needs to be allocated to ensure a reasonable supply of ready processes to consume available processor time 8

6 Memory Management Requirements! Relocation!! Programmer does not know where program will be placed in memory!! it may be swapped to disk and return to main memory at a different location (relocated)!! Memory references in the program must be translated to actual physical memory address!! Protection!! Sharing!! Logical organization!! Physical organization 9

7 Requirements: Protection! Processes should not be able to reference memory locations in another process without permission!! Unless special arrangement is made, e.g., shared memory segment is setup!! Memory protection checking!! performed at run time!! Impossible to check absolute addresses to ensure protection at compile time!! Program might calculate address at run time, e.g., access array element, pointer to a data structure!! By processor (hardware)!! OS cannot anticipate all memory reference of a program!! Too time consuming if done by software 22

8 Requirements: Sharing! Allow several cooperating processes to access same portion of memory!! ex: system call shmget: to allocate shared memory segment!! Allow shared library!! Allow multiple processes running same program to access same copy of the program rather than have their own separate copy 24

9 Multistep Processing of a User Program Programmer s view of a program/process?! 11

10 Multistep Processing of a User Program Compiler seldom knows where an! object will reside, it assumes a! fixed base location (for example, zero).! 12

11 Multistep Processing of a User Program Linker/Linkage editor! * combines multiple object! files into a executable program,! resolving symbols as it goes along.! arranges objects in a program's! address space. Relocating code! that assumes a specific base address! to another base 13

12 Dynamic Linking! Linking postponed until execution time!! Small piece of code, stub, used to locate appropriate memory-resident library routine!! Stub replaces itself with the address of the routine, and executes routine!! Operating system needed to check if routine is in other processes memory address!! Dynamic linking is particularly useful for libraries!! also known as shared libraries 14

13 Demo! To view shared library requires by a program!! ldd /usr/bin/ls!! ldd lab4.out!! libpthread.so.0!! Flag -static will force linker to use static library (.a) instead of shared (.so)!! But static libraries not always installed by default. So if you need static link you have to install static libraries.!! Lab5: libproc (a library to be shared by all students).! 17

14 Multistep Processing of a User Program Loader start up programs:! read executable files into memory,! carry out other required preparatory! tasks so that the process can run!! Unix loader, i.e., handler for execve() :! * validation (permissions, memory! requirements etc.);! Copy program image from disk into memor! Copy command-line arguments on stack! Initialize registers (e.g., stack pointer)! Jump to program entry point (_start)!!! 15

15 Dynamic Loading Mechanism! Allow a program to load, execute and unload library (or binary code) at run time!! loads a library (or other binary) into memory!! retrieves addresses of functions and variables contained in the library!! executes those functions or access those variables!! unloads library from memory.!! Benefits: better memory-space utilization: unused routine is never loaded!! Useful when large amounts of code are needed to handle infrequently occurring cases 16

16 Dynamic Loading (cont d)!!!! void *lib_handle; double (*fn)(int *); int x; char *error; lib_handle = dlopen("./libctest.so.1.0", RTLD_LAZY); if (!lib_handle) { fprintf(stderr, "%s\n", dlerror()); exit(1); } fn = dlsym(lib_handle, "ctest1"); if ((error = dlerror())!= NULL) { fprintf(stderr, "%s\n", error); exit(1); } (*fn)(&x); printf("valx=%d\n",x); dlclose(lib_handle); 17

17 Logical vs. Physical Address Space! Logical address generated by CPU; also referred to as virtual address!! User program deals with logical addresses; it never sees the real physical addresses!! Physical address address seen by memory unit!! Memory-management Unit (MMU): hardware for run time mapping from logical address space to a physical address space!! Many different schemes for the mapping!! First let s see an simple example 20

18 Address protection: base and limit registers 23

19 Requirements: Logical Organization! Programs are written in modules!! Modules can be written and compiled independently!! Different degrees of protection given to modules: read-only, readable/writable!! Share modules among processes!! OS and hardware provide support for deal with such requirement through segmentation 25

20 Requirements: Physical Organization! Keep track of available memory blocks, allocate and reclaim them as processes come and go!! Cannot leave programmer with the responsibility to manage memory!! Memory available for a program plus its data may be insufficient!! Overlaying: program and data are organized in such a way that various modules can be assigned same region of memory, main program responsible for switching modules in and out => costly for programmer!! Programmer does not know how much space will be available, and where 26

21 Roadmap: Memory Allocation Schemes! Contiguous Allocation!! Paging!! Segmentation 27

22 No Memory Abstraction Figure 3-1. Three simple ways of organizing memory with an operating system and one user process. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

23 Multiple Programs Without Memory Abstraction Figure 3-2. Illustration of the relocation problem. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

24 Base and Limit Registers Figure 3-3. Base and limit registers can be used to give each process a separate address space. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

25 Swapping (1) Figure 3-4. Memory allocation changes as processes come into memory and leave it. The shaded regions are unused memory. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

26 Swapping (2) Figure 3-5. (a) Allocating space for growing data segment. (b) Allocating space for growing stack, growing data segment. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

27 Memory Management with Bitmaps Figure 3-6. (a) A part of memory with five processes and three holes. The tick marks show the memory allocation units. The shaded regions (0 in the bitmap) are free. (b) The corresponding bitmap. (c) The same information as a list. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

28 Memory Management with Linked Lists Figure 3-7. Four neighbor combinations for the terminating process, X. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

29 Contiguous Allocation! Main memory usually divided into two partitions:!! Resident operating system, usually held in low memory with interrupt vector!! User processes held in high memory!! Relocation registers used to protect user processes from each other, and from changing operating-system code and data!! 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 28

30 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 29

31 Dynamic Storage-Allocation Problem How to satisfy a request of size n from a list of free holes! First-fit: Allocate the first hole that is big enough!! Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size!! Produces the smallest leftover hole!! Worst-fit: Allocate the largest hole; must also search entire list!! Produces the largest leftover hole 30 First-fit and best-fit better than worst-fit in terms of speed and storage utilization

32 Fragmentation problem! External Fragmentation total memory space exists to satisfy a request, but it is not contiguous!! Internal Fragmentation allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used!! Reduce external fragmentation by compaction!! Shuffle memory contents to place all free memory together in one large block!! Possible only if relocation (final binding to memory address) is done at execution time!! To solve external fragmentation problem!! Allocate noncontiguous memory to process!! Paging, Segmentation, and Combined 31

33 Roadmap: Memory Allocation Schemes! Contiguous Allocation!! Paging!! Segmentation 27

34 Paging! Physical address space of a process can be noncontiguous!! process is allocated physical memory wherever it is available!! Logical address space of a process is still contiguous!! Physical memory divided into fixed-sized blocks called frames!! Frame size is power of 2, between 512 bytes and 8,192 bytes!! Logical memory divided into blocks of same size called pages!! System keeps track of all free frames!! To run a program of size n pages, need to find n free frames!! Set up a page table for logical => physical addresses translation!! Paging scheme address external fragmentation!! internal fragmentation is still possible 32

35 Paging Model of Logical and Physical Memory A process s page table: for logical => physical addresses translation! * contains base address of each page in physical memory 33

36 Logic address under Paging scheme! Logical address (address generated by CPU) is divided into:!! Page number (p) an index into a page table which contains base address of each page in physical memory!! Page offset (d) relative address (within page), combined with base address to define physical memory address that is sent to the memory unit! page number page offset! p d! m - n n!!! For given logical address space 2 m and page size 2 n 34

37 MMU: Paging 35

38 Paging Example byte memory and 4-byte pages

39 Free Frames 37 Before allocation After allocation

40 Implementation of Page Table! Where to store the page tables?!! Register? too small!! 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 page table and one for the actual data/instruction.!! To avoid two memory access problem. 38

41 Associative Memory! a special fast-lookup hardware cache called associative memory or translation look-aside buffers (TLBs)!! Some TLBs store address-space identifiers (ASIDs) in each entry uniquely identifies each process to provide address-space protection!! Associative memory parallel search for all entries!!!!! Page # Frame #! Address translation (p, d)!! If p is in associative register, get frame # out!! Otherwise get frame # from page table in memory 39

42 Paging Hardware With TLB 40

43 Effective Access Time! Associative Lookup = ε time unit (e.g. 20 ns)!! Hit ratio percentage of times that a page number is found in the associative registers; ratio related to number of associative registers!! Hit ratio = α (e.g. 80%)!! Time to access memory (e.g. 100 ns)!! Effective Access Time (EAT)! e.g. EAT = 0.80 x x ( )! (found in TLB) =140 ns (not found in TLB)! 41

44 Memory Protection! Memory protection implemented by associating protection bit with each frame! Valid-invalid bit 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 42

45 Valid (v) or Invalid (i) Bit In A Page Table Most processes do not use whole logic address space Address space: 2 14 (0,, 16383),! Process use only 0 to 10468! Page size: 2KB Alternative: page-table length register (PTLR) 43

46 Shared Pages! Useful for shared code (reentrant code, pure code)!! One copy of read-only code shared among processes (i.e., text editors, compilers, window systems).!! Each process has its own copy of registers and data storage to hold its data!! Shared code page and private data page 44

47 Shared Pages Example 45

48 Structure of the Page Table! To support large logic address space!! up to 2 32 or 2 64!! page table too large to put in a contiguous memory!! Solutions!!!! Hierarchical Paging!! Hashed Page Tables!! Inverted Page Tables 46

49 Hierarchical Page Tables! Break up the logical address space into multiple page tables!!! A simple technique is a two-level page table!! e.g. for large logical address spaces 2 32 to

50 Two-Level Page-Table Scheme 48

51 Two-Level Paging Example! A logical address (on 32-bit machine with 1K page size) is divided into:!! a page number consisting of 22 bits!! a page offset consisting of 10 bits!! Since the page table is paged, the page number is further divided into:!! a 12-bit page number!! a 10-bit page offset!! Thus, a logical address is as follows: page number page offset p i p 2 d where p i is an index into the outer page table, and p 2 is the displacement within the page of the outer page table

52 Address-Translation Scheme 50

53 Three-level Paging Scheme 51

54 Hashed Page Tables! Common in address spaces > 32 bits!!! Virtual page number is hashed into a page table. This page table contains a chain of elements hashing to the same location.!!! Virtual page numbers are compared in this chain searching for a match. If a match is found, the corresponding physical frame is extracted. 52

55 Hashed Page Table 53

56 Inverted Page Table! One entry for each real page of memory!! Entry consists of virtual address of the page stored in that real memory location, with information about 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!! Use hash table to limit the search to one or at most a few page-table entries 54

57 Inverted Page Table Architecture 55

58 Segmentation! Memory-management scheme that supports user view of memory!! A program is a collection of segments. A segment is a logical unit such as:!!! main program,!!! procedure,!!! function,!!! method,!!! object,!!! local variables, global variables,!!! common block,!!! stack,!!! symbol table, arrays 56

59 Logical View of Segmentation user space physical memory space 57

60 Segmentation Architecture! A process s logical address consists of a two-tuple:!!! <segment-number, offset>,!! Segment table maps two-dimensional physical addresses; each table entry has:!! base contains starting physical address where the segments reside in memory!! limit specifies length of the segment!! Segment-table base register (STBR)!! points to 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 58

61 Segmentation Architecture (Cont.)! Protection!! With each entry in segment table associate:!! validation bit = 0 illegal segment!! read/write/execute privileges!! Protection bits associated with segments; code sharing occurs at segment level!! Since segments vary in length, memory allocation is a dynamic storage-allocation problem 59

62 Segmentation Hardware 60

63 Example of Segmentation 61

64 Example: The Intel Pentium! Supports both segmentation, and segmentation with paging!! CPU generates logical address!! Given to segmentation unit!! Which produces linear addresses!! Linear address given to paging unit!! Which generates physical address in main memory 62

65 Logical => Physical Address: in Pentium 63

66 Intel Pentium Segmentation 64

67 Pentium Paging Architecture 65

68 Linux on Pentium Systems Uses six segments:! 1. A segment for kernel code! 2. A segment for kernel data! 3. A segment for user code! 4. A segment for user data! 5. A task-state segment (TSS)! 6. A default local descriptor table (LDT) segment 66

69 Three-level Paging in Linux Linear Address in Linux 67

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