上海交通大学：《操作系统 Operating System》课程教学资源（课件讲稿）OS-Lec17_mainmemory

CHAPTER 8:MAIN MEMORY

CHAPTER 8: MAIN MEMORY

CHAPTER 8:MEmory mAnagement o Background o Swapping o Contiguous Memory allocation o Paging o Structure of the Page Table o Segmentation o Example:The Intel Pentium

CHAPTER 8: MEMORY MANAGEMENT  Background  Swapping  Contiguous Memory Allocation  Paging  Structure of the Page Table  Segmentation  Example: The Intel Pentium

REVIEW o Protection o Address binding ·Static ·Dynamic o Memory Management ·Partition o fixed o variable

REVIEW  Protection  Address binding  Static  Dynamic  Memory Management  Partition  fixed  variable  http://wenku.baidu.com/course/study/77f1dcccda38376ba f1fae94#665ea0c7aa00b52acfc7ca94

MULTISTEP PROCESSING OF A USER PROGRAM source program compiler or compile assembler time object module other object modules linkage editor load load module time system library loader dynamically loaded system library in-memory binary executio dynamic linking time (run memory image time)

MULTISTEP PROCESSING OF A USER PROGRAM

DYNAMIC LOADING o Routine is not loaded until it is called o Better memory-space utilization;unused routine is never loaded o Useful when large amounts of code are needed to handle infrequently occurring cases o No special support from the operating system is required implemented through program design

DYNAMIC LOADING  Routine is not loaded until it is called  Better memory-space utilization; unused routine is never loaded  Useful when large amounts of code are needed to handle infrequently occurring cases  No special support from the operating system is required implemented through program design

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

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

SWAPPING A process can be swapped temporarily out of memory to a backing store,and then brought back into memory for continued execution o Backing store-fast disk large enough to accommodate copies of all memory images for all users;must provide direct access to these memory images o 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 0 Major part of swap time is transfer time;total transfer time is directly proportional to the amount of memory swapped 0 Modified versions of swapping are found on many systems (i.e., UNIX,Linux,and Windows) o System maintains a ready queue of ready-to-run processes which have memory images on disk

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 all memory images for all users; must provide direct access to these memory images  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  Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped  Modified versions of swapping are found on many systems (i.e., UNIX, Linux, and Windows)  System maintains a ready queue of ready-to-run processes which have memory images on disk

SCHEMATIC VIEW OF SWAPPING operating system process P swap out process P2 2) swap in user space backing store main memory

SCHEMATIC VIEW OF SWAPPING

CONTIGUOUS ALLOCATION o Main memory usually into two partitions: Resident operating system,usually held in low memory with interrupt vector User processes then held in high memory o 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

CONTIGUOUS ALLOCATION  Main memory usually into two partitions:  Resident operating system, usually held in low memory with interrupt vector  User processes then 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

CONTIGUOUS ALLOCATION (CONT. o 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

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 process 5 process 8 process 2 OS process 5 process 2 OS process 5 process 2 OS process 5 process 9 process 2 process 9 process 10