聊城大学计算机学院计算机操作系统（1）英文课件 Chapter7.ppt

Memory Management Chapter 7 Memory Management Subdividing memory to accommodate multiple processes Memory needs to be allocated to ensure a reasonable supply of ready processes to consume available processor time Memory Management Requirements 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 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 rum 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 Memory Management Requirements Sharing Allow several processes to access the same portion of memory Better to allow each process access to the same copy of the program rather than have their own separate copy Memory Management Requirements Logical Organization 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 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 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 over