操作系统英文教材课件第9章.pptx

Chapter 9: Virtual Memory虚拟内存;CHAPTER OBJECTIVES;Chapter 9: Virtual Memory;9.1 Background;Virtual memory – separation of user logical memory from physical memory. 虚拟存储－－用户逻辑存储与物理存储分离 （部分加载+交换） Only part of the program needs to be in memory for execution. 仅部分程序必须在内存，以使其运行 Logical address space can therefore be much larger than physical address space. 从而逻辑地址空间远大于物理地址空间 Allows address spaces to be shared by several processes. 允许地址空间被多个进程共享 Allows for more efficient process creation. 允许更有效的进程创建;Virtual memory can be implemented via: Demand paging 请求式分页 Demand segmentation 请求式分段 However, segment-replacement algorithms are more complex than are replacement algorithms because the segments have variable sizes. 但是，段的替换算法比页替换算法复杂，因为段的大小不同 paged segmentation 段页式;Figure Virtual Memory That is Larger Than Physical Memory;9.2 Demand Paging 请求式分页;A demand-paging system is similar to a paging system with swapping. 请求式分页系统类似于分页系统＋对换 But we use a lazy swapper—pager (页调度程序) Swapper vs Pager A pager never swaps a page into memory unless that page will be needed. A swapper manipulates entire processes.;Basic Concepts 基本概念;Valid-Invalid Bit 有效位 “valid” the associated page is both legal and in memory. “invalid” the page either is not valid(that is ,not in the logical address space of the process),or is valid but is currently on the disk.;Initially valid–invalid bit is set to 0 on all entries. During address translation, if valid–invalid bit in page table entry is 0 ? page fault .;Page Table When Some Pages Are Not in Main Memory;Page Fault 缺页 If there is ever a reference to a page, first reference will trap to OS ? page fault 对一个页面的首次访问将自陷到操作系统－－缺页;OS checks an internal table to decide: OS 检查内置页表以确定 Invalid reference ? abort(terminate the process). Valid,Just not in memory ?page it in. Get empty frame. 获取空闲帧 Swap page into frame. 对换页面到该帧 Reset tables, validation bit = 1. 重置页表，有效位置1 Restart instruction 重启指令;Steps in Handling a Page Fault;Pure d