中科大操作系统原理与实现课件8_memory2.pdf

.操作系统原理与设计第8章 Main Memory2陈香兰中国科学技术大学计算机学院2009年09月01日.提纲Paging(分页)Basic MethodHardware supportMemory ProtectionShared PagesStructure of the Page TableHierarchical PagingHashed Page TablesInverted Page Tables.Discrete Memory AllocationIpaging（分页）Iinternal fragmentation one pageIsegmentation（分段）IlogicalIcombined paging segmentation（段页式）.OutlinePaging(分页)Basic MethodHardware supportMemory ProtectionShared PagesStructure of the Page TableHierarchical PagingHashed Page TablesInverted Page Tables.Paging IILogical address space of a process can be noncontiguous;process is allocated physical memory whenever the latter isavailableIBasic Method1.Divide physical memory into fixed-sized blocks called framesIsize is power of 2,between 512B and 8,192BIPage Frame Number,PFN:0,1,.,PFNmax2.Divide logical memory into blocks of same size called pagesILogical Frame Number,LFN:0,1,.,LFNmax3.the backing store is also divided into fixed-sized blocks ofsame size as frames.Paging IIIneed hardware and software support for pagingIKeep track of all free framesITo run a program of size n pages,need to find n free framesand load programISet up a page table to translate logical to physical addressesfor each processIInternal fragmentation 32 bitsIThe virtual page number is hashed into a page table.This page tablecontains a chain of elements hashing to the same location.IVirtual page numbers are compared in this chain searching for a match.If a match is found,the corresponding physical frame is extracted.OutlinePaging(分页)Basic MethodHardware supportMemory ProtectionShared PagesStructure of the Page TableHierarchical PagingHashed Page TablesInverted Page Tables.Inverted Page TableIOne entry for each real page of memoryIEntry consists of the virtual address of the page stored in thatreal memory location,with information about the process thatowns that pageIDecreases memory needed to store each page table,butincreases time needed to search the table when a pagereference occursIUse hash table to limit the search to one or at most a few page-table entries.Inverted Page Table Architecture.小结Paging(分页)Basic MethodHardware supportMemory ProtectionShared PagesStructure of the Page TableHierarchical PagingHashed Page TablesInverted Page Tables.谢谢！