操作系统第七版第八章PPT精品.ppt

Shared Segments Shared code One copy of read-only code (reentrant code，pure code) shared among processes (i.e., text editors, compilers, window systems). reentrant code or pure code is non-self-modifying code, it never changes during execution. Shared code must appear in same location in the logical address space of all processes.（why？） Code segments typically contain references to themselves. A conditional jump and/or a loop, for example. Private code and data Each process keeps a separate copy of the code and data. The pages for the private code and data can appear anywhere in the logical address space. Sharing of Segments 分段与分页的主要区别 页是信息的物理单位，分页是为实现离散分配方式，以消减内存的外碎片，提高内存的利用率；或者说，分页仅仅是由于系统管理的需要，而不是用户的需要； 段是信息的逻辑单位，它包含有一组其意义完整的信息。分段的目的是为了能更好地满足用户的需要； 页的大小固定且由系统决定，把逻辑地址划分为页号和页内地址两部分，是由机器硬件实现的，因为一个系统只有一种大小的页面； 段的长度不固定，决定于用户所编写的程序，通常由编译程序对源程序进行编译时，根据信息的性质来划分； 分页的作业地址空间是一维的，即单一的线性地址空间，程序员只须一个地址记忆符，即可表示一个地址； 分段的作业地址空间是二维的，程序员在标识一个地址时，既需给出段名，由需给出段内地址； Segmentation with Paging – MULTICS(补充) 基本思想 作业先分段，各段再分页，内存划分成与页相等的页框； 每个段都有一个段名； 内存分配以页为单位； 每个作业有一个段表，记录段在内存的起始地址以及段长； 每个段还对应一个页表； 逻辑地址的格式：段号 ，段内偏移量(页号，页内偏移量) 地址变换机构（过程） 内碎片 存储保护 段及页的共享 Segmentation with Paging – MULTICS The MULTICS system solved problems of external fragmentation and lengthy search times by paging the segments. Solution differs from pure segmentation in that the segment-table entry contains not the base address of the segment, but rather the base address of a page table for this segment. MULTICS Address Translation Scheme 8.7 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 Paging units form equivalent of MMU Logical to Physical Address Translation in Pentium 8.7.1 Intel Pentium Segmentation s: Segment number g: the segment is in GDT or LDT p: for Protection s g p 13 1 2 selector 8.7