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Operating Systems Design and Implementation, Third Edition phần 5

Bộ nhớ hệ thống quản lý có thể được chia thành hai loại cơ bản: những người di chuyển quá trình qua lại giữa bộ nhớ chính và đĩa trong quá trình thực hiện (trao đổi và phân trang), và những người không. Loại thứ hai là đơn giản, vì vậy chúng tôi sẽ nghiên cứu đầu tiên. | Simpo PDF Merge and Split Unregistered Version - http www.simpopdf.com 1 Page 374 continued 4.1. Basic Memory Management Memory management systems can be divided into two basic classes those that move processes back and forth between main memory and disk during execution swapping and paging and those that do not. The latter are simpler so we will study them first. Later in the chapter we will examine swapping and paging. Throughout this chapter the reader should keep in mind that swapping and paging are largely artifacts caused by the lack of sufficient main memory to hold all programs and data at once. If main memory ever gets so large that there is truly enough of it the arguments in favor of one kind of memory management scheme or another may become obsolete. On the other hand as mentioned above software seems to grow as fast as memory so efficient memory management may always be needed. In the 1980s there were many universities that ran a timesharing system with dozens of more-or-less satisfied users on a 4 MB VAX. Now Microsoft recommends having at least 128 MB for a single-user Windows XP system. The trend toward multimedia puts even more demands on memory so good memory management is probably going to be needed for the next decade at least. 4.1.1. Monoprogramming without Swapping or Paging The simplest possible memory management scheme is to run just one program at a time sharing the memory between that program and the operating system. Three variations on this theme are shown in Fig. 4-1. The operating system may be at the bottom of memory in RAM Random Access Memory as shown in Fig. 4-1 a or it may be in ROM Read-Only Memory at the top of memory as shown in Fig. 4-1 b or the device drivers may be at the top of memory in a ROM and the rest of the system in RAM down below as shown in Fig. 4-1 c . The first model was formerly used on mainframes and minicomputers but is rarely used any more. The second model is used on some palmtop computers and embedded .