1.Why Data Management?
We have already considered hardware and software in some detail. In this chapter, we turn our attention to a third basic computer resource, data. Many computer applications require that data be stored for subsequent processing. [1] Simply storing the data is not enough, however. A typical computer system, even a small one, can have dozens of disks and taped, each holding data for dozens of different applications. For any given application, one and only one set of data will do. We must be able to store, locate, and retrieve the specific data needed by a given program. That is the concern of data management.
2.Accessing Data
Imagine a single diskette containing several programs. For a particular application, only one of those programs will do. How is a given program selected, loaded, and executed? We learned that the operating system, responding to a user’s command, reds the disk’s index, searches it for the requested program name, extracts the program’s track and sector address, and issues primitive commands to read it into main memory. Later, following a RUN command, the program is given control of the processor.
1.為什么要數(shù)據(jù)管理
許多計(jì)算機(jī)應(yīng)用要求數(shù)據(jù)存放起來，以備后用，然而簡(jiǎn)單地存放數(shù)據(jù)是不夠的。一個(gè)普通的計(jì)算機(jī)系統(tǒng)，甚至一個(gè)小系統(tǒng)，可有幾十個(gè)磁盤和磁帶，每一個(gè)都為幾十個(gè)不同的應(yīng)用保存數(shù)據(jù)，對(duì)任意給定的應(yīng)用，只用到一組數(shù)據(jù)就夠了，必須能存儲(chǔ)、定位和檢索一個(gè)給的程序所需要的特定數(shù)據(jù)，這就是為什么需要數(shù)據(jù)管理的原因。
2.存取數(shù)據(jù)
設(shè)想一個(gè)含有若干程序的軟盤，對(duì)于一個(gè)特定的應(yīng)用，這些程序中只有一個(gè)要工作。如何選定、裝入和招待這個(gè)程序呢？我們知道操作系統(tǒng)響應(yīng)用戶的命令，讀出磁盤的索引，根據(jù)所要求的程序名字查找程序，得到了存放程序的磁道和扇區(qū)的地址，并發(fā)出原始命令，把程序從磁盤讀到主存中，接著，在RUN命令之后，處理器就招待該程序了。