Classification of Computers
1 ．Digital, Analog, and Hybrid Computers
A digital computer is a counting device that operates on discrete data. It operates by directly counting numbers (or digits) that represent numbers, letters，or other special symbols．Just as digital watches directly count off the seconds and minutes in an hour, digital processors also count discrete values to achieve the desired out put results．
In contrast to digital processors, however, there are also analog computers that do not compute directly with numbers．Rather, they deal with variables that are measured along a continuous scale and are recorded to some predetermined degree of accuracy Temperature, for example, may be measured to the nearest tenth of a degree on the Celsius scale, voltage may be measured to the nearest hundredth of a volt, and pressure may be measured to the nearest "pound per square inch” value. Analog computing systems are frequently used to control processes such as those found in an oil refinery where flow temperature measurements are important．
Desirable features of analog and digital machines are sometimes combined to create a hybrid computing system. In a hospital intensive-care unit, for example, analog devices may measure a patient's heart function, temperature, and other vital signs．These measurements may then be converted into numbers and supplied to a digital component in the system. This component is used to monitor the patient's vital signs and to send an immediate signal to the nurse's station if any abnormal readings are detected ．
Analog and hybrid processors obviously perform important specialized tasks．But the overwhelming majority of all computers used for business and scientific applications are digital devices．
2. General-Purpose and Special-Purpose Digital Computers
Digital computers are made for both special and general uses．As the name suggests，a special-purpose computer is one that's designed to perform only one specific task．The program of instructions is wired into or permanently stored in such a machine. Al-though it lacks versatility it does its single task quickly and efficiently.
Special purpose processors designed just to solve complex navigational problems are installed aboard U. S. atomic submarines．Not too long ago, however, special-purpose computers were too expensive for most applications．Only a few might be needed by one organization, and the specialized model might not do anyone else any good. But today, customized microcomputers are produced in large quantities to perform tasks such as monitoring household appliance and controlling the fuel, ignition, and instrument systems in automobiles．Furthermore, rapid progress is now being made in developing the automated design tools that engineers can use to economically build small quantities of complex microcomputer chips for specialized purposes．
A general-purpose computer is one that can store different programs and can thus be used in countless applications．You have seen that by using different instructions such a machine can process a payroll one minute and a billing application the next. New pro-grams can be written, and old programs can be changed o dropped. The versatility of general-purpose stem is limited only by human imagination. And so, unless otherwise noted，all our future discussion of “computer” will be about general purpose digital systems
3 Micro, Mini，Mainframe and Super Computers
All computer system of interest to us is similar that they contain hardware components for input, central processing, and output. They all perform basic machine operations under the direction of stored programs, which can be quickly changed to permit the Processing of a stream of different applications. Of course, widely different sorts of applications require different system resources to process them. In other words, the personal computer used in the home to play Space Invaders would hardly be used in NASA missions control to monitor an actual space shuttle launch.
Modern computers vary in physical size from those that fill rooms to those with CPUs the size of amide. Generally, the larger the system, the greater is its processing speed, storage capacity, and cost. Also, the larger systems are better equipped to handle a greater number of more powerful input and output devices.
Systems on the low end of the size scale are called microcomputers or minicomputers. Microcomputers or personal computers are the smallest general-purpose systems．But they may perform the same operations and use the same program instructions as much larger computers. Minicomputers are also small general-purpose systems. The are topical more powerful and expensive than micros，although the performance of some newer micros may surpass the capabilities or some older minis. In physical size, minis can vary from a desktop model to a unit the size of a small file cab.net.
Continuing up the size scale, mainframe computers are system that may offer faster processing speeds and greater storage capacity than a typical mini．A whole series of mainframe models ranging in size from small to very large are generally lumped to-getter under family designation by mainframe manufacturers．There is quite a bit of overlap possible in the cost, speed, and storage capacity of larger minis and smaller mainframes．
Finally come the supercomputers, designed to process complex scientific applications．These systems are the largest, fastest.
翻译：
计算机的分类
1．数字、模拟和混合计算机
数字计算机是对离散数据进行操作的计算设备。它直接对表示数字、字母或其它专用符号的数进行计算。正如数字表以秒、分来计算小时那样，数字处理器也对离散数值进行计算以获得所需要的输出结果。
还存在一种与数字计算机相反的模拟计算机，它不直接对数进行计算。模拟计算机处理在连续标度上测量的变量并按预先规定的精度进行记录。例如：测量时，温度可精确到1/10摄氏度，电压可精确到1/100伏，压力可精确到“磅／平方英寸”。模拟计算机系统常用于控制诸如炼油厂中的流量和温度测量过程。
有时把模拟计算机和数字计算机的优点结合起来形成一个混合计算机系统。例如在医院使用的监护器中，模拟部件可用于测量病号的心功能、温度和其它生理症状。然后把这些测量信号转换成数字并送给系统的数字部件，该部件用于监视病员的生理症状，当检测到异常信号时，向护士值班室发出报警信号。
很明显，模拟和混合处理器用于完成重要的特定任务。但绝大多数商业和科学的计算机是数字计算机。
2.通用和专用数字计算机
数字计算机可分为专用的和通用的。正如它们的名字所表示的，专用计算机是完成某一专门任务的计算机，其指令程序是固化或永久存储在该机器上的，虽然它缺乏通用性，但它执行单一任务时很快，效率很高。
为了解决复杂的导航问题，美国将几个专用处理器装在了核潜艇上。但是，专用计算机对大多数用户来讲仍然很昂贵。一个部门对专用计算机需求很少，而专用方式不能很好完成其它任务。现在，客户定制的微型计算机己大量生产，它们用于完成诸如监视家庭设施、控制燃料、点火以及汽车中的仪表系统。目前自动设计工具得到了快速发展，工程师可用这些工具为个别用户设计复杂的专用微型计算机芯片。
通用计算机是一种可存储不同程序的，应用无可估量的计算机。己看到了，一台机器使用不同的命令，可在这一分种内计算机工资单，在下一分种开出账单。可以写出新程序而更改或去掉旧程序。通用计算机的通用性只是受人们创造力的限制。因此，除非特别说明，以后所讨论的计算抓都是指通用数字系统。
3.微型、小型、大型计算机和超级计算机
所有目前使用的计算机，它们的输入设备、中央处理器和输出设备的硬件都是相似的。所有这些机器都是在存储程序的指导下完成基本的机器操作，而存储可很性地改变以处理其它不同的任务当然，种类繁多的应用要求不同的系统资源去处理。换句话说，家中玩空中大战的个人计算机决不可能用于宇航局监视航天飞机发射的飞行控制计算机。
现代的计算机，其规模可以是几间屋子那么大，也可以是硬币大小。总的来讲，系统越庞大，其处理速度越高，存储量越大，价格越高。同时，系统越庞大，越应配备更多的、功能更强的输入和输出设备。
规模最小的系统 叫做微型计算机或小型计算机。微型计算机或个人计算是最小的通用系统。但它们使用与大型计算机相同的程序指令来完成同一操作，小型计算机也是小见通用系统。尽管某些新的微型计算机的性能已超过某些旧的小型机，但一般来讲小型机比微型计算机功能更强，更贵。小型机的规模小可为台式，大可为一个小型文件柜。
随着规模的增加，就形成了大型计算机，它比一般的小型机有更快的处理速度和更大的存储容量。大型计算机厂家把它们的主机从规模小的小型机到非常大的大型机设计成系列产品。较大的小型机和较小的大型机之间在价格、速度和存储器容量上全可能有重迭。
最后是超级计算机，它用于处理更复杂的科学问题，这种计算机规模最大、速度最快。
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