英语1

Digital Computer

    The digital computer is a digital system that performsvarious computational tasks.The word "digital" implies that theinformation in computer is represented by variables that take a limited numberof discrete values.These values are processed internally by components that canmaintain a limited number of discrete states.The decimal digits 0,1,2…9,forexample,provide 10 discrete values.The first electronic digitalcomputers,developed in the late of 1940s,were used primarily for numerical computations.Inthis case,dhe discrete elements are the digits.From this application the termdigital computer has emerged.In practice,digital computerfunctions more reliably if only two states ate used.Because of the physicalrestriction of components ,and because human logic tends to be binary,digitalcomponents that are constrained to take dicrete values are are furtherconstrained to take only two values and are said to be binary.

    Digital computers use the binary number system,which hastwo digits:0 and 1.A binary digit is called bit.Information is represented indigital computer in groups of bits .By using various coding techniqus ,groupsof bits can be made to represent not only binary numbers but also otherdiscrete symbols,such as decimal digits or letters of the alphabet.For example,ASCII(American Standard Code For Information Interchange) originally used 7bits ro form a character .By judicious use of binary arrangements and by usingvarious coding techniques ,the groups of bits are used to develop complete setsof instructionsfor performing various type of computations .In contrast to thecommon decimal numbers that employ the base 10 system ,binary numbers use abase 2 system with two digits :0 and 1.The decimal equivalent of a binarynumber can be found by expanding it into a power series with a base of 2.

    A computer system consists of hardwere system andsoftware system .The hardware system is the physical equipment that you can seeand touch,such as thd disks and the screen .The software system is theintangible “control” that governs the computer ;it is the total of all theprograms that can be run on the computer.A program is a list ofinstructions.Programs tell the hardware what to do. The hardware of thecomputer is usually divided into three major parts: input and output devices(I/O devices),a central processing unit (CPU), and memory.They are described inmore detail in Chapter 2.Software can be classified according to its purpose.Application software is designed to accomplish real-world tasks in fields suchas accounting ,entertainment ,and engineering If you’ve ever played a videogame or typed a paper on a word processor,you’ve already had some experienceeith application software programs.System software,on the other hand, controlsthe computer system itself.System software includes not only the complexprogramsused by technicians to create application software in the first placebut also the organizational programs needed to start up the computer and governits use of other programs.They are described in more ditail in Chapter 2 andChapter 6.