【正文】 sbased and requires tremendous data processing. A graphics accelerator can drastically increase the performance of the system. Another ponent, the hard disk, is typically the major cost of a puter system. Therefore, it is not costjustifiable to store all drawings on line on disk. Magic tape is still the most economical medium to use for offline data storage. It is necessary for a CAD system to have a tape backup subsystem. Currently, all levels of puters are used in CAD systems. Personal puters are used in lowcost,2D drafting applications, and with the new power of these puters, hey are also being used for some 3D solid modeling applications. Engineering workstations have been the mainstay in CAD, and are usually a singleuser CAD workstation in a work. Miniputers are used in multiuser CAD systems. Mainframes are used for large multiuser CAD systems to support real time simulation and engineering analysis. Sometimes, in a large corporation, all levels 2 of CAD systems are implemented. All these systems are linked together through a hierarchical puter work. Input. Output (I/O)Devices Figure shows the typical I/O devices used in a CAD system. Input devices are generally used to transfer information from a human or storage medium to a puter where ”CAD functions” are carried out. A keyboard is the standard input device used to transmit alphanumeric data to the system. Function keypads are also used to make input easier. Joysticks, track balls, and mousse are also used to manipulate a cursor. They can be used to position the graphic cursor(. cross hair)on a monitor and feed back the location of an object on the monitor to the puter. Using these devices allows an operator to address terminal locations to interactively in reaching an accurate position. Mouses have been used extensively with windows and pulldown menus. They are easy to use for pointing。(1)model the object on a drawing,(2)digitize the drawing, or (3)scan the object. Digitizing is usually much easier than modeling. A digitizer is a device that translates the XY locations on a drawing than modeling. A digitizer is a device that translates the XY locations on a drawing into a digital and feeds that signal to a puter. Graphics scanners scan a drawing and convert it to a CADsystemreadable format. Some scanners have builtin characterrecognition software that can convert characters on paper into ASCII codes. Another input device is a sketch pad called a graphics tablet. A graphics tablet is a special flat surface on which a user draws with a stylus. The location of the stylus is sent to the puter. The tablet is an absolute coordinate device. It is easy to implement overlay menus on the tablet and pull down menus on the screen to improve the operation. Currently. the tablet is the most popular input device used in CAD other than the keyboard. The standard output device for CAD is a monitor display. Modern monitor displays are rasterscan display monitors. Similar to a TV monitor, an electron gun(Sony Trinitron uses three guns for three basic colors)sends an electronic beam to the front of the monitor. Easy display dot is called a pixel(picture element).For color displays, each pixel is represented by three closely located dots with red, green , and blue colors. The electronic beam selects the color elements and the intensity of each color element. The resolution of the monitor is determined by the number of different colors or gray scales displayable at each pixel, usually measured in bits, A 1bit display can turn each pixel either ON or gray scale has at least v2 bits. A 1bit display can turn each pixel either ON or gray scale has at least 2 bits which allow three light intensities plus an OFF (2*2).Usually, displays are 4 bits(16 colors),8 bits(2