Explain in detail about the Random Scan Display 

 This device using CRT directs the electron beam only to the parts of the screen where a picture is to be drawn. This kind of device draws only a line on the screen given by its two end points. That is why these devices are referred as vector displays (or calligraphic displays). In case of display device, it is not only an object which has to be drawn but it is also important to refresh it before it disappears in a given period of time depending on the persistence. No frame buffer exists for this kind of systems. When the object consists of too many lines, refreshing time might be longer than the persistence of the phosphor. This may lead to flickering of the picture on screen. In such systems, there may be a display processor that reads a display file. Display file consists of a sequence of graphics commands generated by the application program to refresh the screen.