The arrangement of a simple random scan system is shown in the following figure. System stores and application program in the system memory along with a graphics package. With the help of graphics package the Graphics command in the application program are converted into a display file stored in the system memory. And this file helps the system to refresh the screen. When operated as a random-scan display unit, a CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn. Random-scan monitor draw a picture one line at a time and for this reason are also referred to as vector displays (or stroke-writing of calligraphic displays).
The component lines of a picture can be drawn and refreshed by a random-scan system in any specified order Fig. a pen plotter in a similar way and is an example of a random-scan, hard-copy device.
Refresh rate on a random-scan system depends on the number of lines to be displayed. Picture definition is now stored as a set of line-drawing commands in an area of memory referred to as the refresh display file. Sometimes the refresh display file is called the display list, display program,