This invention relates to the video display of images and, more particularly, to improvements in increasing the meaning attached to various coded data words outputted from a data source to a character generator so that variations in the number of characters or attributes or control commands can be had.
Whereas the invention is described herein in conjunction with a video display system incorporating a host computer and individual terminals, it is to be appreciated that the invention is not limited thereto but may also be employed in a stand alone video display terminal or other apparatus where it is desirable to vary the meaning of various coded data words supplied to a character generator from a data source.
In a typical video display system a data source supplies multi-bit coded data words to a video display circuit, which includes a character generator, and which in turn controls the display of characters and the like on a video display screen, such as a cathode ray tube. The coded data words fall into three basic categories; to wit, character codes, control codes and attribute codes. Control codes are special commands, such as end of line (EOL) or end of frame (EOF) which command the video control circuitry to take some action in conjunction with controlling the video display. Attribute codes define special conditions in a field of characters following the attribute code, such as reverse video display or blinking and the like. Character codes in a typical video display terminal serve as addresses for addressing a character memory, such as read only memory (ROM) from which dot patterns are obtained for use in controlling the cathode ray tube to form a dot pattern image of the character to be displayed. Each of these multi-bit data words has a bit pattern which is similar in nature. For example, each may be an eight bit word and, hence, coding circuitry or the like is employed to determine whether the coded data word is representative of a character code or a control code or an attribute code. To maximize the number of codes available for describing characters, the first bit of a coded word representative of a character may have a distinctive binary level, such as a binary "0" level. This leaves seven bits to be used for addressing the character memory thereby providing a possibility of 128 addresses to provide a system having a capability of generating 128 different characters. If the most significant bit is a binary 1 level, then the next most significant bit may be either a binary 1 or a binary 0 and dedicated to identify the coded word as either a control code or an attribute code. The least significant six bits of those data words, then, may be used to provide attribute codes or control codes.
Because at least the most significant bit of such a data word is dedicated to describing that word as a character code, then this leaves in that system, a maximum of 128 available codes to describe characters to be displayed. In many applications, this simply is not enough characters and, hence, this limits the use to which such a display terminal may be employed. In examining these data words, it will be determined that in many applications, the maximum number of attributes being requested by an attribute code is limited to six attributes and that, perhaps, as many as only two control codes are required. This is an inefficient use of the data words that are decoded as being attributes codes or control codes since this leaves a possibility of many as 60 available codes in an eight bit system that are not being used at all.