Text mode - Informatics

Text mode

In text or symbol mode, the image on the monitor screen is displayed as separate characters. Each character is assigned a familiarity, which is given by a dot matrix (Dot Matrix) of size p × q pixels. A typical text mode is 80xx mode. 25 characters, that when using a 9 × 14-pixel character matrix corresponds to a resolution of 720 × 720, 350 pixels. As an example, Fig. 8.7 shows a matrix of 8x8 pixels with A. If we assume that the dark cell (pixel) has a logical unit and the light one has a logical zero, then each row of the symbol matrix can be represented as a binary number (in Figure 8.7 on the right is a binary and hexadecimal code). Therefore, the graphic representation of the symbol can be stored as a set of binary numbers. A set of symbols (letters of English and national alphabets, numbers, a set of special signs and graphic primitives) is called font. Since the character forms are represented by an 8-bit code, their total number is 28 = 256. To store the font, located on the card of the video adapter hardware character generator (Hardware Character Generator), which is a ROM.

Matrix with the image of the symbol A and its description

Fig. 8.7. Matrix with the image of the symbol A and its description

In text mode, the screen and video memory have a matrix organization. Elements of screen matrices are dot matrix (familiarity), and video memory - character codes, located along rows and columns. During scanning of the screen, when approaching the next dot matrix, data is read from the next cell of the video memory. In this case, all rows of a particular dot matrix are read from the same video memory slot. The read data falls into the character generator: its older address inputs receive the code of the current symbol from the video memory, and the lower ones - the number of the current line in the displayed line of familiarity. The output is a bit-by-bit sweep of the current row of the character matrix. A set of strings of symbol matrices is fed to the CRT modulator for illumination, as a result of which a line of symbols appears on the monitor screen. In Fig. 8.8 illustrates the interaction between the video memory and the character generator when the first letter of the screen displays the letter A (code 41) and the last digit is 1 (code 31).

The text adapter has cursor control hardware, the size and position of which is relatively programmable. Usually it is highlighted with a flashing strip.

Two bytes are used to encode the symbol image on the screen: the first one is for specifying the symbol number, the second for specifying the symbol attributes. To attributes include background and symbol colors, inversion, flashing and underlining of the symbol, cursor display. If there is n × t , the amount of video memory required to store the image will be n × m × 2 bytes. This area of ​​video memory is called video page (Video Page). The video page is an analog of the frame buffer in graphic mode, but it has a much smaller volume. In the most common text mode (80x25 characters), the video page size is 4000 bytes, in the 40xx mode, 25 - 2000 bytes. In practice, for the convenience of addressing, the video page is assigned 4 Kbytes = 4096 bytes and 2 Kbytes = 2048 bytes, respectively, and the superfluous bytes (96 and 48) are not used.

The main feature of the text mode is that the addressable element of the screen is not a pixel, but a familiarity. Therefore, in text mode, it is impossible to generate an arbitrary image anywhere

Interaction between video memory and character generator when displaying characters

Fig. 8.8. The interaction of memory and character generator with output symbols

screen - you can only display characters from a given set in the allocated character positions. However, its indisputable advantage is a small amount of memory, high speed of information output, simplicity and compactness of the program code of the output of symbols.

thematic pictures

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