Text mode

Text mode is a computer display mode in which content is internally represented on a computer screen in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of character cells, each of which contains one of the characters of a character set; at the same time, contrasted to graphics mode or other kinds of computer graphics modes. Text mode applications communicate with the user by using command-line interfaces and text user interfaces. Many character sets used in text mode applications also contain a limited set of predefined semi-graphical characters usable for drawing boxes and other rudimentary graphics, which can be used to highlight the content or to simulate widget or control interface objects found in GUI programs. A typical example is the IBM code page 437 character set. An important characteristic of text mode programs is that they assume monospaced fonts, where every character has the same width on screen, which allows them to easily maintain the vertical alignment when displaying semi-graphical characters. This was an analogy of early mechanical printers which had fixed pitch. This way, the output seen on the screen could be sent directly to the printer maintaining the same format. Depending on the environment, the screen buffer can be directly addressable.…

Text mode is a computer display mode in which content is internally represented on a computer screen in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of character cells, each of which contains one of the characters of a character set; at the same time, contrasted to graphics mode or other kinds of computer graphics modes. Text mode applications communicate with the user by using command-line interfaces and text user interfaces. Many character sets used in text mode applications also contain a limited set of predefined semi-graphical characters usable for drawing boxes and other rudimentary graphics, which can be used to highlight the content or to simulate widget or control interface objects found in GUI programs. A typical example is the IBM code page 437 character set. An important characteristic of text mode programs is that they assume monospaced fonts, where every character has the same width on screen, which allows them to easily maintain the vertical alignment when displaying semi-graphical characters. This was an analogy of early mechanical printers which had fixed pitch. This way, the output seen on the screen could be sent directly to the printer maintaining the same format. Depending on the environment, the screen buffer can be directly addressable. Programs that display output on remote video terminals must issue special control sequences to manipulate the screen buffer. The most popular standards for such control sequences are ANSI and VT100. Programs accessing the screen buffer through control sequences may lose synchronization with the actual display so that many text mode programs have a redisplay everything command, often associated with the key combination. ==Technical basis== A video controller implementing a text mode usually uses two distinct areas of memory. Character memory or a pattern table contains a raster font in use, where each character is represented by a dot matrix (a matrix of bits), so the character memory could be considered as a three-dimensional bit array. Display matrix (a text buffer, screen buffer, or nametable) tracks which character is in each cell. In the simple case, the display matrix can be just a matrix of code points (so named character pointer table), but it usually stores for each character position not only a code, but also attributes. In the case of raster scan output, which is the most common for computer monitors, the corresponding video signal is made by the character generator, a special electronic unit similar to devices with the same name used in video technology. The video controller has two registers: scan line counter and dot counter, serving as coordinates in the screen dot matrix. Each of them must be divided by corresponding glyph size to obtain an index in the display matrix; the remainder is an index in glyph matrix. If glyph size equals to 2 n , then it is possible just to use n low bits of a binary register as an index in glyph matrix, and the rest of bits as an index in the display matrix — see the scheme. The character memory resides in read-only memory in some systems. Other systems allow the use of RAM for this purpose, making it possible to redefine the typeface and even the character set for application-specific purposes. The use of RAM-based characters also facilitates some special techniques, such as the implementation of a pixel-graphics frame buffer by reserving some characters for a bitmap and writing pixels directly to their corresponding character memory. In some historical graphics chips, including the TMS9918, the MOS Technology VIC, and the Game Boy graphics hardware, this was actually the canonical way of doing pixel graphics. Text modes often assign attributes to the displayed characters. For example, the VT100 terminal allows each character to be underlined, brightened, blinking or inverse. Color-supporting devices usually allow the color of each character, and often the background color as well, to be selected from a limited palette of colors. These attributes can either coexist with the character indices or use a different memory area called color memory or attribute memory. Some text mode implementations also have the concept of line attributes. For example, the VT100-compatible line of text terminals supports the doubling of the width and height of the characters on individual text lines. ==PC common text modes== Depending on the graphics adapter used, a variety of text modes are available on IBM PC–compatible computers. They are listed on the table below: MDA text could be emphasized with bright, underline, reverse and blinking attributes. Video cards in general are backward compatible, i.e. EGA supports all MDA and CGA modes, VGA supports MDA, CGA and EGA modes. By far the most common text mode used in DOS environments, and initial Windows consoles, is the default 80 columns by 25 rows, or 80×25, with 16 colors. This mode was available on practically all IBM and compatible personal computers. Several programs, such as terminal emulators, used only 80×24 for the main display and reserved the bottom row for a status bar. Two other VGA text modes, 80×43 and 80×50, exist but were very rarely used. The 40-column text modes were never very popular outside games and other applications designed for compatibility with television monitors, and were used only for demonstration purposes or with very old hardware. Character sizes and graphical resolutions for the extended VESA-compatible Super VGA text modes are manufacturer-dependent. Also on these display adapters, available colors can be halved from 16 to 8 when a second customized character set is employed (giving a total repertoire of 512 —instead the common 256— different graphic characters simultaneously displayed on the screen). Some cards (e.g. S3) supported custom very large text modes, like 100×37 or even 160×120. In Linux systems, a program called SVGATextMode is often used with SVGA cards to set up very large console text modes, such as for use with split-screen terminal multiplexers. ==See also== Text-based user interface Teletext Text semigraphics ASCII art Twin Hardware code page VGA text mode Line-oriented printer Characters per line ==References== ==External links== High-Resolution console on Linux ==Further reading== (NB. For example: Signetics 2513 MOS ROM.)

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