Text mode

When text mode is referred to the publication of the first video games and GUI operating systems (Mac OS, TOS, AmigaOS ) customary mode of graphics hardware. No single one of the software image points managed with their respective color, but individual characters, such as letters and digits - Here are - in contrast to graphics mode. These characters are then converted by the graphics hardware on the computer in pixels. While this prevents graphics can be displayed in high resolution, but it also reduces the need for graphics memory tremendously reduces the programming effort considerably, and accelerated programs as per character usually only one or two bytes in the video RAM are required.

Typical resolutions ranging from 40 × 25 characters, which is the largest practically usable resolution for TV screens, on 80 × 25, which is still the mode today, in which PC graphics cards are after boot up to 132 × 50 which were used by text-based spreadsheets and similar programs under DOS, which must represent a lot of information simultaneously. Other variants were 80 × 24 (many mainframe terminals), 64 × 16 ( Tandy TRS 80 Model 1 ) or 22 × 23 ( Commodore VIC-20, which is more rows than columns ).

The individual characters usually have a fixed size of 8 × 8 and 9 × 16 pixels ( in the early days with much higher memory prices even 5 × 7), on PC graphics cards only characters with 8 pixels wide are supported. These are stored in a separate character memory, see also in character generator. Depending on your computer and graphics card model may be an immutable ROM or a variable from the user RAM this character generator; only in the latter case the user can define and use your own character shapes.

The character memory in PC graphics card stores the character as a bitmap, each character requires as many bytes as pixels high. The ROM of the EGA cards there are two fonts with font size 8 × 8 and 8 × 14 pixels, with VGA cards still is added an 8 × 16- character set. Custom fonts, which are available from EGA, can be high 1 to 32 rows of pixels.

Highlighted modes are directly supported by the BIOS text modes; the other can only be achieved through direct programming of the graphics card. The horizontal resolution graphics in text mode VGA card defaults to 720 pixels. Thus, each character is 9 pixels wide, which improves the readability of the text representation, since the distances between the letters a bit bigger. Since only 8 pixels per character stored in the character memory but, the color of the 9th pixel is calculated separately: For most characters this pixel is in the background color, except for the character with the code values ​​C0hex to DFhex. These characters are the 8 pixels of each character is repeated. In order for a seamless transition in graphics and frame character is reached. The ROM character sets are encoded in the rule in the code page 437, which houses the graphics and frame signs, which can enter into these horizontal connections and their right adjacent characters in this field. Both the 9 -pixel wide characters, and the special treatment of the signs C0hex to DFhex can, however, deactivate via direct programming of the graphics card tab.

In the video RAM only the character number is stored for each character. On PC systems, this is always 8 bits in size, it can be as reference 256 different characters; also called an attribute byte is stored for each character, which encodes the color of the character or special character attributes ( bold, underline, blinking, etc.).

Bold type is usually simulated by a lighter color. Thus 16 different foreground colors can be displayed on the color graphics cards. The meaning of the bit 7 can be reprogrammed so that it allows either flashing character representation or 8 more (light ) background colors. From EGA but can be reprogrammed to 16 colors so that a freer choice of color is possible.

A special feature of the EGA / VGA cards is to be able to display two character sets (and thus up to 512 different characters) simultaneously. The bit 3 of the attribute byte determines which character set of the character is to be taken. By default, both character sets are identical. The Linux Console dominated this feature as soon as a screen font with more than 256 characters will be charged. Here, the colors are 8-15 reprogrammed so that they are identical with the colors 0 to 7, so that the characters are not displayed brighter from the second character set. Thus, there is no " bold " more available.

Through the use of special " block character" can be a low-resolution " blocks " graphics display in text mode. If, in addition per character cleverly chosen the foreground and background color, so simple multi-color images can be produced. In the on-screen text, this function was very often used in the video text, this is still being practiced. Some systems also allow the redefinition of the character shapes by the user; in conjunction with hardwareunterstütztem soft scrolling and so-called sprites for foreground elements, this technique allows, for example, the representation of fast moving backgrounds for computer games in text mode, which is then hardly recognizable as such. This approach has been used for example in many games for the 8 -bit Commodore computers of the company.

Computer systems for business use decreed until the early 1980s, mostly only on a text mode. Only with the advent of video games and home computers the graphics mode was usual; some home computers, such as the Schneider / Amstrad CPC and the Commodore Amiga, had no more text mode. The Apple Macintosh models 68000 and the PowerPC series did not have a text mode, based on the BIOS or have emulated computer text mode until today, but he is hardly used by most users since Windows 95 prevailed from 1995. Only when the computer starts, he continues to be active for a short time. When you maximize a command prompt window (Alt Enter, from Windows Vista no longer available), the computer is also set to text mode. Linux or Unix -like operating systems always know in their console to text mode, graphics mode is optional.

Because support for the text mode on modern graphics cards rarely have the resolution and refresh rate of the VGA cards go (some Super - VGA cards dominate text modes with 132 text columns, which corresponds to a graphics resolution of 1056 pixels ), use some operating systems for their text console now also a graphic mode and emulate the behavior of the text mode completely in software. Since this amount of data to be processed is significantly larger, such emulated text console is significantly slower (eg when scrolling ) as a hardware text mode ( see also: framebuffer console of Linux).

Older televisions and VCRs have for on-screen menus, teletext often graphic chips that only have a text mode. This then contain special characters (often mask programmed ) that are used cleverly used, symbols, or even to produce light and shadow effects.

See also: ASCII art, character -oriented user interface

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