System information

4.1 VGA Specification
Graphics Controller The graphics controller is the interface between the video memory
and the attribute controller during active display time. The graphics controller can perform
logical operations on the memory data.
Attribute Controller It contains the color look up table which determines what color will
be displayed for a given pixel value in the video memory.
Video Memory The VGA video memory is also called framebuffer and consists of 256KB
of memory organized as four planes of 64KB. The access to these video planes depends on
the video mode and is described in section 4.1.4. The pixel information inside the video
memory is not linear organized. Instead, the raw data of one pixel can be spread to several
planes and several locations inside a plane.
4.1.2 Video Modes
A video mode is a programmed VGA configuration that produces a graphics frame buffer
format and a screen image with specific characteristics. A VGA configuration is defined by
the register content of the Attribute, Graphics, CRT controller and the Sequencer. Basically,
a distinction is drawn between alphanumerical video modes and graphical video modes. In
alphanumerical video modes, the system writes the ASCII character code and attribute
data to the video memory. In the graphical video modes, the framebuffer contains pixel
information. A video mode is numbered from 1 to 19, but some numbers are omitted and
do not describe a valid video mode. Table H.1 in the appendix H shows all existing video
modes.
Alphanumerical video modes
The alphanumerical video modes are also called text modes. They differ among each other
in screen resolution, color depth and character font. Thereby, the first video memory plane
contains the ASCII character and the second plane stores the attribute value of the related
ASCII character.
01234567
character color
background color
blinking
Figure 4.2: Layout of the attribute byte.
Figure 4.2 depicts the layout of the attribute value. The first nibble defines the character
color, the following three bits define the background color and the most significant bit
activate the blinking of the character. Before a character can be drawn on the screen, the
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