System information

4 Graphic Card Implementation
font of the character has to be loaded. The third video plane contains the fonts of the 256
ASCII characters.
Character
Plane 0
Attribute
Plane 1
Unused
Plane 3
Plane 2
Font 0
Font 1
Font 2
Font 3
Font 4
Font 5
Font 6
Font 7
Font 8
Figure 4.3: Organization of the video planes in alphanumeric mode.
Three default fonts are contained in the read-only memory (ROM) of the VGA card: an
8x8 font, an 8x14 font, and an 8x16 font. These font sizes are related to the running text
mode resolution. Up to eight fonts containing 256 characters can be loaded into the third
video plane. Thereby, two of these fonts can be active at one time.
Graphical Video Mode
The video memory contains the pixels of the screen whereas the organization of video
memory depends on the selected mode. There are three ways how the pixel are organized
in the video memory while using a graphic mode. It depends on the number of planes which
are used for the video mode. Video modes with high resolution uses more planes than low
resolution modes. A video plane can store a maximum of 64 KB, which is not sufficient to
store the pixel information of high resolution modes. The following paragraphs explain the
pixel organization inside the video memory related to the number of the used video planes.
One Plane Modes Modes using only one memory plane can organize the pixels in two
ways. The first one stores the pixels linear into the memory plane. The video memory
is sequential and the first byte contains the color information for the upper left picture
element of the screen. The second way to organize the pixels is to separate the pixel into
two parts. Every part has its own pixel area. Figure 4.4 illustrates this process. The screen
pixels are divided into odd and even pixels. The even pixels are stored in the first pixel
area and the odd in the second pixel area.
Two Plane Modes Some modes use two planes and a pixel is divided into two parts.
The first part is stored in the first plane and the second part in the second plane. It is
important not to confuse the classification of the pixel into two types explained in the one
plane mode with the fragmentation of a pixel into two parts. But there are combination of
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