System information

6.3 Display Screen Inspector
start of the valid screen content inside a video plane. Increasing this pointer will scroll the
screen content to the top. Figure 6.7 illustrates this process.
Video
Mode
Viewable
Screen Content
CRTC
Start Address
Register
non visible
Video Plane
0x0
0x2000
Figure 6.7: Diagram of the viewable part of the video plane. Running an alphanumeric
mode, the CRTC Start Register defines the start pointer of the current screen
content.
This approach has the advantage that the scrolling of a text does not cause a copy of the
existing data to the new position on the screen. Instead, the start position of the screen
inside the video plane is changed and the new content is written behind the end of the last
content. Figure 6.8 shows the output of the terminal program which prints out the screen
content. The parameter -bw enables the black-white mode of the program which ignores the
color information of the screen and prints the characters in cold print, the parameter -shot
activates the screenshot mode of the program which prints out the screen content one-time.
With the aid of the offset parameter (-o), the screen content at a dedicated location inside
the video plane can be print out. Thus, the previous content of the screen can be obtained
by the terminal program. A video plane can store up to 8 screen pages at the same time.
But the graphical video modes also have the possibility to provide hidden screen content.
To improve a fast change to new screen contents, the graphical video modes support hidden
screen pages. The hidden pages can be used for a fast built of the next screen content in
the background or to save the frame of an application running in the background.
6.3.3 Text Highlighting of the Screen
Another important function of the screen content interpretation is the extraction of posi-
tional parameters like cursor position or identification of the activation of a menu item. For
example, the BIOS setup utility marks an active menu item with the exchange of the font
and the background color. Figure 6.10 shows the menu bar of the BIOS setup utility of an
AMI BIOS. In this case, the "Main" menu item is activated.
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