System information

6.3 Display Screen Inspector
content of the screen. Generally, the alphanumeric content of the VGA data is used to
get status information of the host computer, for example, to detect error keywords or to
validate a keyboard entry which was sent by the CHARM. A windows blue screen, for
example, has a specific design [83] or a Linux kernel panic message is printed on the text
console. These events can be detected via their typical screen content. Another advantage
of the VGA plane representation of the screen content is the receipt of the cursor position
and the extraction of highlighting of text contents. For example, running the BIOS setup
utility the text of the actual or activated menu item is normally highlighted. The CHARM
can use this information to move inside the BIOS menu and to select the right settings
automatically. Section 6.3.3 illustrates this approach.
6.3.1 Alphanumerical Representation of the Screen Content
While running a VGA text mode, the content filtering of keywords is very simple. The
first VGA plane contains ASCII characters which can be browsed like normal text files.
Section 7.1.3 discusses an application of this approach. Instead, running a graphical video
mode, the textual or semantic representation of the VGA data has to be find out in another
way. In this case, the inspection of the screen content is more difficult than the inspection
while running an alphanumeric video mode. In principle, any ordinary optical character
recognition (OCR) software can be used to extract the text from the VGA data. But the
CHARM does not have neither the performance nor the memory to get the result of standard
OCR software in adequate time. Due to the special conditions of the VGA content, the
OCR is reduced to a simple pattern recognition software. First, the video data are present
in form of digital pixel values instead of a screenshot with aliasing and distortion effects.
Second, the fonts of the characters are known. The VGA planes are inspected from the
upper left to the lower right corner. Figure 6.3 depicts an example content of a video plane,
whereas dark pixels represent an activated bit and the white ones a deactivated bit. The
bitmap of a VGA plane is scanned for characters of a specific font. A look-up table provides
the characters of the related bitmap pattern (see picture 6.4).
Figure 6.3: Example content of a video
plane.
=A
=B =C
Figure 6.4: Look up table of a font set.
The font look-up tables are related to the operating system which is currently running
on the system. Inspecting the boot up screen, for example, requires the font table of the
specific computer BIOS. With the aid of the DMI information, the CHARM determines the
BIOS version and selects the proper font set for the inspection of the boot screen. Following
the usage of the display screen inspection is described:
85