System information

4 Graphic Card Implementation

The CHARM implements a VGA graphic card. It replaces the primary graphic card of the

host system to get the screen content of the host computer. But the CHARM is not installed

to a local monitor. Instead, the graphic data are processed on the CHARM and send it via

the network to an arbitrary remote computer. The card does not use a commercial graphic

processor chip because the raw graphic data can be better inspected than the video output

signal. For example, in a text mode (see section 4.1.2) the raw graphic data are ASCII

characters. Section 6.3 discusses the usage of scanning the screen content precisely. There

are tree major expansion bus types which are used to connect a graphic card: PCI, AGP

and PCI Express. However, PCI Express drives AGP out of the market. The PCI bus is

widespread and almost the favored bus system for low bandwidth devices. The CHARM

implements a PCI graphic card to be operable in a variety of computer systems.

The ﬁrst sections give an overview of the basic VGA protocol. Afterwards, the implemen-

tation of the VGA functions is illustrated precisely. The last section describes the software

which provides the screen content of the host computer.

4.1 VGA Speciﬁcation

The Video Graphic Array (VGA) is an analog computer display speciﬁcation developed by

IBM in the year 1987. The VGA speciﬁcation has become one of the de facto standards

for PCs. It describes a video subsystem which includes a video buﬀer and a video digital-

to-analog converter (DAC). Thereby, the DAC drives the analog output to the display

connector. The video memory consists of at least 256 KB and its use and mapping depend

on the mode selected. The VGA standard supports display resolution of up to 640x480 pixel

with a color depth of 4 bits. Older video standards like the Enhanced Graphics Adapter

(EGA) or the Monochrome Display Adapter (MDA) are integrated into the VGA standard.

Basically, there are two possibilities to draw the screen content:

• Usage of the BIOS’s video functions.

• Direct access to the VGA register and VGA planes.

The video functions of the BIOS hide the complexity of the VGA protocol. These functions

are explained in section 4.3. The direct access to a graphic card provides more ﬂexibility

and optimization while building the screen content.

Nowadays, this standard is technologically outdated and the graphic cards support screen

resolution in excess of the VGA standard. Furthermore, the graphic cards provide digital

output to install an LCD or TFT monitor. But the basic video system of the most computers

Accelerated Graphics Port.