System information

4 Graphic Card Implementation
today is based on the VGA standard and all VGA video modes are supported by the common
operating systems.
4.1.1 VGA Components
The VGA system has four main functional areas: the Cathode Ray Tube (CRT) controller,
the sequencer, the graphics controller, and the attribute controller. Figure 4.1 shows a
diagram of the VGA functional areas and the connections between the video memory and
the video Digital Analog Converter (DAC). The video DAC produces the analog video
signal for the monitor. Every VGA unit has its own configuration register set. They are
accessible via special I/O ports (see section 4.1.3). Appendix H.2 shows the register of the
VGA components.
Bus Interface
CRT
Controller
Video
RAM
Attribute
Controller
Video
DAC
Graphics
Controller
Sequencer
Analog Output
Figure 4.1: Diagram of the VGA data processing. The arrows describes the data flow.
The following paragraphs give a brief overview of the main VGA units.
CRT Controller The CRT controller generates vertical and horizontal synchronization
signal timings for the electronic beam. It also controls the addressing of the video memory,
the cursor and underline timings.
Sequencer The sequencer generates basic memory timings and the character clock. The
information from the framebuffer is read out and is converted into pixel color information
by the sequencer.
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