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Much the way the system BIOS provides a set of functions that are used by software programs to access the system hardware, the video BIOS provides a set of video-related functions that are used by programs to access the video hardware as well as storing vendor-specific settings such as card name, clock frequencies, VRAM amount & voltages. The video BIOS interfaces software to the video chipset in the same way that the system BIOS does for the system chipset. The ROM also contained a basic font set to upload to the video adapter font RAM, if the video card did not contain a font ROM with this font set instead.
Unlike some other hardware components, the video card usually needs to be active very early during the boot process so that the user can see what is going on. This requires the card to be activated before any operating system begins loading; thus it needs to be activated by the BIOS, the only software that is present at this early stage. The system BIOS loads the video BIOS from the card's ROM into system RAM and transfers control to it early in the boot sequence.
Early PCs contained functions for driving MDA and CGA cards in the system BIOS, and those cards did not have any Video BIOS built in. When the EGA card was first sold in 1984, the Video BIOS was introduced to make these cards compatible with existing PCs whose BIOS did not know how to drive an EGA card. Ever since, EGA/VGA and all enhanced VGA compatible cards have included a Video BIOS.
When the computer is started, some graphics cards (usually certain Nvidia cards) display their vendor, model, BIOS version and amount of graphics memory.
- INT 10 - VIDEO - SET VIDEO MODE
- https://www.usenix.org/legacy/event/usenix05/tech/freenix/full_papers/lo/lo_html/vgabios.html "Traditionally, a VGA card is initialized by software known as the VGA BIOS, which is considered an extension of system BIOS. It is loaded by the system BIOS from an expansion ROM located on the VGA card into a specific address in system memory. Control is then transferred to the VGA BIOS, and it uses the 16-bit callback interface to communicate with the system BIOS."
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