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Sunday, December 4, 2011

VESA BIOS Extensions

VESA BIOS Extensions (VBE) is a VESA standard, currently at version 3, that defines the interface that can be used by software to access compliant video boards at high resolutions and bit depths. This is opposed to the "traditional" int 10h BIOS calls, which are limited to resolutions of 640×480 pixels with 16 color (4-bit) depth or less.
VBE is made available through the video adapter's BIOS, which installs interrupt vectors pointing at itself during system startup. Unfortunately, the older versions of VBE (those bundled with older video cards) provided only a real mode interface, which could not be used without a significant performance penalty from within protected mode operating systems, such as Windows 95 or Linux. This meant that the VBE standard was almost never used for writing video-drivers, and each video board vendor had to invent a proprietary protocol for communicating with their own boards. Despite this, many existing drivers for these video cards still thunk out to the real mode interrupt to initialize screen modes and gain direct access to a card's linear frame buffer, tasks which would otherwise require many hundreds of port writes which change from card to card. Most newer cards implement the more capable VBE 3.0 standard

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