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LDT ? how strange ...
Well, using VBE pmode extension requires the caller to provide 16 bits segments that map things like VGA BIOS code and data areas. But afaik, these segments may be built in GDT aswell (but still, they have to be built, yes).
We cannot just tell you how linear frame buffer is enabled but "use VBE set mode function with the LFB bit set" because its hardware-dependent, and that hardware manufacturer jealously keep these information secret...