If you use vesafb: the trick is to switch to a high-resolution graphic
mode using the video card bios (or a dos tsr program if you boot with
loadlin) before entering the kernel mode (see arch/i386/boot/video.S),
and then get from the bios functions a usable framebuffer. Once the
kernel is booted, you can no longer call bios functions (for they run
in so-called "real-mode", the old 16-bits way of 8086) and therefore
you have to stick to the current video mode (to switch from one video
mode to another requires that you know how to speak to the video card,
and the whole vesafb thing is about using high-resolution modes without
having this information).
--Thomas Pornin
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