What is vesafb?

"vesafb" is a generic driver for a graphic framebuffer on intel boxes.

The idea is simple: Turn on graphics mode at boot time with the help
of the BIOS, and use this as framebuffer device /dev/fb0, like the m68k
(and other) ports do.

This means we decide at boot time whenever we want to run in text or
graphics mode. Switching mode later on (in protected mode) is
impossible; BIOS calls work in real mode only. VESA BIOS Extensions
Version 2.0 are required, because we need a linear frame buffer.

Advantages:

* It provides a nice large console (128 cols + 48 lines with 1024x768)
without using tiny, unreadable fonts.
* You can run XF68_FBDev on top of /dev/fb0 (=> non-accelerated X11
support for every VBE 2.0 compliant graphics board).
* Most important: boot logo :-)

Disadvantages:

* graphic mode is slower than text mode...

Switching modes is done using the vga=... boot parameter.

The value of "vga" can be one of the video mode numbers of the Linux kernel.

The table shows the video mode numbers of the Linux kernel under different resolutions and color depths.

解析度(Resolution)

彩度(Color Depth) | 640x480 800x600 1024x768 1280x1024
---------------------------------------------------------------------------------------------------------------------------
|256 (8 bits) | 0x301 0x303 0x305 0x307
|32k (15 bits) | 0x310 0x313 0x316 0x319
|64k (16 bits) | 0x311 0x314 0x317 0x31A
|16M (24 bits) | 0x312 0x315 0x318 0x31B


Reference :

(1)http://lxr.linux.no/linux+v2.6.21.4/Documentation/fb/vesafb.txt
(2)http://ku777.blogspot.com/2008/04/vga.html