Documentation/fb/vesafb.rst

Source file repositories/reference/linux-study-clean/Documentation/fb/vesafb.rst

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Documentation/fb/vesafb.rst
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===========================================
vesafb - Generic graphic framebuffer driver
===========================================

This 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...


How to use it?
==============

Switching modes is done using the vga=... boot parameter.  Read
Documentation/admin-guide/svga.rst for details.

You should compile in both vgacon (for text mode) and vesafb (for
graphics mode). Which of them takes over the console depends on
whenever the specified mode is text or graphics.

The graphic modes are NOT in the list which you get if you boot with
vga=ask and hit return. The mode you wish to use is derived from the
VESA mode number. Here are those VESA mode numbers:

====== =======  =======  ======== =========
colors 640x480  800x600  1024x768 1280x1024
====== =======  =======  ======== =========
256    0x101    0x103    0x105    0x107
32k    0x110    0x113    0x116    0x119
64k    0x111    0x114    0x117    0x11A
16M    0x112    0x115    0x118    0x11B
====== =======  =======  ======== =========


The video mode number of the Linux kernel is the VESA mode number plus
0x200:

 Linux_kernel_mode_number = VESA_mode_number + 0x200

So the table for the Kernel mode numbers are:

====== =======  =======  ======== =========
colors 640x480  800x600  1024x768 1280x1024
====== =======  =======  ======== =========
256    0x301    0x303    0x305    0x307
32k    0x310    0x313    0x316    0x319
64k    0x311    0x314    0x317    0x31A
16M    0x312    0x315    0x318    0x31B
====== =======  =======  ======== =========

To enable one of those modes you have to specify "vga=ask" in the
lilo.conf file and rerun LILO. Then you can type in the desired

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