今日遇到一个问题，就是刚刚安装完成的rhel6，进入字符模式字体很大，屏幕上能显示的内容太少了，仔细想想应该是分辨率比较小的原因，所以上网查询了一下，大部分都是在grub的kernel后面加上 vga=xxx 就可以搞定，所以一开始设置 vga=791 正常显示，但是据网上介绍 791 = 1024 x 768 的分辨率，我的显示器是宽屏，很显然这个分辨率也相对较小，然后将vga设置为 794，结果启动的时候出现了一个分辨率设定程序，其中是一对vga和vesa相关的东东，最后经过苦苦查找，发现下面文章，终于明白其中原理：

原文出处

我的电脑是ATI X700Pro的显卡，使用fglrx的驱动的情况下在X下1024x768@85Hz很正常，但是要切换到控制台就有问题了，花的！这就是图形模式到文 本模式的切换是分辨率不一致而出现的切换问题。这是就要给内核指定一个分辨率的参数。网上有很多很容易找到的现成答案，比如1024x768,24位色就 是vga=0x318,如果使用lilo装载据说不能处理16进制，就用vga=792,但是这个值是如何得到的，我在网上查了很久，才找到答案。

事实上，给内核的vga的参数值是有VESA(Vedio Electronic Standard Association)规定的SVGA的视频模式信号决定的。计算公式如下：

Linux_kernel_mode_number = VESA_mode_number + 0x200

VESA_mode_number在VESA的标准文档中有如下规定：

4. Super VGA mode numbers

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Standard VGA mode numbers are 7 bits wide and presently range from 00h to 13h.

OEMs have defined extended video modes in the range 14h to 7Fh. Values in the

range 80h to FFh cannot be used, since VGA BIOS function 00h (Set video mode)

interprets bit 7 as a flag to clear/not clear video memory.

Due to the limitations of 7 bit mode numbers, VESA video mode numbers are 15

bits wide. To initialize a Super VGA mode, its number is passed in the BX

register to VESA BIOS function 02h (Set Super VGA mode).

The format of VESA mode numbers is as follows:

D0-D8 = Mode number

If D8 == 0, this is not a VESA defined mode

If D8 == 1, this is a VESA defined mode

D9-D14 = Reserved by VESA for future expansion (= 0)

D15 = Reserved (= 0)

Thus, VESA mode numbers begin at 100h. This mode numbering scheme implements

standard VGA mode numbers as well as OEM-defined mode numbers as subsets of the

VESA mode number. That means that regular VGA modes may be initialized through

VESA BIOS function 02h (Set Super VGA mode), simply by placing the mode number

in BL and clearing the upper byte (BH). OEM-defined modes may be initialized in

the same way.

To date, VESA has defined a 7-bit video mode number, 6Ah, for the 800x600,

16-color, 4-plane graphics mode. The corresponding 15-bit mode number for this

mode is 102h.

The following VESA mode numbers have been defined:

GRAPHICS TEXT

15-bit 7-bit Resolution Colors 15-bit 7-bit Columns Rows

mode mode mode mode

number number number number

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

100h - 640x400 256 108h - 80 60

101h - 640x480 256

109h - 132 25

102h 6Ah 800x600 16 10Ah - 132 43

103h - 800x600 256 10Bh - 132 50

10Ch - 132 60

104h - 1024x768 16

105h - 1024x768 256

106h - 1280x1024 16

107h - 1280x1024 256

VESA Super VGA Standard VS911022-8

10Dh - 320x200 32K (1:5:5:5)

10Eh - 320x200 64K (5:6:5)

10Fh - 320x200 16.8M (8:8:8)

110h - 640x480 32K (1:5:5:5)

111h - 640x480 64K (5:6:5)

112h - 640x480 16.8M (8:8:8)

113h - 800x600 32K (1:5:5:5)

114h - 800x600 64K (5:6:5)

115h - 800x600 16.8M (8:8:8)

116h - 1024x768 32K (1:5:5:5)

117h - 1024x768 64K (5:6:5)

118h - 1024x768 16.8M (8:8:8)

119h - 1280x1024 32K (1:5:5:5)

11Ah - 1280x1024 64K (5:6:5)

11Bh - 1280x1024 16.8M (8:8:8)

以上来自VESA官方文档

这下就很容易理解了，1024x768,16.8M颜色的代码是118h。故对应就是在内核参数中写入vga=0x318或vga=792。