VGA
The glory of 256 colors
The glory of 256 colors
BIOS and VESA
Textmode
Short History
MDA is text mode only (80x25 symbols) with 256 characters in code page 473. CGA is graphical mode with a max resolution of 640x200 with 2 colors and 320x200 with 16 colors.
EGA - 1984
More resolution (640×350) and more colors, 16 colors from a palette of 64.
VGA - 1987
Mode 13h, the best graphic mode ever with 320x200 and 256 colors. Aka Mode X 320x240 for those people that prefer square pixels.
Not much of a standard, more chaos really. Everything 800x600 and up with increasing number of colors is SVGA.
BIOS
To modify the video mode the BIOS is used. It can only be done in real mode. This is done by invoking a interrupt 10h with the INT x86 instruction. The action to perform is put into the AH register.
00h Set Video Mode
0Ch Write Graphics Pixel
0Dh Read Graphics Pixel
10h Set Palette Registers
4Fh VESA Extension Functions
00h Set Video Mode
AH = 0
AL = Video Mode number.
0CH/ODH: Store pixel value / Return pixel value
AH = 0CH / ODH
AL = Pixel value (IN/OUT)
BH = Video page
CX = x coordinate
DX = y coordinate
10h: Set Palette registers
AH = 10h
AL = 12h (subfunction 12h: Update dac)
BX = First register
CX = Number of registers
ES:DX = Address to table of rgb values.
4F02h: Set VBE Video Mode
Reference
Operating Systems Development - Prepare for the Kernel part 1 - 2008
QB Graphics Tutorials - 2000
Free VGA - 1998
Asphixia VGA Trainers - 1995
Introduction To Mode X - 1993
The Fastest Sprite in the West (Lets talk VGA, Part 2)
https://chuckjonesdevblog.wordpress.com/2020/03/22/the-fastest-sprite-in-the-west-lets-talk-vga-part-2/