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Could anybody do me a favour :1,how to change video mode directly without using bios in pmode (particularly,changing to 640*480*16).and 2:how to write a pixel derectly in pmode to the screen in 640*480*16 mode.

Thank you.

to change modes (any standard VGA mode):
use the VGA control registers:

this was the first result in google programing vga registers
http://www.osdever.net/FreeVGA/vga/vga.htm


bonifide osdever has lots of useful tutorials heres a link:
http://www.osdever.net/

Thank you very much!

If you google TabOS you can find an open source OS that I used to learn how start graphics mode in PMode.

See the file, vga.c I think. You'll have to poke around the include files to find defines and structures needed, but all the code and functions are in one place.