I need that for set up video mode in my OS.

return in rm, set video mode with bios, return in pm, or...
write a driver for the video card in pm.

And more precisely, the method for doing the former is:

• disable interrupts
  
• switch paging off (by clearing the PG bit in cr0)
  
• flush the TLB (cr3)
  
• "jump" to a 16 bit code area (using the real-mode code descriptor)
  
• adjust segment registers for real-mode (using the real-mode data descriptor)
  
• load the real-mode IVT
  
• switch to real-mode (clear PE bit in cr0)
  
• enable interrupts
  
• <run real-mode code>
  
• switch back to protected-mode as you first did
  

OK, I now almost understand at all,
Now, i need to know how to load Real mode IVT..
Can you tell me about that?

Same as in protected-mode, use the lidt instruction for that. Since you inherit the IVT the BIOS created before you entered protected-mode, you don't have to set anything up. So an IVT such as this should work:

don't forget to restore PIC to its original state which would probably mean disabling ioapic for a modern os

If you see my post here, there's a vesa demo i coded, that goes to and from realmode for mode switching, from vesa to text mode to vesa etc, every 10 seconds.
Fasm source included.
http://board.flatassembler.net/topic.php?p=143625#143625

Also see Mike Gonta work.

Ok, I understand now
Thanks All, and also to Dex for your code. your code helpful

Hey, wait...
i get little problem just now...

yesterday, i try to write small booloader that load gdt, switch to pmode and then back to rmode and print a char using int 10h
yeah, i'm very glad, it works

just now, i m trying to re implement that to my kernel...
first, bootloader load it to 0500:0000, but bochs said something like this :
....[cpu0]virtual_emm_read32.. blah blah blah...
whats wrong?
then i try changed bootloader to make my kernel loaded at 0600:0000, yeah, i get no errors, but the interrupt not executed
so, i decide to change bootloader again. now it load my kernel at 1000:0000.
i get error like first experiment :p