Can any one here explain what the video? registers
sequencer index (3c4h) and sequencer (3c5h) do exactly?

I know they can be used to select bit planes in EGA/VGA
modes like this:

Code:
mov al, 0x2
mov dx, 0x3C4
out dx, al

mov al, bitplane
mov dx, 0x3C5
out dx, al
What else do these registers do?