use rep stos to write zeroes to the video memory buffer, which should be located at B800:0000 in text mode and A000:0000 in graphics mode.

For example, suppose you're on a 80x25 text mode. There are a total of 2000 positions on the screen, each of which has a character byte and an attribute byte - 4000 bytes in total. So you could have something like

mov ax,0B800

mov es,ax

xor di,di

xor ax,ax

mov cx,2000d

cld

rep stosw

which effectively writes 2000 zero words to B800:0000, clearing the screen.