clear the screen before printing.

os/first.asm

@@ -11,6 +11,9 @@ start:
        mov ax, 0x07c0
        mov ds, ax
        ;; clear the screen
        call clear_screen
        mov si, text_string     ; put string position into si
        call print_string
@@ -19,6 +22,12 @@ start:
        text_string db 'This is my very first OS!', 0
clear_screen:
        mov ah, 0x00            ; "set video mode"
        mov al, 0x03            ; 80x25
        int 0x10
        ret
print_string:                   ; output string in `si` to screen
        mov ah, 0x0e            ; int 0x10 'print char' function