print a message periodically.

os/first.asm

@@ -17,10 +17,26 @@ start:
        mov si, text_string     ; put string position into si
        call print_string
        ;; jump here, infinite loop!
        jmp $
run:
        ;; sleep a bit
        mov cx, 10
        mov dx, 0
        call sleep
        ;; say something so we notice it happened
        mov si, sleepy_string   ; could be done earlier, but maybe we'll print different messages later
        call print_string
        ;; go back to sleep
        jmp run
        text_string db 'This is my very first OS!', 0
        sleepy_string db ' Waking up again...', 0
sleep:
        mov ah, 0x86            ; "wait", specify length in `cx` and `dx` in microseconds (10^-6s)
        int 0x15
        ret
clear_screen:
        mov ah, 0x00            ; "set video mode"