Write character at cursor position

        Entry:
          AH = 09h
          AL = character
          BH = display page - alpha mode
          BL = attributes of char (alpha modes) or color (graphics modes)
                if bit 7 == 1 in graphics mode, character is xor'ed onto
                screen
          CX = number of times to write character
        Return:
          AX,SI,DI,BP destroyed by call