INT 14 - X00 FOSSIL - EXTENDED SERIAL PORT STATUS/CONTROL
        AH = 1Fh
        AL = 00h read modem control register
              Return: BL = modem control register (see below)
                      AH = status
        AL = 01h write modem control register
              BL = modem control register
                  bit 0: data terminal ready
                  bit 1: request to send
                  bit 2: OUT1
                  bit 3: OUT2 (interrupts) enabled
                  bit 4: LOOP
                  bits 5-7 reserved
              Return: AX = status
        DX = port number
SeeAlso: AH=05h"SERIAL I/O"
Notes:  this function is intended to exactly emulate the PS/2 BIOS AH=05h call
        X00 forces BL bit 3 set (interrupts cannot be disabled)