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 (see below)
              Return: AX = status
        DX = port number
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)
SeeAlso: AH=00h,AH=05h"SERIAL I/O"

Bitfields for 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

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