INT 15 - SYSTEM - later PS/2s - QUERY A20 GATE SUPPORT
        AX = 2403h
Return: CF clear if successful
            AH = 00h
            BX = status of A20 gate support
                bit 0: supported with bit 1 of I/O port 92h
                bit 1: supported on keyboard controller
                bit 15: additional data is available (location not yet defined)
        CF set on error
            AH = status
                01h keyboard controller is in secure mode
                86h function not supported
Note:   also supported by Qualitas 386MAX v6.01+
SeeAlso: AX=2402h

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