INT 15 - SYSTEM - later PS/2s - DISABLE A20 GATE
        AX = 2400h
Return: CF clear if successful
            AH = 00h
        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=2401h,AX=2402h,AX=2403h

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