INT 15 - SYSTEM - later PS/2s - GET A20 GATE STATUS

	AX = 2402h
Return: CF clear if successful
	    AH = 00h
	    AL = current state (00h disabled, 01h enabled)
	    CX = ??? (set to 0000h-000Bh or FFFFh by AMI BIOS v1.00.03.AV0M)
		FFFFh if keyboard controller does not become ready within C000h
			 read attempts
	CF set on error
	    AH = status
		01h keyboard controller is in secure mode
		86h function not supported
Notes:	also supported by AMI PCI BIOS and Qualitas 386MAX v6.01+
	IBM classifies this function as optional
SeeAlso: AX=2400h,AX=2401h