INT 15 - SYSTEM - later PSIBM PS/2, any model/2s - GET A20(Address line 20) The 80286 and higher CPUs allow addresses in real mode to extend slightly beyond the one megabyte mark, which causes an incompatibility with some older programs which expect such addresses to wrap back to the beginning of the address space.  For complete compatibility with the 8088, newer machines thus contain circuitry which permits the twenty-first address line (A20) to be disabled.  The CPU then effectively has only twenty address lines in real mode, just as the 8088 does, and addresses which would extend beyond the one megabyte mark wrap to the beginning of the address space.  See also High Memory Area, Real Mode. 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 AMIAmerican Megatrends, Inc.(American Megatrends, Inc.) A hardware, software and firmware company founded in 1985. BIOS(Basic Input/Output System) A set of standardized calls giving low-level access to the hardware.  The BIOS is the lowest software layer above the actual hardware and serves to insulate programs (and operating systems) which use it from the details of accessing the hardware directly. 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 AMIAmerican Megatrends, Inc.(American Megatrends, Inc.) A hardware, software and firmware company founded in 1985. PCI BIOS(Basic Input/Output System) A set of standardized calls giving low-level access to the hardware.  The BIOS is the lowest software layer above the actual hardware and serves to insulate programs (and operating systems) which use it from the details of accessing the hardware directly. and Qualitas 386MAX v6.01+
	IBMInternational Busiuness MachinesInternational Busiuness Machines) A hardware, software and other service technology company founded in 1911. classifies this function as optional
SeeAlso: AX=2400h,AX=2401h