INT 15 - SYSTEM - later PSIBM PS/2, any model/2s - QUERY 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 SUPPORT
	AX = 2403h
Return: CF clear if successful
	    AH = 00h
	    BX = status of 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 support (see #00462)
	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
BUG:	at one point early in processing INT 15/AH=24h, the 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.
	  version 1.00.05.AX1 compares whether AL==03h instead of AL>=03h,
	  thus causing a random branch on any value of AL greater than 3.
SeeAlso: AX=2402h


Bitfields for 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 support status:
Bit(s)	Description	(Table 00462)
 0	supported on keyboard controller
 1	supported with bit 1 of I/O port 92h
 14-2	reserved
 15	additional data is available (location not yet defined)