Int 15 Fn 2403  - System - Later Ps/2s - Query A20 Gate Support            [b]

   AX = 2403h

Return: CF clear if successful
       AH = 00h
       BX = status of A20 gate support (see #0388)
   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
BUG:   at one point early in processing INT 15/AH=24h, the AMI PCI BIOS
     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.

See Also: AX=2402h

Bitfields for A20 gate support status:
Bit(s) Description (Table 0388)
 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)