Int 15 Fn 2401  - System - Later Ps/2s - Enable A20 Gate                   [b]

   AX = 2401h

Return: CF clear if successful
       AH = 00h
   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

See Also: AX=2400h,AX=2402h,PORT 0092h