Int 15 Fn 2400  - System - Later Ps/2s - Disable A20 Gate                  [b]

   AX = 2400h

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
BUG:   AMI BIOS v1.00.03.AV0M never reports an error on failure to disable
     the A20 gate; it simply writes 0 to PORT 0092h

See Also: AX=2401h,AX=2402h,AX=2403h,PORT 0092h