Int 2F Fn 4330  UC - Himem.sys V2.77+ - Get External A20 Handler Address   [m]

   AX = 4330h

Return: AL = 80h if external A20 handler provided
       ES:BX -> external A20 handler
       CL = ???

Note:  HIMEM.SYS calls this function to allow an external program to provide
     an A20 handler (i.e. to support a machine not supported by HIMEM
     itself)

External A20 handler called with:
   AX = function
       0000h disable A20
       0001h enable A20

   Return: AX = status
           0000h failure
           0001h successful

See Also: AX=4308h,AX=4310h