Int 2F Fn 4330  CU - 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 (see #2477)
       CL = A20 detection support
       00h handler is unable to report A20 state
       01h handler supports function 0002h to report A20 state

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)

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

(Table 2477)
Call parameters for external A20 handler are:
   AX = function
       0000h disable A20
       0001h enable A20
       0002h get A20 state

Return: AX = status (functions 0000h and 0001h)
       0000h failure
       0001h successful
   AX = A20 state (function 0002h)
       0000h disabled
       0001h enabled

Note:  HIMEM.SYS only calls function 0002h if the returned CL indicated that
     the handler supports the call