INT 2F UC - HIMEM.SYS v2.77+ - GET EXTERNAL A20 HANDLER ADDRESS
        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
SeeAlso: AX=4308h,AX=4310h

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