INT 11 - BACK&FORTH (before v1.62) API

	AX = FFFEh
	CX = FFFEh
	BX = function
	    00h installation check
		Return: AX = installation state
			    0001h BNFHIGH and BNFLOW both loaded
			    0003h only BNFHIGH loaded
			    else neither loaded
	    01h ???
		Return: DX:AX -> ???
	    02h ???
	    03h ???
	    04h ???
	    05h ??? switches current PSP segment and stack if BNFLOW has not
		    yet announced itself installed
	    06h ???
		Return: AX = ???
SeeAlso: INT 12/AX=FFFEh
Index:	installation check;BACK&FORTH