INT 11 - BACK&FORTH (before v1.62) API
AX = FFFEh
CX = FFFEh
BX = function
00h installation check
Return: AX = 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 = ???

See also: 12FFFE

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