[Q]: Как узнать, в FS ли DOS-сессия?

[A]: Alexander Perezhogin (2:5007/2.1)

 INT 2F C - OS/2 compatibility box - SWITCHING DOS TO BACKGROUND

         AX = 4001h
 Note:   called by OS/2 when the DOS box is about to be placed in the
         background and the video driver should save any necessary state
 SeeAlso: AX=4002h,AX=4005h

 INT 2F C - OS/2 compatibility box - SWITCHING DOS TO FOREGROUND

         AX = 4002h
 Note:   called by OS/2 when the DOS box is about to be placed in the
         foreground and the video driver should restore the previously-saved
         state
 SeeAlso: AX=4001h,AX=4006h