INT 28 - DOS 2+ - DOS IDLE INTERRUPT
   Invoked each time one of the DOS character input functions loops while
   waiting for input.  Since a DOS call is in progress even though DOS is
   actually idle during such input waits, hooking this function is necessary
   to allow a TSR to perform DOS calls while the foreground program is
   waiting for user input.  The INT 28h handler may invoke any INT 21 function
   except functions 00h through 0Ch.  Under DOS 2.x, the critical error flag
   (the byte immediately after the InDOS flag) must be set in order to call
   DOS functions 50h/51h without destroying the DOS stacks.

Notes:  calls to INT 21/AH=3Fh,40h may not use a handle which refers to CON
        at the time of the call, the InDOS flag (see INT 21/AH=34h) is normally
          set to 01h; if larger, DOS is truly busy and should not be reentered
        the default handler is an IRET instruction
        supported in OS/2 compatibility box
SeeAlso: INT 2A/AH=84h