Int 28  C - DOS 2+ - Dos Idle Interrupt                                    [D]

   SS:SP = top of MS-DOS stack for I/O functions

Return: all registers preserved
Desc:  This interrupt is 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 21h function except functions
     00h through 0Ch.

Notes: 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 from
     the INT 28h handler without destroying the DOS stacks.
   calls to INT 21/AH=3Fh,40h from within an INT 28 handler 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
   the _MS-DOS_Programmer's_Reference_ for DOS 5.0 incorrectly documents
     this interrupt as superseded

See Also: INT 21/AH=34h,INT 2A/AH=84h,INT 2F/AX=1680h