INT 28 C - DOS 2+ - DOS IDLE INTERRUPT                                          
	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(Terminate and Stay Resident) A program which remains in memory after terminating in order to provide services to other programs or the user.  The name comes from the name of the DOS function call used to remain in memory after termination. 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	  this interrupt as superseded
	the performance of NetWare Lite servers (and probably other peer-to-
	  peer networks) can be dramatically improved by calling INT 28
	  frequently from an application's idle loop
SeeAlso: INT 21/AH=34h,INT 2A/AH=84h,INT 2F/AX=1680h