INT 28 - DOS Idle Loop / Scheduler (Undocumented)

	- issued by DOS during keyboard poll loop
	- indicates DOS may be carefully re-entered by TSR
	- used by TSR programs to popup and make DOS disk I/O calls
	- supported by OS/2 compatibility box
	- default behavior is to simply perform an IRET
	- any DOS functions above 0Ch may be called while in this handler
	- INT 21,3F and INT 21,40 may not use a handle that refers to
	  CON while in this handler
	- see also  INDOS