DOS
Later compatible systems from other manufacturers include DR DOS ( 1988 ) , ROM-DOS ( 1989 ) , PTS-DOS ( 1993 ) , and FreeDOS ( 1998 ) . 
<triplet> PTS-DOS <subj> DOS <obj> part of the series <subj> DOS <obj> part of the series <subj> DOS <obj> part of the series <subj> DOS <obj> part of the series <triplet> FreeDOS <subj> DOS <obj> part of the series <subj> DOS <obj> part of the series <subj> DOS <obj> part of the series <subj> DOS <obj> part of the series