The early microcomputer OSes were little more than program loaders and only came into play if you had a disk drive. Even MS DOS was not much more than a program loader for most of its existence. It was a little jarring when making the transition from a TRS-80 or Apple ][ or Commodore computer to something running MS-DOS or CP/M that you could no longer just type 10 PRINT "HELLO, WORLD!" to start writing a program.