Not least amongst the charms and virtues of the UNIX Time-sharing System is
the compactness of its source code.
The source code for the permanently
resident “nucleus” of the system when
only a small number of peripheral devices is represented, is comfortably
less than 9,000 lines of code.

It has often been suggested that 1,000
lines of code represents the practical
limit in size for a program which is to
be understood and maintained by a single individual.
Most operating systems either exceed
this limit by one or even two orders of
magnitude, or else offer the user a
very limited set of facilities, i.e.
either the details of the system are
inaccessible to all but the most determined, dedicated and long-suffering
student, or else the system is rather
specialised and of little intrinsic
interest.