RC 4000 was not widely used, but was highly influential, sparking the microkernel concept that dominated operating system research through the 1970s and 1980s.
Monitor was created largely by one programmer, Per Brinch Hansen, who worked at Regnecentralen where the RC 4000 was being designed.
Unix, for instance, uses small interacting programs for many tasks, transferring data through a system called pipelines or pipes.
However, a large amount of fundamental code is integrated into the kernel, notably things like file systems and program control.
Mach also used threading extensively, allowing the external programs, or servers in more modern terms, to easily start up new handlers for incoming requests.