What is that supposed to mean?
Throughout history, people have discovered that their OS doesn't provide the right abstractions to allow some apps to take advantage of the hardware's performance potential. The traditional solution was to add features to the OS (Web server not fast enough? sendfile() to the rescue!), but the exokernel proposes the radical alternative of providing simpler, maximally flexible abstractions and letting apps (or libOSes) implement whatever behavior they want.
It's basically a very early (pretty much a full decade) predecessor to Xen.
In a perfect world, an exokernel OS would be roughly as stale as the architecture it was built for.
Imagine an operating system that could conceivably be 'done', in a development sense.
