A
kernel where the total amount of kernel code, i.e. code executing in the
privileged mode of the hardware, is very small. The term
picokernel was sometimes used to further emphasize small size. The term
nanokernel was coined by Jonathan S. Shapiro in the paper
''The KeyKOS NanoKernel Architecture''. It was a sardonic response to
Mach, which claimed to be a microkernel while Shapiro considered it monolithic, essentially unstructured, and slower than the systems it sought to replace. Subsequent reuse of and response to the term, including the picokernel coinage, suggest that the point was largely missed. Both
nanokernel and
picokernel have subsequently come to have the same meaning expressed by the term microkernel.