The unlicensed clone market has flourished following Nintendo's discontinuation of the NES. Some of the more exotic of these resulting systems have gone beyond the functionality of the original hardware and have included variations such as a portable system with a color LCD (e.g. PocketFami). Others have been produced with certain specialized markets in mind, such as an NES clone that functions as a rather primitive personal computer, which includes a keyboard and basic word processing software. These unauthorized clones have been helped by the invention of the so-called NES-on-a-chip.
The NES was duplicated on which medium in order to make non-clones easy to produce?