Custom MOS has been around since the late 60's when it was used extensively by the military and some farseeing calculator manufacturers. In fact, custom MOS caused the calculator revolution and the development of microprocessors. The 8080 was an evolution from a custom processor.
Custom MOS is a synergyism between the end product developer and the semiconductor technologist. It is a semiconductor product that is designed for a specific need. As a consequence, it is the most efficient and cost effective answer to that need. However, when you also take development time, development costs, and manufacturing guidelines into account, another solution may be more sensible. Custom MOS is anything from a simple sound generator to a complex engine control computer system.
Custom MOS is not limited to a single chip system. It may be a multi-chip system with a mixing of standard components. Techniques can be used to reduce system costs by efficient usage of interconnect.
What are the constraints in building a custom circuit?