<P> Historically, computers evolved from mechanical computers and eventually from vacuum tubes to transistors . However, conceptually computational systems as flexible as a personal computer can be built out of almost anything . For example, a computer can be made out of billiard balls (billiard ball computer); an often quoted example . More realistically, modern computers are made out of transistors made of photolithographed semiconductors . </P> <P> There is active research to make computers out of many promising new types of technology, such as optical computers, DNA computers, neural computers, and quantum computers . Most computers are universal, and are able to calculate any computable function, and are limited only by their memory capacity and operating speed . However different designs of computers can give very different performance for particular problems; for example quantum computers can potentially break some modern encryption algorithms (by quantum factoring) very quickly . </P> <P> There are many types of computer architectures: </P> <Ul> <Li> Quantum computer vs. Chemical computer </Li> <Li> Scalar processor vs. Vector processor </Li> <Li> Non-Uniform Memory Access (NUMA) computers </Li> <Li> Register machine vs. Stack machine </Li> <Li> Harvard architecture vs. von Neumann architecture </Li> <Li> Cellular architecture </Li> </Ul>

When was computer invented and who invented it