The first generation computers are based on vacuum tubes, which were subsequently replaced by discrete semiconductors. The first generation systems used hard coding, whereas in second generation the assembly languages were used, in which pneumonic symbols were used to represent various mathematical and memory operations. The third generation systems provided high level languages and reduced the computer size through the use of integrated circuits. The operating system controlled the peripherals.
Eventually better high level software was made available and Large Scale Integration (LSI) and MSI reduced the size of the computer. The fourth generation systems have higher memory and the VLSI, VHSIC circuits have been put to make the computers still smaller. The microprocessor era started in 70's.
The fifth generation computer era did not stop with the Japanese project on the "fifth generation computer project" for Artificial Intelligence which did not see the light of the day.
For sixth generation, the molecular computers and optically connected systems to improve the speed up to the limit of light are envisaged.