Fourth Generation of Computer : VLSI based Microprocessor Computer

       Early systems used a variety of memory technologies prior to finally settling on magnetic-core memory. The Atanasoff–Berry computer of 1942 stored numerical values as binary numbers in a revolving mechanical drum, with a special circuit to refresh this "dynamic" memory on every revolution. The war-time ENIAC could store 20 numbers, but the vacuum-tube registers used were too expensive to build to store more than a few numbers. Mercury delay-line memory was used by J. Presper Eckert in the EDVAC and UNIVAC I. Eckert and John Mauchly received a patent for delay-line memory in 1953. Bits in a delay line are stored as sound waves in the medium, which travel at a constant rate. The UNIVAC I (1951) used seven memory units, each containing 18 columns of mercury, storing 120 bits each. This provided a memory of 1000 12-character words with an average access time of 300 microseconds.

            

   Williams tubes were the first true random-access memory device. The Williams tube displays a grid of dots on a cathode-ray tube (CRT), creating a small charge of static electricity over each dot. The charge at the location of each of the dots is read by a thin metal sheet just in front of the display. The Ferranti Mark 1 (1951) is considered the first commercial vacuum tube computer. The first mass-produced computer was the IBM 650 (1953).

                                                    [VACUUM TUBES]              

                "Source : https//www.javatpoint.com/first-generation-of-computer"