First generation computers used Thermionic Valves and programming was done in machine languages. The period of the first generation computer was 1951-1958.
All the computers were of very big sizes and programming was very difficult. The input and output devices used were punched card readers and card punches. These were very slow computers.
Electronic Numerical integrator and calculator was the beginning of first generation computers. It was based upon decimal number system rather than binary number system.
The limitations of first generation computers were −
High power consumption
Very big in size
Slow speed
Limited programming capability