Computer history | Electronic computing

From room-sized calculators to stored programs

Early electronic computers made calculations far faster than earlier electromechanical machines. A second idea—storing instructions in memory—made computers easier to reprogram and became a foundation of general-purpose computing.

Period: 1940sSource: Computer History Museum

ENIAC: electronic speed, hands-on programming

ENIAC was publicly unveiled in 1946. Built at the University of Pennsylvania's Moore School, it used roughly 18,000 vacuum tubes, occupied more than 1,000 square feet, and weighed about 30 tons. Its electronic circuits made it dramatically faster at calculation than electromechanical machines.

In its original mode, setting up a new problem involved switches and cables across panels. ENIAC was therefore programmable, but changing a program was a physical task. It should not be confused with the stored-program approach that was developed in parallel and soon influenced later machines.

The stored-program idea

A 1945 report by mathematician John von Neumann described a design in which a computer stores both data and instructions in electronic memory. The computer can fetch an instruction, act on data, and continue through the sequence. Rather than physically rewiring the machine for every task, an operator can load a different program.

The Manchester Small-Scale Experimental Machine—nicknamed the “Baby”—ran a program in 1948 to test a new electronic memory. That demonstration is widely recognized as the first program to run on an electronic stored-program computer. The principle helped turn computers into flexible machines that could be adapted to many kinds of work.

Why this shift mattered

Electronic components increased speed; memory-based instructions increased flexibility. Together, they helped make it practical to use one computer for different problems, from scientific calculations to business tasks. Modern computers have become far smaller and more powerful, but still depend on this basic cycle: fetch instructions from memory, process data, and produce results.