From large and slow to small and fast, the evolution of the computer has changed technology and led to the personal computer of today.

2 + 2 = 4.

This may be a simple calculation for a human being, but it was not always so with computers. In fact, this elementary math problem was part of the first successful program written for a computer that an average person could own. The personal computer had already come a long way, but it still had far to go.

The road from conception to the compact-but-powerful computer of today’s Information Age has been one filled with trial and error. The computer originated with the military in the 1940s, when the United States Army financed the development of ENIAC (Electronic Numerical Integrator and Computer), to perform ballistics calculations. Shortly after, a Remington Rand division created UNIVAC (UNIVersal Automatic Computer) in the ‘50s for business use. But the cost was an astronomical $1 million, and only large companies or government agencies could afford these computers, which required entire rooms for their hardware. Impractical is an understatement: Air conditioning was critical to combat the heat they generated; just one errant dust speck was a program-ending enemy, and computing ability was minuscule by today’s standards. Introduced in 1956, an early hard drive — a whopping 2 feet in diameter — held the equivalent of one MP3 song, thousands of which can now fit on an inches-long iPod.

To use those giants, programmers needed extraordinary patience. They were required to code the program on punch cards, hand them to an operator and often wait days for results. If one card was inserted out of place, minutely mutilated or missing a single character, the computer had to be restarted entirely. Programmers yearned for a practical machine they could actually control. Their wish came true in 1975.

In Albuquerque, N.M., Ed Roberts — a balding, cigarette-puffing former Air Force officer — and the debt-ridden MITS, or Micro Instrumentation and Telemetry Systems, were busily crafting an affordable computer kit, programmable by users.

MITS — started by Roberts as well as civilian contractor Stan Cagle and former military men Bob Zaller and Forrest Mims III (the latter three left the company in 1970) — initially produced model rocket instrument kits, a market that turned out to be too small. So, Roberts, along with his new partner, Bob Yates, decided to develop an inexpensive electronic calculator kit. Soon, however, Texas Instruments took the lead of the industry race by producing an already-assembled calculator that was also cheaper. Undaunted, the MITS duo instead built a kit for a personal computer using a new Intel processor released in 1974 for a multipurpose computer.

The world first got a glimpse of this groundbreaking gadget in January 1975, when Popular Electronics magazine featured it on its cover; the editors named it Altair after a star. But readers did not know at the time that the actual kit was not yet available.