History of General Purpose CPUs - 1950s: Early Designs

1950s: Early Designs

Each of the computer designs of the early 1950s was a unique design; there were no upward-compatible machines or computer architectures with multiple, differing implementations. Programs written for one machine would not run on another kind, even other kinds from the same company. This was not a major drawback at the time because there was not a large body of software developed to run on computers, so starting programming from scratch was not seen as a large barrier.

The design freedom of the time was very important, for designers were very constrained by the cost of electronics, yet just beginning to explore how a computer could best be organized. Some of the basic features introduced during this period included index registers (on the Ferranti Mark 1), a return-address saving instruction (UNIVAC I), immediate operands (IBM 704), and the detection of invalid operations (IBM 650).

By the end of the 1950s commercial builders had developed factory-constructed, truck-deliverable computers. The most widely installed computer was the IBM 650, which used drum memory onto which programs were loaded using either paper tape or punched cards. Some very high-end machines also included core memory which provided higher speeds. Hard disks were also starting to become popular.

A computer is an automatic abacus. The type of number system affects the way it works. In the early 1950s most computers were built for specific numerical processing tasks, and many machines used decimal numbers as their basic number system – that is, the mathematical functions of the machines worked in base-10 instead of base-2 as is common today. These were not merely binary coded decimal. Most machines actually had ten vacuum tubes per digit in each register. Some early Soviet computer designers implemented systems based on ternary logic; that is, a bit could have three states: +1, 0, or -1, corresponding to positive, zero, or negative voltage.

An early project for the U.S. Air Force, BINAC attempted to make a lightweight, simple computer by using binary arithmetic. It deeply impressed the industry.

As late as 1970, major computer languages were unable to standardize their numeric behavior because decimal computers had groups of users too large to alienate.

Even when designers used a binary system, they still had many odd ideas. Some used sign-magnitude arithmetic (-1 = 10001), or ones' complement (-1 = 11110), rather than modern two's complement arithmetic (-1 = 11111). Most computers used six-bit character sets, because they adequately encoded Hollerith cards. It was a major revelation to designers of this period to realize that the data word should be a multiple of the character size. They began to design computers with 12, 24 and 36 bit data words (e.g. see the TX-2).

In this era, Grosch's law dominated computer design: Computer cost increased as the square of its speed.