Technology
The 1401 Mainframe

IBM 1401: The Mainframe ... and the infamous plugboard

Burgess COMMENTARY

Peter Burgess

IBM 1401: The Mainframe

Overview ... Transforming the World ... Technical Breakthroughs ... Cultural Impacts

The Endicott team that developed the 1401 system didn't just develop a new technology; they shifted a technical paradigm. The capabilities of the 1401 machine transformed a global culture of work that had been dedicated to the previous punched card data processing, by introducing the first single, sophisticated and relatively affordable computer system.

BY THE NUMBERS ... 62,500

The IBM 1401 system was capable of reading or writing tapes at speeds up to 62,500 characters per second.

Plugboard

Plugboards were huge, complicated masses of wires that required a skilled programmer to come and rewire between each program that needed to be run. Until the 1401, companies’ only solution to this problem was to keep multiple plugboards available, each one a unique tangled web of a program so that when they needed to run different programs, they would pull out one plugboard and insert another. Note the handle on the plugboard in this image—it's made for two hands. ... Courtesy of Mike Loewen

The IBM 1401 system was introduced to provide large-scale system features to small business computer users who needed more system flexibility to solve their smaller, but unique problems. The 1401 computer brought to these industries a machine with an array of benefits: high-speed card reading and punching; high-speed printing; use of magnetic tape for input, output and extra storage; stored program of instructions; arithmetic and logical ability.

The chief architect of the 1401 computer, Fran Underwood, knew his biggest challenge was overcoming the obstacle of the plugboard, a spaghetti-like array of cords and plugs that tell a machine what to do—move this information here, perform this action on that output and so forth. These control panels, or plugboards as they were commonly known, were nightmarishly constructed Gordian knots of complexity and confusion. They were slow, time-consuming, and above all, expensive.

Underwood recognized he could strip out the cumbersome plugboard control panel and replace it with a simplified stored-program machine, which would allow for rapid application development. But in order to make his case, Underwood had to prove stored programming could be run inexpensively and effectively.

At this stage, stored programming was prohibitively expensive because the instructions it employed required a lot of memory, and resulted in a lot of waste. This was because the commands to the machine comprised fixed word lengths and fixed instruction lengths. That is to say, regardless of whether a given instruction to the machine was three characters long or eight characters long, the space given to each instruction was the same. And whatever space was not used by the instruction was simply lost, completely devoid of information, never to be seen again. This inherent waste required designers and engineers to over-provision on memory, which meant a significant increase in production cost.

Underwood’s approach eliminated this unnecessary waste and cost by using concepts called variable word length and variable instruction length. In theory, these tactics sent a continuous and solid block of instructions to the machine, without all the blank spaces of the fixed method. The only problem was the machines didn’t know where one command stopped and another began, so Underwood incorporated what he called a “word mark” to flag the end of a particular instruction. The innovative word mark, combined with the variable word length and variable instruction length, allowed Underwood to pack an immense amount of information into a fraction of the space that otherwise would have been required using the fixed word length approach. All told, Underwood’s methodologies required 40 percent less core memory than the next best thing IBM had going at the time. And that directly translated into cost savings for the consumer.

Underwood also used alphabetic characters as opposed to the established binary method. “A” for add, “S” for subtract, “M” for move and so on. Of particular note was the “E” for edit function. This edit function, and the other intuitive alphabetic characters, greatly increased the 1401 system’s usability from the customer’s standpoint, and made the 1401 computer vastly superior to other accounting machines.

Less expensive and easier to use? It’s no wonder the 1401 system was such a groundbreaking achievement.