IBM 701
In the early 1950s, Thomas J. Watson Sr. approved James Birkenstock’s proposal to create an innovative computing machine. This machine would use cathode ray tube memory of approximately 20,000 digits and feature a clock cycle capable of multiplying two numbers in one millisecond. Initially named the “Defense Calculator,” the machine came into being during the Korean War at the request of the U.S. Department of Defense. A team led by J. A. Haddad and Nathan Rochester was assigned to its development.
The world had only about twenty functional digital computers. Birkenstock believed that approximately thirty of these machines would find buyers in the defense industry. The creation of the Defense Calculator was relatively rapid thanks to IBM’s experience with its earlier products. The team adopted a minimalist philosophy: simple equipment, streamlined logic design, and the absence of superfluous devices.
The question of word length sparked considerable debate at IBM. Werner Buchholz emphasized the importance of this technical choice, as insufficient length would require using multiple words for a single number. Conversely, excessive length would slow operations and increase costs. After studying existing machines, IBM determined that a length between 10 and 12 digits (35 to 41 bits) would be ideal. Magnetic tape technology suggested that 6 parallel channels represented an optimum for storage. IBM therefore set the word size at 36 bits for its 701.
On April 29, 1952, Thomas J. Watson officially announced the IBM 701 Electronic Data Processing System at an annual meeting. The “Data Processing” designation emphasized its ability to handle different types of data in various contexts. The first model arrived at IBM headquarters in New York in December 1952. Installed in the former SSEC room, it became an attraction for passersby. This machine served technical consulting operations for various clients, with a team of IBM scientists dedicated to solving their problems.
The second machine (serial number 1) was delivered to Los Alamos Scientific Laboratory on April 1st, 1953. Three days were sufficient to make it operational. It functioned until autumn 1956, before a second machine was installed in 1954. Its electrostatic memory, despite its lack of reliability, contained 2048 words of 36 bits. The system comprised the central unit, a card reader, a printer, a magnetic drum storage unit, two magnetic tape drives, and a card punch.
The 701 was the first machine produced in sufficient quantity to meet the needs of an expanding clientele. Its modular design, with detachable components, marked the beginning of a new philosophy at IBM: building expandable machines rather than monolithic systems requiring complete replacement. The standard configuration consisted of eleven elements: two magnetic tape units (each with two drives), a magnetic drum memory unit, a cathode ray tube electrostatic storage unit, an L-shaped arithmetic and control unit with operator panel, a card reader, a printer, a card punch, and three power supply units.
The IBM 701 displayed remarkable compactness. Its logic was housed in 64-pin modules, each carrying a row of 8 vacuum tubes on the front. Logical operations were executed via germanium diodes located at the base of the modules. These modules integrated into a backplane designed to allow their replacement without shutting down the system. IBM was among the first to use backplanes as a means of accommodating different types of functional modules.
The Analytical Control Unit (ACU) housed the arithmetic components, input-output control circuits, and stored program control circuits in a large cabinet. A separate cabinet contained the operator console. The ACU operated with a pulse repetition rate of 1 megahertz. A 12-stage ring generated twelve 1-microsecond pulses forming the basic machine cycle. This speed primarily met the regeneration requirements of the electrostatic storage unit. The latter contained 72 cathode ray tubes storing 10,240 digits, or 2,048 words of 36 bits, with an access time of approximately 12 microseconds. Data could be accessed randomly. The cost per bit approached one dollar, much less than vacuum tube storage. The monthly rental for this unit amounted to approximately $2,600.
The 701’s input-output system performed five functions: selecting a unit for reading, writing, or control; interlocking the operation with program execution; copying data between the electrostatic storage unit and I/O units; synchronizing signals between the selected unit and the CPU; disconnecting the I/O unit once its work was completed.
In 1956, the IBM 737 Magnetic Core Storage Unit replaced the electrostatic storage unit. With a capacity of 4,096 words of 36 bits, it reduced the addition time, including access, from 60 to 36 microseconds. Its monthly rental reached approximately $6,100.
As a binary single-address scientific computer, the IBM 701 had a word size of 36 bits and an instruction set of 33 instructions. The 701’s CPU remained 25% empty to facilitate error correction or the addition of features. Its monthly rental amounted to approximately $16,000. By the end of 1954, 18 machines were installed and fully operational.
Its successor, the 704, filled this space with additional functions. This was followed by a line of increasingly powerful and faster machines, maintaining backward compatibility and incorporating new features. This evolution culminated with the 7094 II and its simplified versions, the 7040 and 7044.