Ben Gurley (PDP-1)

Benjamin Gurley (1926-1963)
Benjamin Gurley (1926-1963)

In 1959, a small and relatively new company (founded in 1957) for logic modules and other laboratory equipment, located in an old woolen mill in Maynard Massachusetts, named Digital Equipment Corporation (DEC), decided to build a computer. Ken Olsen, a cofounder of DEC, remembered: We had a dream of interactive computing. Normal computing was considered big, expensive, awesome, beyond ordinary people. Interactive computing was exciting and fun, and people could interact directly with the computer.

Announced in 1960 (the first machine was delivered in December 1959), DIGITAL’s landmark first computer, the PDP-1 (Programmed Data Processor-1), marked a radical shift in the philosophy of computer design: it was the first commercial computer that focused on interaction with the human user rather than the efficient use of computer cycles. Unlike the computers before it, it was centered around the user and actually it was the engineers, researchers, students, and hackers from a variety of companies and organizations, that brought DEC’s vision of the PDP-1 to life. The DEC people and the machine they created went on to have wide influence.

DEC management assigned the project to their best engineer and colleague from MIT, Ben Gurley. Benjamin Gurley (1926-1963) was a brilliant engineer and designer, who came to DEC from MIT’s Lincoln Laboratory, just like the founders of DEC—Ken and Stan Olsen brothers, and Harlan Anderson. While at MIT, Gurley worked on the TX-0 and TX-2 computers created at MIT in the second half of the 1950s (he designed the cathode ray tube display and light-pen of the TX-0), thus his computer design work at MIT greatly influenced the development of the PDP-1. The TX-0 and TX-2 computers, designed by Wesley Clark (the creator of the LINC computer), were (besides all) among the first transistor-driven computers in the world.

PDP-1 in Datamation November/December issue of 1959
PDP-1 in Datamation November/December issue of 1959

The work on PDP-1 began in the summer of 1959. Gurley formed a group to work on the computer with fellow engineers and former Lincoln Laboratory employees—Ken and Stan Olsen, Dick Best, Bob Savell, and Harlan Anderson. Amazingly, Gurley managed to build nearly the entire system himself in three-and-a-half months, making liberal use of DEC’s existing system building block product line—laboratory modules. He once joked that his assignment was “to make [the PDP-1] from inventory.” The truth, however, is that Gurley designed roughly half of the modules used in the PDP-1 from scratch.

The prototype was ready in November 1959, and the machine was featured in the November/December issue of the magazine Datamation (see the nearby image). The article outlining the many features of the new machine generated much advance interest in its appearance at Boston’s Eastern Joint Computer Conference.

The PDP-1 was a solid-state digital computer, built mostly of DEC 1000-series System Building Blocks, using Micro-Alloy and Micro-Alloy-Diffused transistors (it uses 2700 transistors and 3000 diodes) with a rated switching speed of 5 MHz. PDP-1 has an 18-bit word and 4 kilowords as standard main memory (equivalent to 9 kilobytes), upgradable to 64 kilowords (144 kB). The magnetic core memory’s cycle time was 5 microseconds, thus most arithmetic instructions took 10 microseconds (100000 operations per second) because they had two memory cycles: one for the instruction, and one for the operand data fetch.

PDP-1 was designed to operate with many types of input-output devices (among them typewriter, paper tape, cathode ray tube, light pen, magnetic tape, etc., see the list below) with no internal machine changes.

It is a single address, single instruction, stored program computer with powerful program features. It supports fully parallel processing and is unusually versatile. It is easy to install, operate and maintain. Conventional 110-volt power is used, neither air conditioning nor floor reinforcement is necessary, and preventive maintenance is provided for by built-in marginal checking circuits.

PDP-1 computer
Digital Equipment Corporation’s PDP-1 computer

The PDP-1 used punched paper tape as its primary storage medium. Unlike punched card decks, which could be sorted and re-ordered, paper tape was difficult to physically edit. This inspired the creation of text-editing programs such as Expensive Typewriter and TECO.

The PDP-1 (see PDP-1 manual) was the first computer in Digital Equipment Corporation’s successful 14-members PDP series (The last model, PDP-16, was launched in 1972. The brilliant PDP-11 was produced up to 1996.). It is famous for being the computer most important in the creation of hacker culture at MIT, BBN, and elsewhere. The PDP-1 (with the optional high-resolution graphical display) was also the original hardware for playing history’s first game on a minicomputer, Steve Russell’s Spacewar!

Being the world’s first commercial interactive computer, the remarkable PDP-1 was used by its purchasers to pioneer timesharing systems, making it possible for smaller businesses and laboratories to have access to much more computing power than ever before.

The entire computer occupies below 2 sq. m. of floor space, dimensions of the system module are 1.5 x 2.5 x 1 m. PDP-1 consists of four equipment frames, one of which is used as the operating station.

The Central Processor contains the control, arithmetic, and memory addressing elements, and the memory buffer register. The word length is 18 binary digits. Instructions are performed in multiples of the memory cycle time of five microseconds. Add, subtract, deposit, and load, for example, are two-cycle instructions requiring 10 microseconds. Multiplication requires an average of 20 microseconds. Program features include single-address instructions, multiple-step indirect addressing, and logical arithmetic commands. Console features include flip-flop indicators grouped for convenient octal reading, six program flags for automatic setting and computer sensing, and six sense switches for manual setting and computer setting.

The coincident-current, magnetic core memory of a standard PDP-1 holds 4096 words of 18 bits each. The memory capacity may be readily expanded, in increments of 4096 words, to a maximum of 65536 words. The read-rewrite time of the memory is five microseconds, the basic computer rate. Driving currents are automatically adjusted to compensate for temperature variations between 50 and 110 degrees Fahrenheit. The core memory storage may be supplemented by up to 24 magnetic tape transports.

PDP-1 is designed to operate a variety of buffered input-output devices. Standard equipment consists of a perforated tape reader with a read speed of 400 lines per second, an alphanumeric typewriter for online operation in both input and output, and a perforated tape punch (alphanumeric or binary) with a speed of 63 lines per second. A variety of optional equipment is available, including the following:

  • Precision CRT Display Type 30
  • Ultra-Precision CRT Display Type 31
  • Symbol Generator Type 33
  • Light Pen Type 32
  • Oscilloscope Display Type 34
  • Card Punch Control Type 40-1
  • Card Reader and Control Type 421
  • Magnetic Tape Transport Type 50
  • Programmed Magnetic Tape Control Type 51
  • Automatic Magnetic Tape Control Type 52
  • Automatic Magnetic Tape Control Type 510
  • Parallel Drum Type 23
  • Automatic Line Printer and Control Type 64
  • 18-bit Real Time Clock
  • 18-bit Output Relay Buffer Type 140
  • Multiplexed A-D Converter Type 138/139
Digital Equipment Corporation's PDP-1 computer
Digital Equipment Corporation’s PDP-1 computer

All in-out operations are performed through the In-Out Register or through the high-speed input-output channels.

The software included diagnostics routines, an assembler, a debugger, an editor, conversion routines for punching tapes, etc.

Despite its importance for the philosophy of computer design and its significant impact, the PDP-1 seems to be a commercial failure for DEC and was produced in relatively small quantities (53 machines, last built in 1969). The initial cost of a fully equipped system was $120000 ($85000 for minimal models), at a time when other computers sold for well over 1 million dollars.

Ben Gurley headed computer engineering in DEC until he left in 1962 to serve as vice president of Information International Inc., a consulting firm that created PDP-1 applications. His death in November 1963 at the hands of a deranged former colleague from DEC was a tragic loss to computer engineering (Gurley was murdered by a single rifle shot as he sat down to dinner with his wife and children at their home in Concord, Mass. The bullet entered the window and pierced his temple; he fell to the floor and died there in minutes, at the feet of his 5 children.)