Fred Yearian, pictured in front of PDP-7 serial no. 129.
I recently sat down with Fred Yearian, a former Boeing engineer, and Jeff Kaylin, an engineer at Living Computers, to talk about their restoration work on Yerian’s PDP-7 at Living Computers: Museum + Labs. The PDP-7 (short for ‘Programmed Data Processor’) was introduced by The Digital Equipment Corporation (DEC) in 1964.
In the early 1960s, the term ‘computer’ was synonymous with large, costly machines that were often difficult to understand. DEC coined the name PDP as a way to avoid these negative connotations.
Across the country, DEC found customers for the PDP-7 in places such as Bell Labs, the Pacific Northwest National Laboratory, and the Jet Propulsion Lab. The PDP-7 was also popular with universities, with units sent to Oxford University, MIT, the Helsinki University of Technology, and Tokyo University, among others.
To find a PDP-7—particularly one that is operational—is exceptionally rare. For years, there were thought to be four PDP-7s left in the world. Of these, two PDP-7s resided outside the US, with one under restoration in Oslo, Norway and the other a part of a private collection in Australia.
The third and fourth known PDP-7s remained stateside, with one in storage at the Computing History Museum in Mountain View, CA and the other at the Living Computers: Museum + Labs in Seattle, WA.
LCM+L’s PDP-7 was originally installed at the University of Oregon’s nuclear physics department. After Professor Harlan Lefevre accepted delivery of the system, his lab spent the next three years writing the necessary software required to carry out their research. Once complete, the system proved remarkably reliable: over the next four decades, it logged more than 60,000 hours of use and enabled 23 graduate students to earn their PhDs.
Up until the discovery of Yearian’s machine, LCM+L’s PDP-7 was believed to be the only operational PDP-7 left in the world. Chuckling to himself, Yearian recalls hearing this history presented during his first visit to LCM+L.
“I walked in the computer museum, and someone said ‘Oh, this is the only [PDP-7] that’s still working’.
And I said, ‘Well actually, I got one in my basement!’”
Yearian, a graduate of the University of Washington’s School of Electrical Engineering, has worked as an electrical engineer in the Pacific Northwest for decades. He spent a large portion of his career at Boeing, helping run flight simulations to train astronauts at the Kent Space Center. Throughout his career, he grew adept in working with numerous systems, including SDS 930s, Varian Data Machines, and Tektronix equipment.
It was this breadth of knowledge that led Yearian to be called to Boeing’s Development Center in 1973 to complete what he described as, “a super tech type job, fixing computers in one place or another.”
During his visit, he spotted a PDP-7 among the other machines. It is believed that the Development Center’s PDP-7 was interconnected with an SDS 940, forming a system to display processed data.
Little did he know that years later, in 1979, Yearian would find that same PDP-7 in the now-defunct Boeing Surplus Store. He often visited the Surplus Store to buy stocks of wire and other equipment for different projects.
When he first inquired about buying the PDP-7, he was told that he couldn’t purchase the system because someone else was considering stripping it for parts, including the paper tape system.
But on a later visit to the Surplus Store, Yearian noticed that the PDP-7 was still there. He asked again and he was able to purchase the system for $500, load it into a truck, and take it home.
The PDP-7 was housed in Yearian’s basement, and after he first moved it in, he did some troubleshooting. Soon after, he discovered the source of the issue. He described, “There was an intermittent problem with it. Boeing had had [the PDP-7] a long while, and they never did find [the source of the problem]. I troubleshot it from an old Tektronix 45 and found it—terminator in the wrong place.”
After getting it running, Yearian wrote a program in BASIC on another computer and loaded it on the PDP-7, verifying that everything was working correctly. Following these initial tests, over 25 years passed before Yearian’s visit to LCM+L in March 2017.
After communicating to tour guides during his first visit to LCM+L that he had a PDP-7 at home, Yearian was connected with LCM+L’s engineering team.
“We had some skepticism about it because we’ve had cases in the past where people will come to the museum and say they have something and are either incorrect or they have non-working parts," said Stephen Jones, Engineering Manager at Living Computers.
"But after talking with Fred that first visit," said Jones, "we were convinced he had a complete and possibly running machine.”
On November 7th, 2018, after over a year of correspondence following Yearian’s initial visit, a handful of Living Computers engineers including Jeff Kaylin, Cynde Moya, and Stephen Jones arrived at Yearian’s house to extract the PDP-7 from his work room and load it onto a truck bound for the museum.
From there, the team began working to load the UNIX Version 0 operating system onto the machine. This goal was an homage to the fact that Ken Thompson wrote UNIX Version 0 on an old PDP-7 at Bell Labs in 1969. UNIX also turned 50 this year, which made the restoration project all the more timely.
In conjunction with loading UNIX Version 0, Living Computers engineers Jeff Kaylin and Josh Dersch have been working on additions to the PDP-7’s Direct Memory Access and Input/Output interfaces, which allow data to be passed into main memory.
The additions include a disk emulator named the JK09 that interfaces with a custom modification that was installed while the machine was used at Boeing. Also, a device driver (likely the first new driver for UNIX Version 0 in the last 45 years!) was added so that the kernel could use the new drive.
After countless hours of work, the team booted UNIX v0 on Yearian’s PDP-7 on Monday October 28th, 2019 for the first time. The engineers logged in as user 'dmr' as a tribute to Dennis M. Ritchie, who created UNIX with Ken Thompson in 1969.
Output from the PDP-7, including the message: "Hello from the PDP-7 running UNIX v0"
Before the end of our conversation, Yearian described what he sees as the legacy of his PDP-7, in the way that it can show people another piece of computing history.
“[I want people] to learn that there are other things than IBM computers… [In my career, there were] a lot of little stories along the way.”
Visitors can see Yearian’s PDP-7 and the restoration project at Living Computers: Museum + Labs, on view now. Learn more about visiting LCM+L here.