PDP-11/73 Modernization: Overcoming Hardware/Software Challenges to Host Public Website
Clip title: My Computer is now 4× Faster! That’s Why It Won’t Boot… Author / channel: Dave’s Garage URL: https://www.youtube.com/watch?v=EzfHqE9-rbY
Summary
The video chronicles a fascinating journey of bringing a vintage 73 computer, a relic from the mid-1970s, into the modern internet age. The main topic revolves around the intricate challenges and eventual triumph of upgrading, troubleshooting, and integrating this decades-old hardware and its 2.11BSD Unix operating system with contemporary technologies to serve a public website and host an AI chatbot named “Gary.” The speaker, Dave, highlights the appealing aspect of working within an existing architecture to significantly enhance its capabilities.
The adventure began with the acquisition of a Mentec processor board, promising a substantial 400% speed increase and 4MB of fast onboard memory for the PDP-11. However, this upgrade initiated a cascade of technical hurdles. Initial attempts to boot revealed incompatibilities with the SCSI disk controller, necessitating meticulous hardware modifications, including soldering jumpers to properly configure the backplane’s bus grant arrangements. Once the controller was recognized, a new problem emerged: the system couldn’t boot from the SD card, traced back to an incompatible GUID Partition Table (GPT) layout, requiring a switch to a Master Boot Record (MBR) partition.
Further challenges arose with the operating system itself. The BSD kernel’s floating-point unit (FPU) assumptions clashed with the new processor, requiring modifications to enable a software floating-point simulator (FPSIM) and untangle related kernel configurations. Fitting the modified kernel into the PDP-11’s limited 64KB (or 128KB with memory overlays) address space proved another complex task, with ChatGPT assisting in generating appropriate makefiles. Even after successfully compiling a custom kernel and setting up networking, the system failed to reliably boot in its main chassis due to a timing incompatibility in the Mentec boot firmware. This critical issue was diagnosed by creating a custom diagnostic bootstrap using ChatGPT to generate octal machine code, which was then painstakingly loaded and analyzed through the serial console. The solution involved patching the firmware chip with increased polling iterations to allow the disk controller sufficient time to initialize.
With the PDP-11 finally booting reliably and connected to the internet, Dave integrated it into a sophisticated service architecture. Modern components like Cloudflare, Caddy, and Varnish handle public DNS, HTTPS, and caching of static content, protecting the vintage machine from repetitive load. The PDP-11 serves dynamic elements and live kernel activity, while the AI chatbot “Gary” leverages powerful modern GPUs on a separate Linux machine for inference. This distributed approach allows the PDP-11 to contribute meaningfully without being overwhelmed. The project’s key takeaway is the successful synergy of diverse computing generations: the PDP-11 handles its niche tasks, modern systems manage web traffic and heavy AI processing, and tools like ChatGPT accelerate complex debugging and code generation, ultimately giving a 40-year-old machine a practical and engaging new purpose.
Video Description & Links
Description
A Mentec CPU made this PDP-11 about 4× faster. That is why it would not boot. Once it did, it went on the internet as pdp1173.com, serving a live UNIX process list and the front end for Gary, a curmudgeonly wizard whose answers come from a pair of modern GPUs.
The machine is still a PDP-11. The software still sees 2.11BSD. The faster board just ran the firmware’s wait loops before the disk controller was ready.
Visit it: https://pdp1173.com
• A Mentec processor with 4MB on the board, roughly 4× my 11/73 and 8× my 11/44 on integer benchmarks • Bus-grant jumpers, a ZuluSCSI that was reading an EFI partition, and a kernel that had to be told to fake floating point • A diagnostic bootstrap pasted into ODT in octal, because the machine would not boot any other way • The MSCP firmware’s polling limits, 40 and 100, raised to 256 in a RAM copy, then abandoned for a controller that answers in time • Why a cached page from a PDP is the only way this site survives a Facebook link • Gary: the page and the interface run on the PDP, the model runs on GPUs over a socket, and the credentials never touch 2.11BSD
Chapters 0:00 The machine, the site, and Gary 0:00 The Mentec, and why faster is not simpler 0:00 Spare backplane, bus grants, and a disk full of zeros 0:00 Floating point, overlays, and a kernel that fits in 64K 0:00 It boots on the bench, then refuses in the chassis 0:00 Pasting a bootstrap into ODT 0:00 The firmware was counting instructions, not time 0:00 The CMD controller, and the numbers 0:00 pdp1173.com: cache, failover, and a live process list 0:00 Gary, and the 10% the GPUs do 0:00 Facebook appends a query string and breaks the path
If it will not boot, it does not get a job. This one finally has one.
Task Manager, the way I wanted it: https://tmog.org
Drop a question mark on a real question and I will look for it to discuss on Shop Talk! Check it out here: https://www.youtube.com/watch?v=c3EEs-O3bGE
URLs
Related Concepts
- 73 — Wikipedia
- 2.11BSD
- Unix — Wikipedia
- vintage computing — Wikipedia
- hardware modernization
- system integration — Wikipedia
- AI chatbot — Wikipedia
- Boot Firmware — Wikipedia
- Cloudflare — Wikipedia