Simulating a minicomputer (PDP-11) on an Atmel MCU



The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from around 1970 until the 1990s.

Image credit: Wikipedia

According to Wikipedia, the PDP-11 offered a number of uniquely innovative features and was easier to program than its predecessors due to the inclusion of additional general-purpose registers. Perhaps most importantly, the very first officially named version of Unix ran on the PDP-11/20 in 1970.

Recently, an engineer named Dave Cheney kicked off a project to simulate the PDP-11 using a board powered by Atmel’s versatile ATmega2560 microcontroller (MCU) and a custom-built SPI SRAM shield. Combined, the two components form a platform aptly dubbed “AVR11.”

“Today the simulator boots V6 Unix and can execute some simple commands. [Yes], there are some remaining bugs in the mmu which cause the simulator to fail when larger programs (/usr/bin/cc and /usr/games/chess for example) are executed,” Cheney explained in a blog post describing the project.

“The hardware emulated is somewhere between a PDP11/40 and PDP11/45. The EIS option (MUL and DIV) is properly emulated, but FIS (floating point is not). Only a single RK05 drive is simulated, backed by a file on the micro SD card.”

Cheney says he ultimately plans on improving the accuracy of the simulator so it can run V7 Unix, 2.9/2.11 BSD, RSX-11M and even the original DEC diagnostics. In terms of speed, Cheney confirms the simulator is approximately 10x slower the an original 11/40.

“I was never expecting to be amazed with the speed of this simulator, especially at this early stage. However, on a performance per watt basis, I think it’s hard to beat AVR11. The PDP-11 that this simulator models is spartan, even by the standards of the early 70s, yet still consumed over 2 kilowatts of power for the CPU and memory (256kb),” he continued.

Image Credit: Wikipedia

“The 2.5 megabyte RK05 boot drive was another 600 watts. Real Unix installations would have three or more drives, so there goes another 1200-1800 watts. Compared to that, the AVR11 draws well under the 500ma limit of a USB port. Although I lack equipment to measure the current draw I estimate it to be around 100ma at 5 volts which is 0.5 watts.”

Interested in learning more about simulating a PDP-11 with an Atmel MCU? You can check out Dave Cheney’s project page here or download the relevant code from GitHub.

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