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Al Williams

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Open Propeller

August 20, 2014

You've probably picked up on the fact that I like thinking about unusual CPU architectures. I've dabbled myself with a few strange ones (including One-Der) that I’ve talked about before.

The guys over at Parallax (the people who became famous for making the Basic Stamp) rolled out an innovative CPU a few years back called the Propeller. The Propeller gets its name from a novel scheme it uses to avoid dealing with interrupts.

If you are a low-level hardware guy, you might think, "But I like interrupts". The fact is interrupts are really terrible. They often introduce nondeterminism and other problems. However, it is the best way to reduce response times and as an industry, we understand how to manage them pretty well.

Parallax had a different strategy. The Propeller has multiple CPU cores that connect to a central hub. Each core can access any of the I/O pins and shared memory. However, each core also has private resources. You can think of the central hub as a motor shaft that spins (I guess, spins a propeller). Each CPU core is conceptually around the circumference of the hub and when the propeller points to it, the core can access the shared memory. The other cores have to make do with their private work until the propeller spins around to them.

So instead of dedicating an interrupt service routine (ISR) to an external stimulus and having your CPU execute the ISR, with the propeller you dedicate a core (known as a cog) to the ISR task at all times. That's all it does is handles that I/O operation.

At least, that's one way to use the CPU. Each core has video generators and other goodies and there have been a lot of projects to emulate old computers and video games using the propeller (for example see the Pocket Mini Computer and this list of links.

The reason I'm bringing this up is I've noticed Parallax open sourced the Propeller 1 design. The open source release includes the Verilog for the CPU (targeted at Altera devices), the IDE, and several programming languages. Parallax says they've done this to inspire others to learn and create, to support higher education, and to open up the design to community contributions.

I think this is really great. Although the Propeller isn't a conventional CPU, it is a full-featured CPU that is in practical use. There are very few commercial CPUs where you can actually go download and read the Verilog (or modify it). This has great implications for people who want to learn about developing CPUs, and for educators that might be able to build a class around something like this (extending a propeller with some special I/O or instructions, for example).

If you want to play with the device, the development boards aren't that expensive, or you can load it into an FPGA (presumably with a little work if it isn't one of the ones supported out of the box). There's also a simulator available if you just want to experiment. I haven't tried it, but I'd assume you could simulate the design in the Altera tools too if you are stout enough to write a good enough test bench.

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