An update on progress with this experiment, mostly for anyone else keen to try.
To review, I'm tinkering with a couple of veteran Super 7 boards with AMD K6-2 processors, one 300 MHz and one 500 MHz.
The idea is to answer the question:
Q: "Are these arthritic old guys are still useful with a modern light Linux distribution?"
A: No.
Q: "OK, with an OLD light Linux distro?"
A: Yes, with limitations.
EXECUTIVE SUMMARY:
Forget Bionic, even with a non-PAE kernel. The reason is below.
Any of the following Puppy breeds will work:
Puppy 2.16*
Puppy 4.1.2 Retro with kernel 2.6.21.7
Puppy 4.31 (not 2016 update)
Puppy Lucid 5.2.5 (not 5.2.8 or 5.2.8.7)
Puppy Wary 5.5
*Puppy 2.16 choked on one of my video cards, but worked great on the other, an S3 Vierge.
RECOMMENDATIONS:
For light server use, without X, any of the above pups will suit. DSL 4.4.10 also looks promising.
For desktop use, Puppy 2.16 is surprisingly snappy at 500 MHz. I couldn't test it at 300 MHz. You won't be doing any but the most basic web browsing, though. The default is Seamonkey 1.0.8 which will roll over and play dead for the modern web. You can try updating it, but you'll run into the problem described below.
I don't recommend any of the later pups for desktop use with a K6-2, unless you have a lot of patience.
However, if you have a K6-III, or especially a K6-III+, you might try Wary 5.5. Those CPUs are faster for a given clock speed, and also overclock nicely. I've run a III+/450 at 600 MHz, and it never broke a sweat.
It's sacrilege to say it, but the best choice for K6 desktop use is Windows 98. That's what the K6 was optimized for.
THE GORY DETAILS:
AMD K6-2, K6-III, and K6-III+ processors all need a Linux kernel before about 2.6.35.
Non-PAE isn't enough; it has to also avoid the Intel cmov opcode, of which the K6 knoweth naught.
Some distros might boot to a desktop, but you find that some programs, including most (all?) post-2012 web browsers, crash with an "illegal instruction" error.
You might be able to compile a more modern distro yourself, leaving out PAE and cmov. That's above my pay grade, however.