My Apple ][ shipped with "Beneath Apple DOS", the Apple ][ programmer's guide, and the Apple ][ Reference Manual which included a full fold-out schematic!
The original Apple ][ was more than a computer, it was like a degree in computer science if you actually studied what shipped with it.
D5 AA 96 forever.
Piracy was already a big deal back then and pretty much every single software publisher on the Apple ][ had developed their own copy protection mechanism by simply hijacking the low-level vectors of Apple DOS.
Some of these copy protections were quite frankly amazing in ingenuity.
The boot sequence was pretty unmistakable (clear screen, switch to 80 columns, cursor in the upper left corner) but quite novel at the time.
Here it is, if you want to experience it yourself:
Q̶u̶e̶u̶e̶ Cue the person looking at source code mentioning something along the lines of "this is real engineering".
Well it's assembly, so I'm looking at it and thinking, "Yup, that's assembly!"
Keep in mind that the Apple II originally shipped with 4 kB.
I especially like this: http://www.pouet.net/prod.php?which=30244 
Everything (including graphics and music) fits in 200kb (177kb exactly)
While it was made almost 10 years ago it is still very impressive.
 If you can't or are afraid to run executable (antivirus programs might complain due to compression program they used), here's video of it: https://www.youtube.com/watch?v=KMfyrnu3n_c
You get an FPS complete with 3D engine... dynamic lightning IIRC (!), music, textures...
... in 96kb.
Granted, most of it is procedural but still it's a very very impressive piece of code.
I've always thought that demoscene guys were on a whole different engineering league
My first computer.
TIL the name of that band  was suggested by the June Fairchild, the Cheech and Chong "Ajax Lady" .
The RESET key was so unfortunately placed, and earlier models had very soft springs, which Apple eventually stiffened in later models, so there was a whole industry of after market RESET key protectors  (like a square plastic tube that slides over the key so you have to stick your finger down into it to RESET), to prevent you from having to type 3D0G so often.
The other thing that tripped up newbies was the power light on the keyboard that wasn't a switch (you had to reach around the back left of the unit and fumble for it).
While I appreciate that some of this code could possibly be re-purposed for embedded systems or micro-controllers with similar memory constraints, it seems these problems have been well solved several times over in the past 38 years. Yes, there's a surprising amount of 65xx development still going on, but again, is this going to fill any gaps that free software doesn't already cover? Note, I'm not saying this isn't interesting - it very much is.
But, at the risk of sounding churlish, is this hubris, or is there some justification for the constraint:
> ... we are pleased to make available the 1978 source
> code of Apple II DOS for non-commercial use.
Permission like this is very clarifying and welcome, and has nothing to do with what you run on your Debian box.
Debian reference was perhaps not useful -- more a comparison for vintage of code in use.
I appreciate that it's a grey area, and I grew up coding 6502 & 6510 assembly on C64, hence the interest in reading through the pdf's provided.
It's more the 'non-commercial usage is okay' caveat for the release. I genuinely don't know if there's a possibility of commercial use, or, beyond that question, why the restriction would be placed (it seems extremely unlikely it's for financial reasons).