Old school source code?

[ZeoDefender]

Does anyone know of any companies that have released the source code for some 1st-4th generation games? This is probably a long shot, but I'd love to spend an afternoon digging through the code for something like Castlevania and seeing how it all comes together.

[AppleQueso]

You can find a lot of disassemblies if you look around.

[MrPopo]

Atari and NES era stuff doesn't have any source code beyond the disassemblies (as in, they were programmed in assembly). The biggest thing you miss out on a disassembly is the comments the devs originally added to remind themselves what the hell this block did (which is non-trivial in assembly).

PC games of the era have had their source code released, most notably the id FPSs and Duke 3D. But as far as I know the only console game to ever had its source code released was the 3DO port of Star Control 2 (which was then turned into the Ur-Quan Masters project).

[Anapan]

I've seen a few Atari 2600 games' source code - comments included (don't remember where).
! I just found the source code to Space Funky B.O.B. on my hard drive while looking for the Atari games' source code! (don't even remember downloading it, but if you can't find it online I can upload it).
I remember reading that someone dissassembled and then commented the source to Metroid (NES), tho I don't think it'd be as friendly to read as the original.
Other games that I know had their source code released were mostly DOS games. Zorlim's Pizza Worm, Jump 'n' Bump, Abuse...
There's a bunch listed here.

[MrPopo]

I remember reading that someone dissassembled and then commented the source to Metroid (NES), tho I don't think it'd be as friendly to read as the original.

Well, for an American it's probably more friendly than whatever romanji comments were in the original.

[Key-Glyph]

Not exactly what you're looking for, but Jordan Mechner (Karateka, Prince of Persia) is big on releasing source code in his blog, if you're interested.

Here's his entry on Prince of Persia's source code, with a link to the files at the end.

[Overload]

I'm pretty sure the source code for DOOM is publicly available.