I am Jean-Baptiste "JBQ" "Djaybee" Queru, original coder of the ZX124 megademo, and these are my personal notes about the licensing constraints associated with that code. These notes might not (and probably do not) reflect the thoughts of other people who might have been involved in the code you received, whether they distributed it to you, modified it, or contributed to my version. There's still a reasonable chance that they don't entirely disagree with me, since they wouldn't have been involved otherwise.
On the one hand, I understand that corporate creators of Open Source Software might prefer the Apache 2.0 license, and that has influenced me in the past into picking that license for my personal projects.
On the other hand, I now try to prefer the GPL family for my own Open Source projects, because I believe that recipients and users of software should be given the legal rights, technical means, and necessary information, that they need in order to maintain that software themselves.
I try to choose the most restrictive of the GPL variants when I believe that the additional restrictions are relevant to the way my code might be used.
While the GPL licenses are well designed for desktop environments, with some good attempts at covering embedded cases and server cases, I find them somewhat confusing in the case of retrocomputing, because development and execution environments for retrocomputing aren't exact matches for their desktop counterparts. This document aims to clarify my interpretation of the GPL, to help align my expectations with those of recipients of my software, though the exact license text remains the ultimate binding one.
At the time of writing, the current versions of the GPL are LGPLv3, GPLv3 and AGPLv3, which were orriginally published in 2007. This document refers to those versions, and specifically to AGPLv3.
It is understood that not all retrocomputing targets can reasonably display Appropriate Legal Notices as defined in GPL.
As a rough guideline, targets with less than 32kB of total RAM will rarely be expected to display such notices, while machines with 1MB or more of total RAM will typically have no difficulty displaying such notices. Between those extremes, the choice will depend on the exact characteristics of both the target and the actual program.
The source code in its preferred form is specifically expected to contain full revision history with meaningful fine-grained change messages, typically in git or in something newer with similar capabilities. Snapshots (a.k.a. tarballs) might be acceptable when their usage is justified (e.g. to provide smaller downloads side-by-side with full revision history).
The source code is expected to contain image files for graphics when such files are used, and all design documentation including screen layouts and timing calculations.
The Standard Interfaces might be the addresses of hardware registers
or the mechanisms to invoke BIOS or OS methods (e.g. the knowledge
that on Atari 2600, address $1B is the graphics for player 0, often
called GRP0, or that on Atari ST, XBIOS $26 invokes a subroutine in
supervisor mode and is often called Supexec).
In the case of retrocomputing, Major Components also include actual target hardware, BIOSes and OSes associated with such hardware (whether stored in ROM or otherwise), emulators, assemblers and compilers used to build retrocomputing applications, toolchain configuration files and libraries for each target (even when distributed separately from other tools,) and relevant packaging tools.
System Libraries include any common interface declaration files for
each target, e.g. files that declaring that GRP0 is $1B, or that
XBIOS is $E while Supexec is $26.
When the program is meant to be used in a public terminal, such as a gaming cabinet installed in an amusement arcade, whether coin-operated or not, the resulting terminal should be considered a User Product.
Additionally, using such a terminal will be considered a
Remote Network Interaction. While I realize that this is quite
a stretch from the letter of AGPLv3, it's also consistent with the
spirit of GPL in general where users of software must be allowed to
maintain it and modify it, even in situations where they use
software that is not distributed to them.
See README.md for more details.
In practice, it is expected that any user of the program can receive a copy of the Corresponding Source, whether the usage happens on hardware that the user owns or not. It is also expected that recipients of tangible products including the program can replace the program with their modified version.
Finally, as a matter of development practice, I try to reduce or eliminate my code's reliance on external interface declarations and prefer to recreate such declarations myself in a self-contained way. I also try to reduce my code's reliance on BIOSes and OSes associated with the target, though full elimination is not always possible.