> Some games don't have the compatiblity tag. Probably they can be used by any Atari
> 2600 machine.
> Still need to check more gamaes and other machines like Amiga, MSX, Atari 800, Super
> Nintendo, Master System, etc...
> I hope I'm right about this.
the compatibility flag is not an easy concept because it is a compromise between being as user friendly as we can with non-expert users and being flexible enough to satisfy more expert users. E.g. if you launch a2600 emulation with a game that has NTSC or no compatibility flags the games load and run with no issues, both if you run it as "mame a2600 gamename" and as "mame a2600 -cart gamename". if you do the same with a game that only has PAL compatibility then mame will have two different behaviors depending on the command line options
* expert usage: if you launch emulation with "mame a2600 -cart gamename" and gamename has PAL compatibility, the emulator gives you a warning at command line but it launches anyway the game, because it assumes that you are experienced enough to understand that if you get a black screen at start the cause is related to the warning you have been exposed to
* noob usage: if you launch emulation with "mame a2600 gamename" (without the -cart switch) and gamename has PAL compatibility, the emulator assumes you are just here to play and thus refuses to load the game (mame actually *filters out* the game from the software list in this case, giving a best match message)
the latter scenario prevents you to sit at a black screen in systems like a2600 and a7800, but for instance gives you no way to see the "wrong region" messages that many genesis and snes games contain, which might be instead interesting for developers (and thus experienced users have a way to load every game in every console region, by adding the media switch -cart)
feel free to keep asking for clarifications: I understand very well that it's hard to digest all the available options at once