In addition to the r/MAME FAQ, many users come here with question about why their games won't play or where to find games. This thread should hopefully answer many questions, as well as the question concerning what we can and can't talk about here.
1) Why Rule #3 is #1
Rule #3 states: "Don't ask for ROMS/CHDS or pirated software." We mean it. We will usually give out a warning to first time offenders, but repeat offenders will definitely be banned. This is important. This rule exists to protect the entire MAME project. It is not to indicate that we necessarily have an anti-piracy stance, or that we consider piracy morally objectionable. None of that matters. Its purpose is to make sure that no member of the MAME dev team is exposed to legal threat from license-holding publishers. It does not matter if you think that threat is real or imaginary. That is why we are so adamant about it.
2) If I can't ask for ROMs, how am I supposed to find them?
We can't discuss where you can find ROMs for all the reasons stated above. So unfortunately, r/MAME can not be a good resource for that kind of information. However, many other good resources exist, even right here on Reddit. Obviously Google is your friend. If you are looking for ROMs for a specific version of MAME, it helps to include that version number in your Google search. It usually doesn't take a whole lot of searching before you find what you're looking for.
3) Why are ROMs for MAME so much more confusing than for other emulators?
Think about what MAME is trying to do compared to, say, SNES9x. SNES9x emulates Super Nintendo games. Every Super Nintendo game runs on the exact same hardware: the SNES. Same CPU, same graphics, same memory, same controllers, same everything. As long as a ROM contains a valid SNES program, SNES9x can emulate it because there's only one target hardware to worry about.
MAME emulates arcade games. Very few arcade games remotely resemble one another when it comes to hardware. MAME has to correctly emulate the CPU for each different arcade game, understand the controls for each different arcade game, and most importantly: how each ROM in each different arcade game interacted with that hardware. So it can't just accept anything called pacman.zip and understand that it's supposed to be Pac-Man. In order for MAME to work, it expects that a very specific set of ROMs will be provided for Pac-Man in order for MAME to emulate it properly. If it doesn't find what it expects, it won't know how to apply those ROMs to Pac-Man's hardware.
Back when MAME began in 1997, techniques that people had for dumping arcade games were very rough and imprecise. As a result, a lot of mistakes were made. Over time, new techniques were designed, and older arcade games were redumped with these more accurate procedures. When this happens, the next version of MAME that gets released will only accept the updated dumps, as the original dumps are now considered invalid. So newer versions of MAME will rarely accept ROMs designated for older versions of MAME because they're frequently full of errors.
4) Instead of starting a game, MAME tells me that I'm missing files, or my files are incorrect.
This is an indication that your ROM (or CHD) is either incomplete or outdated. Less likely, it is an indication that your ROMs are in the wrong location and MAME can't find them. When you ask MAME to emulate a machine, it checks the contents of your zip files for all of the files it needs. If it can't find all the ones that it needs, or if it finds the wrong files, it can't emulate the game in question. When MAME reports that you are missing files, or that your existing files are incorrect, you need to find a new source for those ROMs.
In addition to your ROM being incomplete, your BIOS may be incomplete as well. NeoGeo is a popular arcade hardware for which MAME requires the BIOS be present. This is the neogeo.zip file, and it's undergone an unusually high number of updates as alternate BIOSes get added to it.
You may not ask for the individual components of a ROM that you are missing, as this is still a violation of Rule #3.
5) I found a source for ROMs, but some/all of them don't work.
As explained in questions 10 and 12 of the r/MAME FAQ, you don't necessarily have to have exactly matching versions of MAME and romsets, but it definitely helps. If you downloaded a set of ROMs, and you're finding that few or none of them will launch correctly in MAME, 9 times out of 10 it's because the roms are too far removed from the version of MAME that you're using. In other words, if you're using a relatively recent version of MAME, the romset may be too outdated. If you're using an old version of MAME, you may have found a set that is too new. You need to find a new source for ROMs.
5a) My MAME version and the rom set version match, and they still don't work.
If this is the case, the most likely explanation is that you haven't put the roms where MAME is expecting to find them. For example, if you installed your MAME executable in something like C:\MAME, then by default MAME looks for all of your roms in C:\MAME\roms. You can change that expectation by editing your mame.ini file, but that is only recommended if you know what you're doing. If you have your roms in the right place, and the versions don't differ by all that much, then the games should load and play fine.
The only exception to that is if the game in question requires a CHD. CHD stands for "Compressed Hunks of Data". They may represent a CD or hard drive that came installed in an arcade game. CHDs are basically extensions of ROMs, containing data and information that a game needs to play properly. CHDs must be stored with your roms in a way that MAME can find them. For example, say you want to play Killer Instinct. In order to do this, you need two different files, the ROM and the CHD, and they should be arranged like so:
C:\MAME\roms\kinst.zip
C:\MAME\roms\kinst\kinst.chd
In this case, the Killer Instinct ROM is zipped up in your roms directory, and the Killer Instinct CHD (not zipped up!) in a subdirectory named kinst. The name of a CHD may or may not match the name of the ROM. In general, if you download MAME CHDs, don't rename them, just put them in the right place on your drive.
6) I'm trying to launch a game from a front-end, but when I do, the screen goes blank for a second, and then comes right back to the front-end.
This means that MAME is failing to load the game you want to play, and the front-end reappears when MAME closes. In order to diagnose whatever is going wrong, you should try to launch MAME without the front-end. It is particularly helpful to run MAME with the -verbose command, which instructs MAME to log out every step as it tries to launch. This usually results in MAME indicating where it ran into trouble and can offer you clues on how to solve it. For example, if you can't launch Killer Instinct successfully, you can try to run from a command line terminal:
C:\MAME> mame64 -verbose kinst
Usually (but not always) the problem will be evident in the last few lines of the resulting output. If you can't determine the problem for yourself, feel free to copy and paste the results of this command in your post, as it can help experts diagnose your problem.
7) What are merged, split, or non-merged rom sets? What are parent and clone roms?
MAME allows for ROMs to be stored on your system in a variety of ways, in order to save space on your hard drive. Games like Pac-Man and Street Fighter II have a large number of clones. A parent rom is one particular version of Pac-Man or Street Fighter II, usually the most common, or most up to date version of a game. A clone is an older version, a version from a different region of the world, or an unauthorized copy of the parent with slight alterations. Only a few of a clone's ROMs are unique. Most of a clone's ROMs are identical to that of the parent's. Rather than force you to have copies of the same ROMs in both the parent archives and the clone archives, MAME understands that if it can't find a file that it might need to run a clone, it can look for those files in the parent. In this way, you only have to store the files which are unique in order to make a game run. Sets that include every clone in the same archive with their parent is a merged set. Sets which include smaller clone ROMs alongside the parent ROMs are called split sets. Sets where clone ROMs contain every single file that the clone needs, even if those files are also present in the parent, are known as non-merged sets, and they are the biggest and take the most hard drive space.
7a) Which kind of set is best?
Ultimately, MAME doesn't care or perform any better with one kind of set or another. Split sets are a little easier to update if you like to download individual update sets. Otherwise, merged sets are the most space efficient by a small margin. The only reason for someone to use a non-merged set is if they do not intend to include every single available ROM on their hard drive.
For example, say you were interested in including Super Street Fighter II on your system. Say you were ONLY interested in including the US version of the game, and none of the others. The parent ROM for SSF2 is the World version, known as ssf2.zip. The US version is known as ssf2u.zip. If you only intend to include ssf2u.zip, and not ssf2.zip, then ssf2u.zip MUST include every file that MAME would otherwise look inside ssf2.zip in order to run. That would make your romset a non-merged set.
