Rom files vs Cartridge game data

depends, the rom data on most cartridges were not protected for snes that is, bet there were cartridges with DRM wich needed patching.
then there are special chip cartridges, the rom might be dumped 1:1 but that doesn't mean it's gonna be able to play it by just booting this rom.
i'm sure for other systems, but 1:1 dumping is most likely not possible on newer systems then snes for the DRM would trigger.

 

I'm not quite sure where you read that, but it's basically wrong. ROM dumps from a cart are taken 1:1, including on the 3DS. Dumpers might go and scrub 0s and garbage data from a ROM to make the file size smaller (since nobody wants to download a 4GB game where only 2GB of data is used or whatever), but it's not missing any of the actual game data. You can still get "full" dumps that include the garbage and 0s, but there's no functional difference between a ROM that's also been scrubbed.

 

1:1 can mean different things to different people in different scenarios, and that might change with time.

Though examples are probably good.
I suppose we can skip going in depth about interleaving. We did however see some variations on this theme with those nice gigaleaks wherein Nintendo's ROM reference collection was different to what those in the rest of the world know.

Bad dumps. Dumping with some older setups was not a precise art that it might be today for some. To that end if a connection came loose in a nice home made dumper then a small run of data might have been different. When the SNES collection was redumped the other year a few were noted in this (a spiderman game being among those if memory serves), and it happened at various points throughout the lifetime of other newer devices as well (the DS had quite a few). Groups like no-intro seek to make as accurate collections as they can here.

I also suppose we can skip any user added data -- a ROM file might just be that, however an emulator/flash cart might really appreciate being told more data about the makeup of the original cart. There is a subtle variation of this on the DS. Here for ROMs prior to the DSi launch then there was a blank space in the header. After the DSi became a thing then this area was used to house some data on the ROM so that Nintendo could authenticate it in the DSi (and later 3ds). Earlier dumpers however did not care about this and continued to leave that blank. This is also why the Scene release list of DS games has a bunch of redumps late in the day, however the extra info if of no use to any flash cart user, ROM hacker, emulator author, fancy FPGA system recreator (unless they want to go 1:1 and include protections that nobody cares about), emulator player or game historian will likely ever care about. So not 1:1 and justifiably redumped in any Scene rule set ( https://scenerules.org/ ) or preservationist effort but nobody actually really cares, and indeed it is probably more annoying for people as different "versions" might make patching things harder in the case of ROM hacks.

The DSi also has a further twist in that it had a special DSi version of some games that had extra features for being on a DSi. However if presented to a DS (or something pretending to be one) it would present a cut down DS version of it. Dump that and you might well be able to play the DS version exactly as it was but it would not be 1:1 for the purposes of preservation, and those looking to use the DSi extras. Note other than nicer wireless there is no real benefit to being on the DSi for anything -- the DSi was a failure.

Encryption is a thing.
If you can't read the proper data then things can be protected from filthy pirates (usually for about 5 minutes but different discussion there), hackers and whatnot. Sometimes dumpers will skip encryption to make life easier for emulators/flash carts. This would fail to be 1:1 for some purposes but most won't care.

Padding is a thing.
Most discs and ROM chips have a fixed size. Games have no requirement to be that size however.
This extra data is data that is literally there on a cart/optical disc/whatever. Some will trim this, and if the data is not easily trimmed then might be trimmed further, or indeed even shrank down such that you need a completely hacked system to make use of it.
The Wii "scrubbing" methods featuring such a thing where any padding between files early on the disc was removed and files put end to end, different to baseline where just the padding was replaced with 00 and thus the iso would compress down nicely where before the random data made them hard to meaningfully compress. The Wii also featuring further things where isos were packed into all manner of weird and wonderful formats ( https://wit.wiimm.de/ ) for use with different loaders, and sometimes distribution. Today we have tools that can recreate lost data but for a hot moment...

The real fun.
Hacker and user added methods and dumping "failures".
Hacker and user added methods.
Three main forms.
1) To work on a given flash cart/emulator setup. Golden Sun Team on the DS probably being the most infamous here but there are others -- the 360 for instance often had data it needed to fake anti piracy checks buried in parts of the game not used and the (hacked) firmware on the DVD drive could use. Extras, encryption, anti piracy protection removal from code itself (might be a perk but not 1:1) all being things seen and sometimes necessary to do.

2) ROM ripping. Yes annoyingly it is the same term some quite rightly use as a synonym for dumping. Classically though a ROM rip saw parts of the game removed or decreased in quality to reduce space used (less storage, less bandwidth, more games on cart/disc/drive...).
Not so commonly seen nowadays but was seen on the DS in a few forms. Most were user derived affairs (I have a nice list of ones I made many years ago https://ezflash.sosuke.com/viewtopic.php?t=457 ) but there were a few Scene groups that compressed the binary in the ROM (so not 1:1 and arguably even made things slower to load but smaller on the cart).
For the original xbox this was basically standard as well, mostly as single layer discs were cheap. Caused a lot of them to have to be redumped when the xbox 360 rolled around and did want those 1:1 or essentially so dumps to be played on that.

3) While bad dumping was covered earlier there is a subtle variation. For this we probably want to go to the original xbox. In it there was a 8 gig hard drive, and by the time the OS and whatever else was taken care of you were looking at 2 or 3 gigs available to play with. Dual layer DVDs were a thing by this point, indeed many games used them. If you ripped the files by alphabetical order you might find yourself changing the file order on the disc. Burn this back to a disc and those files carefully picked to be next to each other on the disc and take a fraction of a second less to get to now need a longer read and your game lags (assuming you were not running it from the hard drive which was faster than DVD anyway).

Trainers are also a thing. Here a group might patch a game to have a nice menu to make some cheats be in the game.


So yeah a brief overview of a few things. Chances are your 8-16 bit era games are good though, and most of the times any errors that made it through are a few pixels or something messed up for a random NPC nobody cares about -- errors in code tend to make things crash, and many times people have dumped their own versions over the years anyway.
Whether they are the same... well there are different versions to consider -- https://www.zeldadungeon.net/wiki/Ocarina_of_Time_Versions as well as regions (if you were used to playing PAL games then playing NTSC might be a rather different experience, and vice versa, and it is not always PAL being inferior slowdown and borders but maybe less censorship or bug fixes).

 

Pay no attention to these idiots! Upon ripping a cartridge to a rom, pixel quality is diminished, easter eggs are removed and loading times (which are in fact stored on the cartridge with fluxx technology) are increased. I mean... When playing the cartridge, Mario is like a strong, muscular hero and the princess is a hot peach. Put that same game on a rom and it's like you're playing a censored version.
Also : pc games' partition systems mean their info is stored in different clusters of 0's and 1's, so make sure to play old games in FAT32 or lower or you're missing pretty much everything.


... Just kidding. The previous posters are right. But I couldn't resist.

 

If I write on paper "1" and ask you to do the same, it will be almost impossible for us to have drawn the exact same shape, but it's likely many people will agree they are both valid ways of representing "1"

The 3 systems you mentioned do not have any real enforcement of "original licensed games" (the security chip on NES/SNES is easily made inoffensive or bypassed by connecting the chip from an original game, the megadrive has TMSS which both now and even then was figured out, and some models of those consoles are even "factory unlocked"), so not only 1:1 backups can be programmed to repros or flashcards*, but so can anything that is a valid program for that console, even if it's not an exact copy of an original game!

* that's a little more complicated because, especially with the NES, an original game card is not just (2) roms but also a huge variety of supporting electronics, indeed some of these "modded games" that were discussed are "mapper hacks" to make them work on a different PCB

Tell that to people making multicarts

 

People do play demos, crippled versions, ripped versions and with bad emulators (plus all the hacks and cheats but that is a different matter).

Anyway again true 1:1 means every bit is the same on the iso as it would be on the cart as it went to the factory to be made or pressed on a disc.
Not everybody needs that level, indeed very few do. However if you can do it then it makes sense to do so.
Data the game does not use might be trimmed to save space (while today we have nice multi terabyte hard drives and SD cards not so far behind it was not always so -- my first GBA flash cart was 32 megabytes in size and there are single GBA games that size, trim a bit off or compress so I can fit 3 games maybe on that and that represents something useful).
Not everything in a game is used either -- https://tcrf.net/The_Cutting_Room_Floor makes a habit of finding things the developers left behind when making a game.
Some would go further and remove parts of the game -- if I never play multiplayer and it takes up a lot of space I might remove that from a game if I can (hard on some systems, trivial on others, and naturally varies by game) and free up space and maybe improve load times.
As it would appear on the disc or cart might also include a bunch of encryption that is hard to work around. If you can render the game in plain text such that an emulator or hacked console can actually make use of it where fully encrypted is harder (or even impossible*) then you might decrypt it even if it takes you away from the "as it was coming out of the factory" ideal.
Same if the game includes anti piracy. Might be worth removing those checks.

*the reason you can't burn a disc that works on a stock console for many things is because every maker of dvd burner agreed to not allow certain areas of discs to be burned.


Generally you can always go from "as it was in the factory" to anything else (some encryption is harder) and "as it was in the factory" is also more future proof in case a new system comes out that will not work with some of the modifications. "as it was in the factory" aka 1:1 is also a fixed standard that everybody can start from and go to do whatever they like, whatever that might be where someone having some random sliced up version might not have the data for my patch (to say nothing of it being harder to patch in general).
This is why some really push for "1:1" ROMs to be the version of things shared.