nesdev.org

Making an iNES analyser, I saw that there was a bit for indicating the presence of a "trainer" : a 512 byte long area located at the beginning of a rom file.

But what exactly is a trainer, and most over all what is its utility?

It's 512 bytes of code preloaded at $7000 (in cartridge RAM) before the game starts. The code is used to emulate mappers via software, thus all ROMs using them are hacked.

Ok

Does this is used by hackers because it's hard, and expensive to find and install mapper chips on boards?

It's also used for the "Swim Cheat" version of Super Mario Bros, a hacked rom where you press Select to make mario swim.
It's basically a way to do a tiny rom expansion for games without needing to hack them, but to actually use such a thing on hardware requires the cartridge WRAM to be preloaded with the trainer. So games with a trainer are always games which don't natively have cartridge WRAM.
So the trainer is for specialized copiers or emulators only.

It's not really about "emulating a mapper", it's about hacking the bankswitching code to call WRAM code instead.

It was used by Famicom pirates in the 80s to sell games that otherwise wouldn't work on their hardware.

Dwedit wrote:It's not really about "emulating a mapper", it's about hacking the bankswitching code to call WRAM code instead.

Well that WRAM code processes the input into discrete writes for the copier hardware and will dump kilobytes of PRG ROM out $2007 to run CHR bankswitched games on CHR-RAM.

Most games with WRAM can still have trainers, of course it's more work to move things, but it happens.

Ok, thank for all of you

kyuusaku wrote: Most games with WRAM can still have trainers, of course it's more work to move things, but it happens.

Also it is important to note that games with WRAM don't always have 8KB of it, some just have 2KB. I think Gradius 2 is one of them and I have seen versions of it floating around with a trainer for a MMC3 hack that makes it run although not very well.