I'm looking for a Carnivore2 or Megaflashroom

I should also mention: you can configure C2 to bypass the menu by default and auto-start a configuration you choose, which makes it similar to MFR experience-wise. You use c2man instead of opfxsd to switch the flashed ROM (s) in this case. It's not the same experience but it's close. I had one machine where failing keyboard membrane made it impossible to access some of MFR config menu options and C2 used this way is a fine substitute.

Conversely with MFR you can get a quite usable menu experience, you can flash a whole list of ROMs and then easily pick your preferred one at boot time. It's not as flexible/powerful as the C2 menu but still quite handy

thanks bsittler for your experience, I was just looking for experience of someone who has used both! As already written, I like the Carnivore2 for the greater hardware features (Ram + FM Pack) to be used in my MSX2 Philips NMS8280, but I was worried about what other users wrote that the C2 has problems with some ROMs, games and software, so I don't do you find all this ?? thank you

ok, I did not know that the C2 does not function as the MFR which simulates the original ROM card .. How does the C2 work?

Once you choose a configuration from the menu, or if you use auto-start with a default configuration, C2 does indeed function like the original ROM cartridge, more or less the same as MFR does. Biggest difference is you can have multiple different ROM images and switch to a different one the next time without reflashing. In a way it's like the opfxsd option to build a multirom collection with a menu, just the MSX-DOS tools used to prepare it differ slightly and the C2 has additional features relating to audio, slot/sub-slot assignments, and autostart. It also lets the Nextor/MSX-DOS booting exist as a menu item, whereas with MFR you have to choose in advance with a key combo either to bypass flashed ROM and use Nextor/MSX-DOS or to use the flashed ROM/ROM collection+menu

Also neither C2 nor MFR are a completely accurate simulator for just "custom megarom", instead they both have additional features and complexity that greatly improve the compatibility and ease of use, and for example simulate or include some audio hardware otherwise found in specific cartridges (FM and SCC in C2, SCC in MFR) and supply flexible RAM.

If you need actual custom megarom with nothing else, but with less flexibility and a bit trickier to use, you might look at http://www.msxcalamar.com/catalog/index.php?id_product=33&controller=product which is an implementation of https://github.com/RBSC/MultiMapperCartridge and can pretend to be one of several common mapper configurations but has no Nextor, no SD slot, no RAM, and no audio chip or simulation thereof. To change the contents you need to power off the machine, remove and open the cartridge, reconfigure the jumpers, then run a special program to write new contents (so you'll want MSX-DOS another way for this, eg MFR or C2 in a separate slot), them repeat the process (power down, remove shell, reconfigure jumpers to match the mapper of the software you flashed, then reassemble) and you have a playable cartridge quite similar to the original hardware though without SCC audio. Multi mapper is also a straightforward solution when you need to run the s software from a sub-slot of an expanded slot and can't use a multi-function cartridge like C2 or MFR that itself expands a primary slot

However for ease of use and "one stop satisfaction" I think C2, MFR, or any OCM-based MSX is a far easier to use solution. Multi mapper is great for testing under-development homebrew if you're contemplating physical cartridge production later and just want to ensure the software works correctly "stand-alone"

Edit: edited to fix some typos, re-add accidentally deleted first paragraph, and fix a broken link. Apologies, was typing on the phone before and apparently did an awful job of it

thank you all for the very helpful answers! before choosing i would like to understand better the management of files and roms in both cartridges. thank you