Feel free to add coop support to NSMB yourself
I am pretty sure the work behind this project is way more than you'd expect
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Youtube - GitHub - NSMB Central

Sorry for the bump but I figured this might be helpful for shadey or TheGameratorT when/if they come back or if the source code is released and someone is ambitious.

Firstly thank you so much, I've got so much enjoyment from this with my sibling over the past three years who is now 9 and we have got every Star Coin and Secret Exit possible in this hack. Feel free to ignore this post if you do not care because I don't want to sound ungrateful. I never thought something like this would be possible and I'd love to understand the wizardry behind this.

But now to the point, I would just list the bugs that we found along the way. I've tried to make it very clear and easy to read. Note when issues were encountered, I tried DSONE EOS, Wood, YSMenu and nds-bootstrap/B4DS to fix them but generally made no difference.

W2-4 Secret Exit
* Problem - flag does not spawn and exit cannot be completed.

All cannons
* Problem - freeze when a player enters.
* Fix - changing to DSi Mode if playing in TWiLightMenu++.

W7 boss
* Problem - freeze with horizontal blue stripes scanning across screen or a red screen crash in TWiLightMenu++.
* Fix - changing to DSi Mode with TWiLightMenu++ fixed the issue.

W4-5 final star coin
* Problem - Need to break bricks with bob-ombs.
* Normal behaviour - A pipe spawning bob-ombs normally hangs over this site, but here, no bob-ombs come out.
* Steps taken to try address issue - we tried many times to carry a bob-omb from elsewhere but it would seem like we would always barely miss.
* Fix - maybe place a bob-omb closer to where the pipe leading to the third star coin is.

W4-6 final star coin
* Problem - Need to hit a red switch above the raft.
* Normal behaviour - In single player, can normally stand on Dorrie's head but since a raft is used, it cannot be reached.
* Steps taken to try address issue - we tried using a mini mushroom and wall jumping, but again would barely miss.
* Fix - maybe another platform or bringing Dorrie back.

W8-8 second/third star coin
* Problem - blocks do not break with the fruit bomb things and rocks which break blocks only fall in the first quarter of the level.
* Steps taken to try address issue - dying at the checkpoint did not help.
* Fix - change brick blocks to empty brown ones?

Final level
* Problem - second switch that turns the level upside down doesn't do anything. Therefore cannot go through upside down door and cannot finish the level.

Occasional crashes and desyncs
In some levels, death meant no return. eg. W8-8, w7-5
W6-6 has less time for some reason.

There are some other issues, but these are the major ones which impede progress.

Just kind of curious if there's any way to bring the other player back once they have no lives

I had trouble having the 3ds (modded and using ntr boot to play) and a ds lite (not modded and no r4 card) connecting becuase the 3ds would have a black error message "connection interupted", but after trying twillight menu it still wouldnt work now having a red error message on both twillight and ntr now saying "error: data abort!" Giving me the options to
1. Reset game 2. Dumb ram 3. Ram viewer 4. Quit game, what should i do other than repatching the files .

NSMB Co-op V2.0!

Xdelta Patches - Download patches for your language: https://github.com/ShaneDoyle/nsmb-coop/releases/tag/v2.3

Report Issues - Found a bug? Report it here: https://github.com/ShaneDoyle/nsmb-coop/issues

Language	Patch Filename
English	nsmb-coop_v2.3_en.xdelta
French	nsmb-coop_v2.3_fr.xdelta
German	nsmb-coop_v2.3_ge.xdelta
Italian	nsmb-coop_v2.3_it.xdelta
Japanese	nsmb-coop_v2.3_jp.xdelta
Portuguese	nsmb-coop_v2.3_pt.xdelta
Spanish	nsmb-coop_v2.3_sp.xdelta
Korean	nsmb-coop_v2.3_ko.xdelta
Chinese	nsmb-coop_v2.3_ch.xdelta

How to Apply Xdelta Patches:

What you need:
• A clean US copy of New Super Mario Bros. DS (.nds file)
• The xdelta patch file for your language
• xdelta3 patcher tool (available for Windows, Mac, and Linux)

Step-by-step instructions:
1). Download xdelta3 from https://github.com/jmacd/xdelta/releases (or use online patchers like https://www.marcrobledo.com/RomPatcher.js/)
2). Place your clean NSMB DS ROM and the downloaded .xdelta patch in the same folder
3). Open command prompt/terminal in that folder
4). Run:

xdelta3 -d -s "original_rom.nds" "patch_file.xdelta" "nsmb-coop.nds"

5). Your patched ROM will be created as "nsmb-coop.nds"

Alternative (GUI): Use Delta Patcher (Windows) or MultiPatch (Mac) for a user-friendly interface.

Coming Soon:

Singleplayer play option - Play the co-op version by yourself!

Save file syncing - Keep your progress in sync automatically!

Core Features:

Complete Co-op Adventure - Play through every single level with a friend using local wireless.

2-Player Boss Fights - All bosses have been modified for cooperative battles.

World Map Navigation - Mario controls the world map, but both players choose levels together.

Smart Respawn - When you die, watch your friend play and jump back in by pressing A (except during boss fights).

Polished Experience - Countless improvements to make everything work smoothly with 2 players!

How to play:

1). Download version 2.0 from the downloads section.
2). Choose your language version or use the patch file with your clean US copy of New Super Mario Bros. DS.
3). Put the game on 2 DS systems and select the same save file on both.
4). Connect with your friend using local wireless.
5). Have fun playing together!

Known Issues:

Mario VS Luigi Mode - The competitive mode isn't available in this co-op version.

Performance - Some levels might slow down a bit. Staying close to your partner and using DSi CPU mode helps!

Audio - You might notice a few sound effects missing here and there.

FAQ:

Will this work on my device? - If it can play .nds files, it can run the game! But you'll need real DS hardware or the melonDS emulator for wireless multiplayer.

Do I need the US version? - Yes, this mod is made specifically for the US version of New Super Mario Bros. DS.

What languages can I play in? - English, French, German, Italian, Japanese, Spanish, Portuguese, Korean and Chinese.

How do I turn on widescreen? - Press L+R+X on the title screen (only works on 3DS with nds-bootstrap, check out https://wiki.ds-homebrew.com/twilightmenu/installing-rtcom).

Credits:

Version 2.0 represents months of hard work and hundreds of improvements. It now supports multiple languages and includes tons of new features. The complete source code is available on GitHub (https://github.com/ShaneDoyle/nsmb-coop) for anyone interested in how it works or wanting to contribute.

 TheGameratorT - Lead developer, engine modifications, research, and extensive testing.

 Shadey - Co-developer, early development, gameplay systems, and continued testing and debugging.

 Arisotura - melonDS development made testing and debugging significantly easier.

Isaac0-dev - Flagpole mechanics prototyping and contributions.

gamemasterplc - Original widescreen code.

Mamma Mia Team - NSMB research and tools.

And thanks for everyone who initially supported this project, as well as the DS homebrew and NSMB modding communities!

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NSMB Hacking Discord

Hi TheGameratorT

I tried to apply the patch nsmb-coop_v2.3_en.xdelta to the USA rom of NSMB, then started in my NDS with R4 card, but i get this error: "Saved data could not be read. Turn the power off & reinsert the game card.
The size of the patched rom results 22.096.520 byte. (It works only on a NDS emulator).
Version 1.01 of your patch works in my NDS instead.
But how to play with 2.3 version?
Also: by using this site www.marcrobledo.com/RomPatcher.js/ i get this error: "not implemented: secondary compressor".

Hello Thunderstorm,

Please use xdelta3 or DeltaPatcher to patch the ROM.

Delete the save data before trying again.
What R4 card model do you have?
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NSMB Hacking Discord

Hello TheGameratorT and thanks for your reply,

- I have already used also Delta Patcher to patch the game: no results.
- Already deleted also the save files: no results.
- Where to download xdelta3?
- One most recent from Aliexpress, alother one old named R4 Ultra.

oh this looks great can i play with emulator with ethernet cable and DeSmuME
that would be great if you could reply thank you

You cannot play this hack on DeSmuME because it doesn't have multiplayer support, use MelonDS instead.
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My YouTube Channel NSMB Central Discord Server

ok thanks i actually didn't know that