Although no one has found multiplayer code in Mario’s Tennis so far, I decided to take a look for myself. So first I modified an emulator to to help me identify data and code sections. Then I spent a while playing the game to collect enough data and to identify unused code paths. Next, I wrote a disassembler that was able to disassemble the whole game by using the collected data.
After taking a first look at the unused code, I was surprised to find quite much code that looked like proper multiplayer code. So I spent some time doing an in-depth analysis of the code and eventually I was able to reintegrate it into the game. A first test revealed that the controls of one of the players were inverted. This was probably done by the developers for easier testing. Unfortunately, the proper code bits were probably removed by a compile switch, so I had to rewrite them myself.
So far, I got 2 of 3 game modes (P1 vs P2 and P1/COM vs P2/COM) working properly. The third mode (P1/P2 vs COM/COM) is buggy and I probably won’t fix it anytime soon, because debugging and testing is too time-consuming without an emulator with link cable support. But once I have the necessary tools, I will try to fix it. Till then enjoy the first version of my patch. To apply the patch you have to use the tool bspatch.

Attachments:

1. multiplayer_v1.bspatch

You’re the best. Very cool as expected. Still testing it, just got my cable.

Amazing work. Mario’s Tennis was always a good game for showing off what the VB could do, so adding (re-enabling) multiplayer makes it even better.

I’ve been playing 2 player with my daughter. Whole new game! Thanks. I’m glad we have another major title now for 2 player. Amazing work!

Wow!!! Thanks for that!

Thanks.
I uploaded a video for those that can not test it:

Thank You kind sir!

Really nice of you!

Are there any links to the patching tool?

EDIT found one.
https://www.romhacking.net/utilities/929/

Simply incredible!!! Thank you, M.K.!!!

I must agree that this is amazing. I only discovered this forum a few months ago but it has definetly revived the love I have always had for this console. I appreciate all the hard work I see that this community has/is putting in.

Thanks for this patch! Now I guess I need a second Flashboy though lol. Will be testing it when I get back home from the USA

Awesome Patch, M.K.! Still haven’t been able to try it on real hardware, but I’m so stoked for some 2P Mario Tennis action when I get everything I need.

So cool! I never thought I’ll would see a patch like this.
From the video that was posted, it looks like its working perfectly.

This looks incredible. And I love the surprise drop. Just like Beyoncé.

Thanks for sharing! I can’t wait to try this out with the link cable

Wow! Outstanding as usual. I can’t believe what kind of effort you put into this. Chapeau, Martin!

Wow,Fantastic!Thanks for this patch!

been wanting this for years, thanks!

It deserves a new label then… 😉
But perhaps two VB instead of the GameBoys…

Attachments:

1. 

   mt_link1200dpi-scaled.jpg
   

Maybe this image stylized. 😉
https://s-media-cache-ak0.pinimg.com/originals/80/2f/d2/802fd25762eed25bebeea079ea7bb081.jpg

Here’s one link-marked with a bit more VB:ish feel.

Attachments:

1. 

   mt_link1200dpi_2-scaled.jpg