The Gens emulator is one of the best and most widespread emulators of the Sega Genesis console for Microsoft Windows, and it is open source (the latest version, as well as the source code is available from the project’s download page).
One pet peeve I’ve had with Gens is that it uses the Tab key to reset the console. This makes it impossible to assign Tab to any button in a game. Because for some particular games I actually wanted to use Tab as a function, this bothered me. As the Gens source code was generously made available, it was a minor task to recompile it and change the key assignment. So I changed the Reset key combination to Ctrl+R.
If, like me, you also prefer to have the Tab key available for use, you can get my modified version here. The archive contains just the executable and is based on the latest source code (version 2.14). No other changes have been made. Just replace the gens.exe you have with this one (make a backup of the old one first), and you should be good to go.
Cloaked Thargoid 
THANK YOU SO MUCH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I accidentally pressed tab while playing shinobi, and lost my game!! TWICE, yesterday and today. i tried to change the reset button on the emulator and could not , i tried to look on the internet, but didn’t find anything, so i find that page, downloaded it and it’s working! thank you so much!! it made the difference for me, now I’m going. I will have fun playing!! thank you!!