es_systems.cfg

   <system>
    <name>megadrive</name>
    <fullname>Sega Genesis</fullname>
    <path>/home/pi/RetroPie/roms/megadrive</path>
    <extension>.smd .bin .gen .md .sg .zip .SMD .BIN .GEN .MD .SG .ZIP</extension>
    <command>/opt/retropie/supplementary/runcommand/runcommand.sh 0 _SYS_ megadrive %ROM%</command>
    <platform>genesis</platform>
    <theme>genesis</theme>
</system>