GYM (Genesis YM2612) is an obsolete audio file format for storage and playback of audio data captured from Sega Genesis (Mega Drive) games. The format was introduced by the Genecyst emulator and initially consisted of a simple raw data log. The initial format was also designed around the NTSC framerate and was thus not suitable for capturing audio from PAL systems.
Later emulators and players apparently introduced new features such as headers, frequency compatibility and compression, but no information appears to be available on these variant formats. The CYM format used by the Callus arcade emulator is similar in structure, but logs data from a Yamaha YM2151 chip.
The GYM format contains only four different instructions:
|0x00||No Operation (e.g. do nothing for 1/60th of a second)|
|0x01 R D||Write data D on YM port 0, register R|
|0x02 R D||Write data D on YM port 1, register R|
|0x03 D||Write data D on PSG port|