Specifications
The GYM file format contains only four different instructions, each represented by one byte with 0 to 2 bytes of arguments:
0x00 Do nothing for 1/60th of a second (NOP) 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 on PSG port the data DRead more about this topic: GYM