CYM File Format
The name stands for Capcom YM2151 because the file contains the data stream sent to the Yamaha YM2151 sound chip and Capcom's CPS-1 hardware arrangement was the most prevalent use of the YM2151 in emulation at the time. The data is logged to a file through the use of emulators running a ROM image.
Logging to the CYM format is available in CPS-1 emulators like MAME and Kawaks and was originally a simple raw YM2151 data log. Given that only raw YM2151 was logged, data from other chips on the system (such as the MSM6295 commonly used for sampled audio like drums and voices) were missing.
CYM was eventually made obsolete when VGM added support for YM2151 events and eventually the accompanying audio chips often found on systems with the YM2151.
The CYM file format contains a sequence of data writes to the YM2151, represented by one byte for the register address and one byte for the data. If the register address given is 0, however, there is no accompanying data byte and instead players should advance one frame (1/60th of a second if NTSC, 1/50th of a second if PAL).