MOD is a computer file format used primarily to represent music, and was the first module file format. MOD files use the “.MOD” file extension, except on the Amiga which doesn't rely on filename extensions; instead, it reads a file's header to determine filetype. A MOD file contains a set of instruments in the form of samples, a number of patterns indicating how and when the samples are to be played, and a list of what patterns to play in what order. The original version of the MOD format featured four channels of simultaneous audio playback, corresponding to the capabilities of the original Amiga chipset, and up to 15 instruments. Later variations of the format have extended this to up to 32 channels and 31 instruments. The format was designed to be directly playable on the Amiga without additional processing: for example, samples are stored in 8-bit PCM format ready to be played on the Amiga DACs, and pattern data is not packed. Playback required very little CPU time on an Amiga, and many games used MOD files for their background music.