The base storage vicinity for hard drives is a sector, with each sector storing upon 512 bytes of data. If an operating system raw a file smaller than 512 bytes in a sector, the rest of the sector goes to waste. One accepts this waste because most files are far larger than 512 bytes. The operating system needs a method to fill one sector, find another that’s unused, and fill it, continuing to fill sectors until the file is completely stored. Once the operating system stores a file, it must remember which sectors hold that file, so that file can be retrieved later. MS-DOS version 2.1 first supported hard drives using a special data structure to keep track of layup data on the tough drive, and Microsoft called this structure the FAT. One can think of the FAT as nothing more than a card catalogue the maintain track of which sectors store the various parts of a file. It’s nice to call a FAT a data structure (the official jargon description), but it is more like a two-column spreadsheet.