Put simply, a file system turns a vast homogenous array of bits in a storage medium into a highly organized hierarchical structure more akin to the human mode of categorically oriented thinking. A file system also must be fast and efficient, making good use of the underlying technology to provide rapid responses to user requests to retrieve and store data. Each file system occupies a storage device such as a floppy disk or CD-ROM drive, or an area on a hard drive called a volume. These volumes are labeled with drive names such as A:, B:, C:, D:, and so on. File systems have evolved to offer many more advanced features, such as long filenames, file security via permissions and ownership, network volume mapping, virtual files, and file/directory compression. This evolution has led to a number of file-system types. Windows NT supports the largest set of file-system types, providing support for both the latest innovative file systems and the older, simpler types. Windows 98 supports the next-largest set, with Windows 95 and Windows 3.11 bringing up the rear. Things became more confusing as support for some file-system types was dropped in later versions of the operating system (for example, HPFS was dropped in NT 4.0). The following sections clarify the various file-system types and operating-system versions that support them.