(Hierarchical File System) The file system used in the Macintosh. The first version, known as "Mac OS Standard," was introduced in 1985. HFS+, an enhanced version, came out in 1998 in preparation for the upcoming Mac OS X operating system. Known as "Mac OS Extended," HFS+ supports Unicode and dramatically increased file size from 2GB to 16TB. In 2003, journaling was added (see journaling file system), and case-sensitive file names were introduced under the HFSX option. Case-sensitive file names are a standard feature of Unix, and Mac OS X is based on Unix (see Mac OS X).

Data and Resource Forks
The design of HFS departed from other file systems of that day with support for two types of structures: the "data fork" and "resource fork." The data fork is like other file system structures. Data are accessed by an offset into the file; for example: OPEN FILE and READ FROM BYTE 13,904.

The resource fork functions like a mini-database, holding executable code and program structures such as icons, menus and sounds. Instead of storing an executable program as a monolithic block, having the program's resources in separate structures allows them to be edited independently and more easily localized into different languages. In addition, data files can use the two forks with the resource fork acting as a sub-file system. For example, a word processor would naturally have its text in the data fork, but could store images in the resource fork. See file system and hierarchical file system.

See HFS.