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HFS(Hierarchical File System) The previous file system in the Mac, which was replaced with the Apple File System (see APFS). The first version of HFS, known as "Mac OS Standard," was introduced in 1985. HFS+ ("Mac OS Extended") came out in 1998 in preparation for Mac OS X, featuring Unicode support and a 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 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 APFS, file system and hierarchical file system.