Resource fork

The resource fork is a fork or section of a file on Apple's classic Mac OS operating system, which was also carried over to the modern macOS for compatibility, used to store structured data along with the unstructured data stored within the data fork.

A resource fork stores information in a specific form, containing details such as icon bitmaps, the shapes of windows, definitions of menus and their contents, and application code (machine code). For example, a word processing file might store its text in the data fork, while storing any embedded images in the same file's resource fork. The resource fork is used mostly by executables, but every file is able to have a resource fork.

Originally conceived and implemented by programmer Bruce Horn, the resource fork was used for three purposes with Macintosh file system. First, it was used to store all graphical data on disk until it was needed, then retrieved, drawn on the screen, and thrown away. This software variant of virtual memory helped Apple to reduce memory requirements from 1 MB in the Apple Lisa to 128 KB in Macintosh. Second, because all the pictures and text were stored separately in a resource fork, it could be used to allow a non-programmer to translate an application for a foreign market, a process called internationalization and localization. And finally, it could be used to distribute nearly all of the components of an application in a single file, reducing clutter and simplifying application installation and removal.

The resource fork is implemented in all of the file systems used for system drives on the Macintosh (MFS, HFS and HFS Plus). The presence of a resource fork makes it easy to store a variety of additional information, such as allowing the system to display the correct icon for a file and open it without the need for a file extension in the file name. While access to the data fork works like file access on any other operating system — pick a file, pick a byte offset, read some data — access to the resource fork works more like extracting structured records from a database. (Microsoft Windows also has a concept of "resources", but these are completely unrelated from resources in Mac OS.)

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