Macintosh File System - Overview

Macintosh File System

Macintosh File System (MFS) is a volume format (or disk file system) created by Apple Computer for storing files on 400K floppy disks. MFS was introduced with the Macintosh 128K in January 1984.
MFS was notable both for introducing resource forks to allow storage of structured data, and for storing metadata needed to support the graphical user interface of Mac OS. MFS allows file names to be up to 255 characters in length, although Finder does not allow users to create names longer than 63 characters (31 characters in later versions). MFS is called a flat file system because it does not support a hierarchy of directories.

Folders existed as a concept on the original MFS-based Macintosh, but worked completely differently from the way they do on modern systems. They were visible in Finder windows, but not in the open and save dialog boxes. There was always one empty folder on the volume, and if it was altered in any way (such as by adding or renaming files), it would be replaced with a new Empty Folder. MFS stored all of the file and directory listing information in a single file. The Finder created the "illusion" of folders, by storing all files as a directory handle/file handle pair. To display the contents of a particular folder, MFS would scan the directory for all files in that handle. There was no need to find a separate file containing the directory listing.dd.

The Macintosh File System did not support volumes over 20 megabytes in size, or about 1,400 files. While this is small by today's standards, it seemed very expansive when all Apple Macintosh computers at the time had a 400 kilobyte floppy drive.
Apple introduced Hierarchical File System as a replacement for MFS in September 1985. In Mac OS 7.6.1, Apple removed support for writing to MFS volumes, and in Mac OS 8 support for MFS volumes was removed altogether. Although Mac OS X has no built-in support for MFS, an example VFS plug-in from Apple called MFS Lives provides read-only access to MFS volumes.