File Systems : Hierarchical File System (HFS, for Mac OS) Prepared by : Mohammad Azzuri bin Zaidi UFH150010

Introduction What is file system? Organizes large numbers of files on some sort of data storage medium (most commonly a disk drive, CD-ROM or DVD). Store data organized as files as well as a collection of functions that can be performed on files. Typical operations include: Create, Delete, Open, Close, Read, Write.

Hierarchical File System HFS is a proprietary file system developed by Apple in September 1985 for use in computer systems running Mac OS. It was the primary filesystem format used on the Macintosh Plus and later models, until Mac OS 8.1, when HFS was replaced by HFS Plus. Organizing files on a Macintosh hard disk. When a hard disk is formatted for a Macintosh computer, the hierarchical file system is used to create a directory that can expand as new files and folders are added to the disk

The structure 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

The design The HFS volume is made up of five sections: Boot Blocks – Blocks 0 and 1 are used to startup an Operating System Master Directory Block (MDB) – Block 2 contains the MDB which holds various details about the volume including the location of the Bitmap information. A duplicate of the MDB is kept at the last block of the volume, which is called the Alternate MDB. Bitmap – file used to track the block usage. In the file, a ‘1’ represents that the block is in use. A ‘0’ shows the block is free. Since the Bitmap size varies on the volume size, the ending block varies. Extent Overflow – B-Tree file containing the information of which blocks belong to which file once the first three extents are used from the Catalog File. This file stores information on bad blocks as well. Catalog File – stores information on files and directories in a B-Tree file.

The directories Files are grouped into directories (also called folders), which themselves may be grouped into other directories. Each directory has a number associated with it called the directory ID. The Finder works with the File Manager to maintain the organisation of files and folders on a volume. The hierarchical relationship of folders within folders on the desktop corresponds directly to the hierarchical directory structure maintained on the volume. The volume is known as the root directory, and the folders are known as subdirectories, or simply as directories.

Revolution HFS was introduced to replace the Macintosh File System (MFS). The MFS kept a flat file storing the directory and file structure. Searching the disk was slow on larger disks. MFS was more intended for use on floppies. HFS uses 32-bit integers for addressing and can support larger disks than MFS. In one area, 32-bit integers were not used, so the maximum number of files is 65,535 (the largest number represented by 16 bits). HFS came about early on in the life of the Macintosh line when disks had relatively small capacities. Limitations appeared in this format when disk drives grew very large in size. Mac OS Extended (or HFS+ as it was originally called) is similar to HFS but some of its internal structures were changed to accommodate the changing needs of modern personal computing. The major differences between the two are outlined below. HFS is best for small volumes or those that may be used on older systems (before OS8.1). Mac OS Extended is best for larger volumes as long as they will not be used on older systems. With the introduction of OS X 10.6, Apple dropped support to format or write HFS disks and images, which are only supported as read-only volumes.

Comparison Standard HFS Compatible with all Mac OS versions File names limited to 31 characters File sizes limited to 2GB Large volumes with small files waste a lot of space. Mac OS Extended (HFS+) Compatible with Mac OS8.1 and later 255 character Unicode file names No file size limit Less wasted space

Operating system support

Resources