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Important: The information in this document is obsolete and should not be used for new development.

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Inside Macintosh: Files /
Chapter 2 - File Manager

Data Organization on Volumes

This section describes how data is organized on HFS volumes. In general, an application that simply manipulates data stored in files does not need to know how that data is organized on a volume or on the physical storage medium containing that volume. The organization described in this section is maintained by the File Manager for its own uses. Some specialized applications and file-system utilities, however, do need to know exactly how file data is stored on a disk.

This section is provided primarily for informational purposes. The organization of data on volumes is subject to change. Before you use this information to read or modify the data stored on a volume, be sure to check that the drSigWord field in the master directory block (described in "Master Directory Blocks" beginning on page 2-59) identifies that volume as an HFS volume.
Much of the information describing the files and directories on an HFS volume is read into memory when the volume is mounted. (For example, most of the volume's master directory block is read into memory as a volume control block.) For a description of how that data is organized in memory, see "Data Organization in Memory" beginning on page 2-76.

The File Manager uses a number of interrelated structures to manage the organization of data on disk and in memory. For this reason, it is easy to lose sight of the simple and elegant scheme that underlies these structures. As you read through this section and the next, you should keep these points in mind:

Disk and Volume Organization
Boot Blocks
Master Directory Blocks
Volume Bitmaps
Catalog Files
Extents Overflow Files

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© Apple Computer, Inc.
2 JUL 1996