Since the Volume Bitmap must have a bit to represent each allocation block, its size is determined by the size of the volume itself. Each allocation block on the volume is represented by a bit in the map, if the bit is set then the block is in use, if it is clear then the block is free to be used. Logical block 3 is the starting block of the Volume Bitmap, which keeps track of which allocation blocks are in use and which are free.This is intended mainly for use by disk utilities and is only updated when either the Catalog File or Extents Overflow File grow in size. There is also a duplicate of the MDB called the Alternate Master Directory Block (aka Alternate MDB) located at the opposite end of the volume in the second to last logical block. This defines a wide variety of data about the volume itself, for example date & time stamps for when the volume was created, the location of the other volume structures such as the Volume Bitmap or the size of logical structures such as allocation blocks. Logical block 2 contains the Master Directory Block (aka MDB).
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For example, the names of the System and Shell (usually the Finder) files which are loaded at startup.
What is hfs file system mac os x#
HFS is still supported by current versions of Mac OS, but starting with Mac OS X an HFS volume cannot be used for booting.
What is hfs file system plus#
In 1998, Apple introduced HFS Plus to address inefficient allocation of disk space in HFS and to add other improvements.
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While HFS like most other file systems may be seen as a proprietary format, because it was so well documented there are usually solutions available to access HFS formatted disks from most modern operating systems. For example, HFS permits filenames up to 31 characters in length, supports metadata and dual forked (separate data and resource forks per file) files. Because Macintosh computers use richer data than other commonly available file systems such as FAT used by DOS or the original Unix file system would allow, Apple developed a new more appropriate file system, rather than adopting an existing specification. It superseded the Macintosh File System (MFS) which was a flat file system, used only on the earliest Mac models. HFS was introduced in September 1985 as a new file system for Macintosh computers.