Editing Apple ProDOS (section)

==File system==
ProDOS uses the same [[file system]] as the earlier [[Apple SOS]] for the [[Apple III]].  The SOS/ProDOS file system is native to [[Apple SOS]], ProDOS 8, ProDOS 16, and [[Apple GS/OS|GS/OS]].  Some [[classic Mac OS]] versions also come with a [[Apple File Exchange|file system translator]] to handle this file system.

A volume is allocated in 512-byte blocks. (5.25" floppy disks are still formatted using 256-byte sectors, as this is the format required by the controller ROM to boot the disk. ProDOS simply treats pairs of 256-byte sectors as a single block on such drives.) A volume can have a capacity of up to 32 megabytes,  and each file can be up to 16 megabytes. Each volume (floppy disk or hard drive partition) has a "volume name", a filename which is used as the base [[directory (computing)|directory]] name; having two volumes with the same volume name can result in conflicts. If necessary, ProDOS searches all available drives to find a named volume. [[Directory (computing)|Subdirectories]] are supported, and the concept of a "prefix" (working directory or current path) was provided to make working with subdirectories easier.

File, directory, and volume names can be 1 to 15 characters, starting with a letter, then containing more letters, digits or periods. Each file entry also contains the 16-bit (2-byte) pointer to the block containing the beginning of the file (or its block index); a 16-bit block count; a 24-bit (3-byte) file size; an 8-bit (1-byte) [[file format|filetype]]; a 16-bit auxiliary type (the meaning of which depends upon the filetype); creation and modification [[timestamp]]s; and data related to how the file is stored on the volume. [[Sparse file]]s are supported, but files are never "sparsified" by removing zero-filled blocks. The volume header contains similar information as relevant to volumes.

Directories (including the [[root directory]]) are sequentially indexed, with each block starting with the address of the previous block (or zero if none) and the subsequent block (or zero if none). The root directory on most disks is initialized to 4 blocks, allowing 51 entries (excluding the volume header). It never changes in size, except by manual intervention with special tools. Subdirectories begin at one block, and grow automatically as needed.

Normal files are progressively indexed. Single-block files (under 513 bytes) have no index block; the directory entry points directly to the block of file data. Files with between 2 and 256 blocks (513 bytes to 128 KB) of data have a single index block, to which the directory entry points, which contains a list of up to 256 data block addresses. Larger files have a master index block containing a list of up to 256 index block addresses. When the Apple IIgs was introduced, a new storage format was introduced for files with two forks, as was typical for IIgs system and program files; the directory entry points to an informational block that tells the computer the storage format of the two forks. These files cannot be read or written natively by ProDOS 8, though the volume itself remains compatible.

The volume has a bitmap of used blocks. Other than this, there is no central [[File Allocation Table|file allocation table]].

A ProDOS 8 volume formatted by Apple's tools has a [[boot sector]] that supports booting both ProDOS and SOS depending on what computer it is booted on. Block 0 is the Apple II boot block and block 1 boots SOS. This allows a disk to be used to boot on either Apple II or Apple III computers by putting both operating system kernels in the top directory: the Apple II boot sector looks for the file {{mono|PRODOS}} and the Apple III boot sector looks for the file {{mono|SOS.KERNEL}}.<ref>{{cite web | url = http://apple2history.org/history/ah15.html#SOS | work = Apple II History | title = Chapter 15: DOS 3.3, ProDOS & Beyond | date = 2001-07-07 | first = Steven | last = Weyhrich | access-date = 2003-06-27 | archive-url = https://web.archive.org/web/20080917065143/http://apple2history.org/history/ah15.html#SOS | archive-date = 2008-09-17 | url-status = dead }}  Weyhrich source: {{cite journal | title = Road Maps To Apple II Disks: DOS 3.3, CP/M, Pascal, and ProDOS | first = Skillman | last = Hunter | journal = [[Call-A.P.P.L.E.]] |date=February 1985 | pages = 10–21 | location = [[Renton, Washington]] | publisher = [[Apple Pugetsound Program Library Exchange]] | issn = 8755-4909 | oclc = 8898855 }}</ref> Third-party formatting utilities often did not provide the SOS boot block, and some would even mark block 1 available for user data.

ProDOS has no kernel support for other file systems.  If necessary, a conversion utility on the main system disk is used to transfer files individually between ProDOS and older [[Apple DOS]] 3.3 disks. Because they use a different low-level disk format than DOS 3.3 and ProDOS, transferring data from DOS 3.2 disks to ProDOS is a two-step process using a DOS 3.3 disk as an intermediary (utilizing the DOS 3.3 utility MUFFIN or similar).