linux/Documentation/filesystems/affs.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3=============================
   4Overview of Amiga Filesystems
   5=============================
   6
   7Not all varieties of the Amiga filesystems are supported for reading and
   8writing. The Amiga currently knows six different filesystems:
   9
  10==============  ===============================================================
  11DOS\0           The old or original filesystem, not really suited for
  12                hard disks and normally not used on them, either.
  13                Supported read/write.
  14
  15DOS\1           The original Fast File System. Supported read/write.
  16
  17DOS\2           The old "international" filesystem. International means that
  18                a bug has been fixed so that accented ("international") letters
  19                in file names are case-insensitive, as they ought to be.
  20                Supported read/write.
  21
  22DOS\3           The "international" Fast File System.  Supported read/write.
  23
  24DOS\4           The original filesystem with directory cache. The directory
  25                cache speeds up directory accesses on floppies considerably,
  26                but slows down file creation/deletion. Doesn't make much
  27                sense on hard disks. Supported read only.
  28
  29DOS\5           The Fast File System with directory cache. Supported read only.
  30==============  ===============================================================
  31
  32All of the above filesystems allow block sizes from 512 to 32K bytes.
  33Supported block sizes are: 512, 1024, 2048 and 4096 bytes. Larger blocks
  34speed up almost everything at the expense of wasted disk space. The speed
  35gain above 4K seems not really worth the price, so you don't lose too
  36much here, either.
  37
  38The muFS (multi user File System) equivalents of the above file systems
  39are supported, too.
  40
  41Mount options for the AFFS
  42==========================
  43
  44protect
  45                If this option is set, the protection bits cannot be altered.
  46
  47setuid[=uid]
  48                This sets the owner of all files and directories in the file
  49                system to uid or the uid of the current user, respectively.
  50
  51setgid[=gid]
  52                Same as above, but for gid.
  53
  54mode=mode
  55                Sets the mode flags to the given (octal) value, regardless
  56                of the original permissions. Directories will get an x
  57                permission if the corresponding r bit is set.
  58                This is useful since most of the plain AmigaOS files
  59                will map to 600.
  60
  61nofilenametruncate
  62                The file system will return an error when filename exceeds
  63                standard maximum filename length (30 characters).
  64
  65reserved=num
  66                Sets the number of reserved blocks at the start of the
  67                partition to num. You should never need this option.
  68                Default is 2.
  69
  70root=block
  71                Sets the block number of the root block. This should never
  72                be necessary.
  73
  74bs=blksize
  75                Sets the blocksize to blksize. Valid block sizes are 512,
  76                1024, 2048 and 4096. Like the root option, this should
  77                never be necessary, as the affs can figure it out itself.
  78
  79quiet
  80                The file system will not return an error for disallowed
  81                mode changes.
  82
  83verbose
  84                The volume name, file system type and block size will
  85                be written to the syslog when the filesystem is mounted.
  86
  87mufs
  88                The filesystem is really a muFS, also it doesn't
  89                identify itself as one. This option is necessary if
  90                the filesystem wasn't formatted as muFS, but is used
  91                as one.
  92
  93prefix=path
  94                Path will be prefixed to every absolute path name of
  95                symbolic links on an AFFS partition. Default = "/".
  96                (See below.)
  97
  98volume=name
  99                When symbolic links with an absolute path are created
 100                on an AFFS partition, name will be prepended as the
 101                volume name. Default = "" (empty string).
 102                (See below.)
 103
 104Handling of the Users/Groups and protection flags
 105=================================================
 106
 107Amiga -> Linux:
 108
 109The Amiga protection flags RWEDRWEDHSPARWED are handled as follows:
 110
 111  - R maps to r for user, group and others. On directories, R implies x.
 112
 113  - W maps to w.
 114
 115  - E maps to x.
 116
 117  - D is ignored.
 118
 119  - H, S and P are always retained and ignored under Linux.
 120
 121  - A is cleared when a file is written to.
 122
 123User id and group id will be used unless set[gu]id are given as mount
 124options. Since most of the Amiga file systems are single user systems
 125they will be owned by root. The root directory (the mount point) of the
 126Amiga filesystem will be owned by the user who actually mounts the
 127filesystem (the root directory doesn't have uid/gid fields).
 128
 129Linux -> Amiga:
 130
 131The Linux rwxrwxrwx file mode is handled as follows:
 132
 133  - r permission will allow R for user, group and others.
 134
 135  - w permission will allow W for user, group and others.
 136
 137  - x permission of the user will allow E for plain files.
 138
 139  - D will be allowed for user, group and others.
 140
 141  - All other flags (suid, sgid, ...) are ignored and will
 142    not be retained.
 143
 144Newly created files and directories will get the user and group ID
 145of the current user and a mode according to the umask.
 146
 147Symbolic links
 148==============
 149
 150Although the Amiga and Linux file systems resemble each other, there
 151are some, not always subtle, differences. One of them becomes apparent
 152with symbolic links. While Linux has a file system with exactly one
 153root directory, the Amiga has a separate root directory for each
 154file system (for example, partition, floppy disk, ...). With the Amiga,
 155these entities are called "volumes". They have symbolic names which
 156can be used to access them. Thus, symbolic links can point to a
 157different volume. AFFS turns the volume name into a directory name
 158and prepends the prefix path (see prefix option) to it.
 159
 160Example:
 161You mount all your Amiga partitions under /amiga/<volume> (where
 162<volume> is the name of the volume), and you give the option
 163"prefix=/amiga/" when mounting all your AFFS partitions. (They
 164might be "User", "WB" and "Graphics", the mount points /amiga/User,
 165/amiga/WB and /amiga/Graphics). A symbolic link referring to
 166"User:sc/include/dos/dos.h" will be followed to
 167"/amiga/User/sc/include/dos/dos.h".
 168
 169Examples
 170========
 171
 172Command line::
 173
 174    mount  Archive/Amiga/Workbench3.1.adf /mnt -t affs -o loop,verbose
 175    mount  /dev/sda3 /Amiga -t affs
 176
 177/etc/fstab entry::
 178
 179    /dev/sdb5   /amiga/Workbench    affs    noauto,user,exec,verbose 0 0
 180
 181IMPORTANT NOTE
 182==============
 183
 184If you boot Windows 95 (don't know about 3.x, 98 and NT) while you
 185have an Amiga harddisk connected to your PC, it will overwrite
 186the bytes 0x00dc..0x00df of block 0 with garbage, thus invalidating
 187the Rigid Disk Block. Sheer luck has it that this is an unused
 188area of the RDB, so only the checksum doesn't match anymore.
 189Linux will ignore this garbage and recognize the RDB anyway, but
 190before you connect that drive to your Amiga again, you must
 191restore or repair your RDB. So please do make a backup copy of it
 192before booting Windows!
 193
 194If the damage is already done, the following should fix the RDB
 195(where <disk> is the device name).
 196
 197DO AT YOUR OWN RISK::
 198
 199  dd if=/dev/<disk> of=rdb.tmp count=1
 200  cp rdb.tmp rdb.fixed
 201  dd if=/dev/zero of=rdb.fixed bs=1 seek=220 count=4
 202  dd if=rdb.fixed of=/dev/<disk>
 203
 204Bugs, Restrictions, Caveats
 205===========================
 206
 207Quite a few things may not work as advertised. Not everything is
 208tested, though several hundred MB have been read and written using
 209this fs. For a most up-to-date list of bugs please consult
 210fs/affs/Changes.
 211
 212By default, filenames are truncated to 30 characters without warning.
 213'nofilenametruncate' mount option can change that behavior.
 214
 215Case is ignored by the affs in filename matching, but Linux shells
 216do care about the case. Example (with /wb being an affs mounted fs)::
 217
 218    rm /wb/WRONGCASE
 219
 220will remove /mnt/wrongcase, but::
 221
 222    rm /wb/WR*
 223
 224will not since the names are matched by the shell.
 225
 226The block allocation is designed for hard disk partitions. If more
 227than 1 process writes to a (small) diskette, the blocks are allocated
 228in an ugly way (but the real AFFS doesn't do much better). This
 229is also true when space gets tight.
 230
 231You cannot execute programs on an OFS (Old File System), since the
 232program files cannot be memory mapped due to the 488 byte blocks.
 233For the same reason you cannot mount an image on such a filesystem
 234via the loopback device.
 235
 236The bitmap valid flag in the root block may not be accurate when the
 237system crashes while an affs partition is mounted. There's currently
 238no way to fix a garbled filesystem without an Amiga (disk validator)
 239or manually (who would do this?). Maybe later.
 240
 241If you mount affs partitions on system startup, you may want to tell
 242fsck that the fs should not be checked (place a '0' in the sixth field
 243of /etc/fstab).
 244
 245It's not possible to read floppy disks with a normal PC or workstation
 246due to an incompatibility with the Amiga floppy controller.
 247
 248If you are interested in an Amiga Emulator for Linux, look at
 249
 250http://web.archive.org/web/%2E/http://www.freiburg.linux.de/~uae/
 251