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-ROMFS - ROM FILE SYSTEM
-
-This is a quite dumb, read only filesystem, mainly for initial RAM
-disks of installation disks. It has grown up by the need of having
-modules linked at boot time. Using this filesystem, you get a very
-similar feature, and even the possibility of a small kernel, with a
-file system which doesn't take up useful memory from the router
-functions in the basement of your office.
-
-For comparison, both the older minix and xiafs (the latter is now
-defunct) filesystems, compiled as module need more than 20000 bytes,
-while romfs is less than a page, about 4000 bytes (assuming i586
-code). Under the same conditions, the msdos filesystem would need
-about 30K (and does not support device nodes or symlinks), while the
-nfs module with nfsroot is about 57K. Furthermore, as a bit unfair
-comparison, an actual rescue disk used up 3202 blocks with ext2, while
-with romfs, it needed 3079 blocks.
-
-To create such a file system, you'll need a user program named
-genromfs. It is available on http://romfs.sourceforge.net/
-
-As the name suggests, romfs could be also used (space-efficiently) on
-various read-only media, like (E)EPROM disks if someone will have the
-motivation.. :)
-
-However, the main purpose of romfs is to have a very small kernel,
-which has only this filesystem linked in, and then can load any module
-later, with the current module utilities. It can also be used to run
-some program to decide if you need SCSI devices, and even IDE or
-floppy drives can be loaded later if you use the "initrd"--initial
-RAM disk--feature of the kernel. This would not be really news
-flash, but with romfs, you can even spare off your ext2 or minix or
-maybe even affs filesystem until you really know that you need it.
-
-For example, a distribution boot disk can contain only the cd disk
-drivers (and possibly the SCSI drivers), and the ISO 9660 filesystem
-module. The kernel can be small enough, since it doesn't have other
-filesystems, like the quite large ext2fs module, which can then be
-loaded off the CD at a later stage of the installation. Another use
-would be for a recovery disk, when you are reinstalling a workstation
-from the network, and you will have all the tools/modules available
-from a nearby server, so you don't want to carry two disks for this
-purpose, just because it won't fit into ext2.
-
-romfs operates on block devices as you can expect, and the underlying
-structure is very simple. Every accessible structure begins on 16
-byte boundaries for fast access. The minimum space a file will take
-is 32 bytes (this is an empty file, with a less than 16 character
-name). The maximum overhead for any non-empty file is the header, and
-the 16 byte padding for the name and the contents, also 16+14+15 = 45
-bytes. This is quite rare however, since most file names are longer
-than 3 bytes, and shorter than 15 bytes.
-
-The layout of the filesystem is the following:
-
-offset content
-
- +---+---+---+---+
- 0 | - | r | o | m | \
- +---+---+---+---+ The ASCII representation of those bytes
- 4 | 1 | f | s | - | / (i.e. "-rom1fs-")
- +---+---+---+---+
- 8 | full size | The number of accessible bytes in this fs.
- +---+---+---+---+
- 12 | checksum | The checksum of the FIRST 512 BYTES.
- +---+---+---+---+
- 16 | volume name | The zero terminated name of the volume,
- : : padded to 16 byte boundary.
- +---+---+---+---+
- xx | file |
- : headers :
-
-Every multi byte value (32 bit words, I'll use the longwords term from
-now on) must be in big endian order.
-
-The first eight bytes identify the filesystem, even for the casual
-inspector. After that, in the 3rd longword, it contains the number of
-bytes accessible from the start of this filesystem. The 4th longword
-is the checksum of the first 512 bytes (or the number of bytes
-accessible, whichever is smaller). The applied algorithm is the same
-as in the AFFS filesystem, namely a simple sum of the longwords
-(assuming bigendian quantities again). For details, please consult
-the source. This algorithm was chosen because although it's not quite
-reliable, it does not require any tables, and it is very simple.
-
-The following bytes are now part of the file system; each file header
-must begin on a 16 byte boundary.
-
-offset content
-
- +---+---+---+---+
- 0 | next filehdr|X| The offset of the next file header
- +---+---+---+---+ (zero if no more files)
- 4 | spec.info | Info for directories/hard links/devices
- +---+---+---+---+
- 8 | size | The size of this file in bytes
- +---+---+---+---+
- 12 | checksum | Covering the meta data, including the file
- +---+---+---+---+ name, and padding
- 16 | file name | The zero terminated name of the file,
- : : padded to 16 byte boundary
- +---+---+---+---+
- xx | file data |
- : :
-
-Since the file headers begin always at a 16 byte boundary, the lowest
-4 bits would be always zero in the next filehdr pointer. These four
-bits are used for the mode information. Bits 0..2 specify the type of
-the file; while bit 4 shows if the file is executable or not. The
-permissions are assumed to be world readable, if this bit is not set,
-and world executable if it is; except the character and block devices,
-they are never accessible for other than owner. The owner of every
-file is user and group 0, this should never be a problem for the
-intended use. The mapping of the 8 possible values to file types is
-the following:
-
- mapping spec.info means
- 0 hard link link destination [file header]
- 1 directory first file's header
- 2 regular file unused, must be zero [MBZ]
- 3 symbolic link unused, MBZ (file data is the link content)
- 4 block device 16/16 bits major/minor number
- 5 char device - " -
- 6 socket unused, MBZ
- 7 fifo unused, MBZ
-
-Note that hard links are specifically marked in this filesystem, but
-they will behave as you can expect (i.e. share the inode number).
-Note also that it is your responsibility to not create hard link
-loops, and creating all the . and .. links for directories. This is
-normally done correctly by the genromfs program. Please refrain from
-using the executable bits for special purposes on the socket and fifo
-special files, they may have other uses in the future. Additionally,
-please remember that only regular files, and symlinks are supposed to
-have a nonzero size field; they contain the number of bytes available
-directly after the (padded) file name.
-
-Another thing to note is that romfs works on file headers and data
-aligned to 16 byte boundaries, but most hardware devices and the block
-device drivers are unable to cope with smaller than block-sized data.
-To overcome this limitation, the whole size of the file system must be
-padded to an 1024 byte boundary.
-
-If you have any problems or suggestions concerning this file system,
-please contact me. However, think twice before wanting me to add
-features and code, because the primary and most important advantage of
-this file system is the small code. On the other hand, don't be
-alarmed, I'm not getting that much romfs related mail. Now I can
-understand why Avery wrote poems in the ARCnet docs to get some more
-feedback. :)
-
-romfs has also a mailing list, and to date, it hasn't received any
-traffic, so you are welcome to join it to discuss your ideas. :)
-
-It's run by ezmlm, so you can subscribe to it by sending a message
-to romfs-subscribe@shadow.banki.hu, the content is irrelevant.
-
-Pending issues:
-
-- Permissions and owner information are pretty essential features of a
-Un*x like system, but romfs does not provide the full possibilities.
-I have never found this limiting, but others might.
-
-- The file system is read only, so it can be very small, but in case
-one would want to write _anything_ to a file system, he still needs
-a writable file system, thus negating the size advantages. Possible
-solutions: implement write access as a compile-time option, or a new,
-similarly small writable filesystem for RAM disks.
-
-- Since the files are only required to have alignment on a 16 byte
-boundary, it is currently possibly suboptimal to read or execute files
-from the filesystem. It might be resolved by reordering file data to
-have most of it (i.e. except the start and the end) laying at "natural"
-boundaries, thus it would be possible to directly map a big portion of
-the file contents to the mm subsystem.
-
-- Compression might be an useful feature, but memory is quite a
-limiting factor in my eyes.
-
-- Where it is used?
-
-- Does it work on other architectures than intel and motorola?
-
-
-Have fun,
-Janos Farkas <chexum@shadow.banki.hu>