Editing FAT filesystem and Linux (section)

== {{Not a typo|umsdos}} ==

=== {{anchor|--LINUX-.---}}{{anchor|Data structures of umsdos}} Data structure ===
The ''{{Not a typo|umsdos}}'' FAT filesystem driver stores all of the extra information relating to Unix file semantics in what, to another FAT filesystem driver, appears to be just a normal file in each directory and subdirectory, named <code>--LINUX-.---</code>.<ref name="StanfieldSmith"/><ref name="Smith1"/><ref name="McCune"/><ref name="Smith2"/><ref name="Danesh"/>

In the absence of this file in any given directory, and thus by default, the ''{{Not a typo|umsdos}}'' filesystem driver provides the same semantics as the ''{{Not a typo|msdos}}'' filesystem driver does for the directory: only 8.3 filenames and no extra Unix file semantics.  To enable the ''{{Not a typo|umsdos}}'' driver's extra abilities, it is necessary to create that file in the directory and synchronize its internal data with the normal FAT data for any existing entries already in the directory.   This is done with a tool called <code>umssync</code>.<ref name="StanfieldSmith"/><ref name="Smith1"/><ref name="McCune"/>

This is the utility program that is run, across every directory on the disc volume, every time that one switches from running Windows to running Linux, in order for the ''{{Not a typo|umsdos}}'' filesystem driver to incorporate any changes made to files and directories by Windows into its private data structures in its <code>--LINUX-.---</code> file.  By default, the <code>umssync</code> tool creates <code>--LINUX-.---</code> files in directories if they do not already exist, resulting in such a file in every directory in the disc volume.  When switching between Windows and Linux this behaviour is not often considered desirable.  Therefore, the normal mode of operation when invoking <code>umssync</code> after switching from Windows to Linux (which is usually done by running the tool at Linux boot time from a startup script) is to employ the <code>-c</code> option to the command, which prevents the creation of any new <code>--LINUX-.---</code> files in directories that do not already possess them.<ref name="StanfieldSmith"/><ref name="Smith1"/><ref name="McCune"/>

=== Installing Linux on FAT {{anchor| on and booting it from FAT volumes using umsdos}} ===
As mentioned, ''{{Not a typo|umsdos}}'' permits installing Linux on, and then bootstrapping and running it from, a FAT format disc volume.  The advantage of this is that it permits the use of Linux on a computer where [[DOS]] is already installed, without requiring that the hard disc be [[partition (computing)|repartitioned]].  Linux is not bootstrapped directly from a [[Volume Boot Record]] in such a scenario.  Instead DOS is first bootstrapped, and [[loadlin]] or {{Not a typo|linld}} is used to then bootstrap Linux from DOS.<ref name="McCune"/>

The convention for such an installation is for the Linux [[root directory]] to be a subdirectory of the root directory of the DOS [[system partition and boot partition|boot volume]], e.g. <code>C:\LINUX</code> .  The various Linux top-level directories are thus, to DOS, directories such as <code>C:\LINUX\ETC</code> (for <code>/etc</code>), <code>C:\LINUX\BIN</code> (for <code>/bin</code>), <code>C:\LINUX\LIB</code> (for <code>/lib</code>), and so forth.  The ''{{Not a typo|umsdos}}'' filesystem driver automatically prepends the <code>C:\LINUX\</code> to all pathnames.  The location of the Linux root directory is supplied to the ''{{Not a typo|umsdos}}'' filesystem driver in the first place via an option to the <code>loadlin</code> command.  So, for example, <code>loadlin</code> would be invoked with a command line such as <code>loadlin c:\linux\boot\vmlinuz rw root=c:\linux</code> .<ref name="McCune"/><ref name="Danesh">{{cite book |title=Making Linux work: essential tips & techniques |series=1001 Tips Series |author-first=Arman |author-last=Danesh |publisher=Cengage Learning |year=2002 |isbn=978-1-884133-78-7 |pages=[https://archive.org/details/makinglinuxworke00dane/page/19 19] |url=https://archive.org/details/makinglinuxworke00dane/page/19 }}</ref>

The installation of Linux into such a directory in the first place simply involves unpacking files from an archive into that directory and its subdirectories.  Such an installation also generally requires the use of a [[swap file]] rather than a [[swap partition]] for Linux, however this is related to the desire not to repartition the hard disc and unrelated to the ''{{Not a typo|umsdos}}'' filesystem driver per se.<ref name="McCune"/>