SchOS/docs/architecture/fat.md

FAT

FAT12, FAT16 and FAT32 with VFAT long names, read and write. kernel/fs/fat/fat.c, behind the same fs_ops vtable as SchFS.

It exists for one reason: it is the filesystem every other system can read. A USB stick written here opens on Linux, Windows and macOS, and so does the EFI system partition. The first user is the hardware report, which has to leave a machine that may have no network on something you can carry to another one.

mount -t vfat /dev/usb0p1 /mnt          # msdos and fat are accepted as well
mount -t vfat -o ro /dev/sata0p1 /boot/efi
umount /mnt

The disks and partitions the block layer found are in /dev under the names it gave them (usb0, usb0p1, sata0p2, nvme0p1), generated like the rest of devtmpfs: a stick appears when it is plugged in and goes when it is pulled.

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Written from the specification, matched to Linux

The on-disk rules come from Microsoft's FAT specification (1.03). Where the specification leaves a choice, the choice is Linux's vfat, so that a volume written here and one written by Linux look alike:

• The FAT type is decided by the cluster count and nothing else: under 4085 is FAT12, under 65525 FAT16, otherwise FAT32. The "FAT32" label in the boot sector is a comment.
• Short names. A name that fits 8.3 in one case (readme.txt, README.TXT) is stored as a short entry alone, with the lowercase bits in byte 12 that Windows NT and Linux use. A name that does not (ReadMe.txt, Reports, big numbers.log) gets long-name entries plus an alias: its uppercase self when that loses nothing (REPORTS), otherwise BIGNUM~1.LOG, counting up until it is unique.
• Lookups ignore case (ASCII and Latin-1), and a name with a long entry still answers to its alias, so PROGRA~1 works.
• Times are stored as UTC. FAT has no time zone, and that is what Linux assumes when it is not told one.
• The dirty bit. Set in the boot sector while a volume is mounted read-write, cleared on unmount, sync-on-shutdown and reboot. A volume found with it set is reported (fsck.fat reads the same bit).
• Every FAT copy is written, unless the volume has mirroring turned off, in which case only the active one is. FSInfo's free count and next-free hint are read and kept up to date.
• Refused, with Linux's errno: device nodes, symlinks and sockets (EPERM, through fs_ops.refuse_types), hard links (EPERM, fs_ops.no_hardlinks), and names containing " * / : < > ? \ | (EINVAL).

Checked against mkfs.vfat-made FAT12, FAT16 (MBR) and FAT32 (superfloppy and GPT ESP) volumes: written here, then fsck.fat -n and mtools on the host.

Inodes, on a filesystem that has none

A FAT file is its directory entry: name, first cluster, size and times are all in those 32 bytes. The inode number says where to find it:

object	number
the root	1
a directory	2 << 40 | its first cluster
a file	1 << 40 | byte position of its short entry / 32
a file created here, or one whose number is taken	3 << 40 | a counter

A directory is numbered by its cluster because .. must be able to find it, and .. holds the parent's cluster and nothing about where the parent's entry is. A file is numbered by its entry, as Linux does it.

The VFS renames by linking the new name to the same number and then unlinking the old one, so a file's entry can move while its number stays. A small per-volume table records every number whose entry is not where the number says. It is consulted first in both directions, and anything nobody holds can be dropped from it, since the next lookup by name finds the entry afresh.

I/O

Through a cache of its own (64 lines of 4 KB per volume), not the buffer cache. FAT addresses sectors, a partition need not be a whole number of 4 KB blocks, and the buffer cache's last block of one would read and write into the next partition. blkdev_xfer takes sectors and refuses a transfer past the end rather than panicking.

Every operation that changes something writes its dirty lines back before it returns. A stick pulled out between two commands holds both of them. sync and fsync also flush the drive's own cache. Unmount, shutdown, reboot and the power button sync and mark the volume clean.

A new cluster is zeroed in the cache before use. FAT has no holes, and a directory ends at the first entry whose first byte is zero.

What it does not do

• Names longer than 29 bytes. FAT allows 255 UTF-16 units, but the VFS's DIRSIZ caps a path component, so a longer name is cut short on the way in. Lifting that is a VFS change.
• Permissions and owners. Every file shows as 0755 root:root, like Linux vfat with its defaults. The read-only attribute is kept but not enforced.
• exFAT. Large SD cards (SDXC, over 32 GB) ship with it. It is a different filesystem, not a FAT variant.
• Code pages. Short names are read as CP437 and written as ASCII; anything else goes in a long name.
• Directories don't move, which is the VFS's rule for every filesystem.
• No fsck. A volume found dirty is reported, not repaired.