Storage and mounting

The filesystem is a directory tree stored on a block device. mnt grafts one device’s tree onto a directory of another so paths can cross between them. File data is allocated dynamically from a shared cluster pool, so a file grows to whatever free space allows (see Memory maps for the on-disk layout).

Devices

Device	Backing store	Capacity
0	on-chip EEPROM (the root fs, whole 1 KB)	8 nodes, up to ~832 B/file
2	external I2C EEPROM (24Cxx) at 0x50	64 nodes, 256 B/file

Device 0 is the on-chip EEPROM and is mounted at / automatically. It is small — eight nodes total: the root directory plus seven files or directories. Device 2 lives on a real 24Cxx part wired to the TWI/I2C bus, addressed with a 16-bit word address; it is non-volatile across firmware reflashes and far larger.

Files and directories

$ mkd docs
$ new docs/readme
$ say hi >> docs/readme
$ cat docs/readme
hi
$ cp docs/readme /            # copy a file into a directory
$ mv docs/readme /tmpdir      # move a node into a directory (may cross devices)
$ rm docs                     # remove a node (a directory is removed recursively)

Names are at most 8 characters. cp copies a file into a directory; mv moves any node into a directory; both work across devices. rm deletes a file, or a directory and its whole subtree; rm <dir>/* wipes a directory’s contents but keeps the directory. fmt wipes a whole volume.

Mounting

mnt <dev> <path> formats the device if it is blank, then makes its tree appear at <path>. umnt <path> detaches it.

$ mkd ext
$ mnt 2 /ext         # the external EEPROM now appears under /ext
$ say data >> /ext/note
$ cat /ext/note
data
$ umnt /ext          # /ext is an empty directory again
$ mnt 2 /ext         # remount: the data is still on the device
$ cat /ext/note
data

Data written through a mount point lives on the mounted device and persists across umnt/mnt — and, for the external EEPROM, across reflashing the firmware.