A disk consists of a linear array of 512 byte sectors addressed by a
sector number. MMFS aggregates these sectors into blocks which are
typically 128, 256 or 512 KiB in size. These blocks are the basic unit
of addressing and allocation for MMFS. File data is transferred
between the disk and memory in whole blocks, but metadata is accessed
in smaller segments.
This disk is divided into four areas, the directory, the freelist, the
block allocation tables (BATs) and the data area. The following
sections describe these in detail.
The directory occupies the first one or two blocks of the disk. It
consists of an array of directory entries. Each directory entry
contains the following fields:
type
Entry type:
MMFS_TYPE_EMPTY Unused entry. Available for allocation.
MMFS_TYPE_VOLUME Volume label. The
data field contains a volume label that describes
the format of the filesystem.
MMFS_TYPE_FILE File. A standard data file.
MMFS_TYPE_RESERVED Reserved entry. An entry that
exists only to occupy a directory slot. Used to protect the volume
label against overwriting while updating adjacent entries.
bat
The block number of the Block Allocation Table for this file.
size
File size in bytes. For streamed files this reflects the number of
data blocks in the BAT. For random access files this is the offset of
the last byte of the last block in the BAT.
created
Creation time. A timestamp in seconds since the epoch implemented by
the system wallclock. If no wallclock is present then this merely
records the time since the last system restart.
state
File state. This records the state of the file and aids in system
recovery. The possible states are:
MMFS_STATE_CREATING: The file is open for creation
and is being actively extended.
MMFS_STATE_CREATED: The file has been closed and
will no longer be extendable. However, random access files may be
extended in this state.
MMFS_STATE_DELETING: The file is being deleted.
checksum
Checksum over directory entry. This ensures that the directory entry
is correct and consistent.
data
Per-entry data. The contents of this field depend on the entry
type. For the volume label it contains the filesystem format
parameters. For files it may contain user-specified data. It is unused
in other entry types.
name
File name. A zero terminated string naming the file. This field is 64
bytes long so, with zero termination, filenames may be a maximum of
63 bytes. There are no limits on the characters allowed.
The data field of a volume label contains the disk format
parameters. This consists of the following sub-fields:
signature1
Volume label signature. This is used, together with
signature2, to ensure that this is a valid volume
label. If these two fields do no contain the expected values then the
disk is presumed to be new or corrupt and the filesystem will reformat
it.
version
Filesystem version number. Together with the revision number this is
used to determine which version of the filesystem formatted this
disk.
revision
Filesystem revision number.
sector_size
The size of sectors on this disk. This should match the sector size
reported by the disk device itself. At present only sectors of 512
bytes are supported.
phys_block_size
Physical block size. This is the size of the physical blocks supported
by this disk. This may differ from the sector size in some cases.
block_size
Size of MMFS blocks in sectors.
disk_size
Total number of blocks on the disk. If the number of sectors on the
disk is not an exact multiple of the block_size then the last partial
block will be unused.
rootdir_size
Size of the directory in blocks.
freelist_start
Block address of the first block of the freelist. This will be just
after the directory.
freelist_size
Size of the freelist in blocks. This is calculated from
disk_size so that the freelist is large enough to
contain all the blocks on the disk.
bat_size
Size of each Block Allocation Table in blocks. The number of blocks per
BAT is set during the formatting process.
bat_count
Number of BATs. The number of BATs is set during the formatting
process and defaults to 200.
direntry_size
Size of a directory entry in bytes. This is currently fixed at 256
bytes. It is present to permit changes to the directory entry size in
the future.
name_size
Size of the name field in a directory entry. This is currently fixed at 64
bytes. It is present to permit changes to the directory entry size in
the future.
data_size
Size of the data field in a directory entry. This is currently fixed at
160 bytes. It is present to permit changes to the directory entry size
in the future.
