Hardware Reference
In-Depth Information
directories, and other data structures for managing its information. Each volume is
organized as a linear sequence of
clusters
, with the cluster size being fixed for
each volume and ranging from 512 bytes to 64 KB, depending on the volume size.
Clusters are referred to by their offset from the start of the volume using 64-bit
numbers.
The main data structure in each volume is the
MFT
(
Master File Table
),
which has an entry for each file and directory in the volume. These entries are
analogous to the i-nodes in
UNIX
. The MFT is itself a file, and as such can be
placed anywhere within the volume. This property is useful in case there are
defective disk blocks at the beginning of the volume where the MFT would nor-
mally be stored.
UNIX
systems normally store certain key information at the start
of each volume and in the (extremely unlikely) case that one of these blocks is
irreparably damaged, the entire volume has to be repositioned.
The MFT is shown in Fig. 6-42. It begins with a header containing infor-
mation about the volume, such as (pointers to) the root directory, the boot file, the
bad-block file, the free-list administration, etc. After that comes an entry per file
or directory, 1 KB except when the cluster size is 2 KB or more. Each entry con-
tains all the metadata (administrative information) about the file or directory. Sev-
eral formats are allowed, one of which is shown in Fig. 6-42.
Standard
information
MS-DOS
name
File name
Security
Data
MFT entry for one file
MFT
header
Master
file
table
Figure 6-42.
The Windows 7 master file table.
The standard information field contains information such as the time stamps
needed by POSIX, the hard link count, the read-only and archive bits, etc. It is a
fixed-length field and always present. The file name is of variable length, up to
255 Unicode characters. In order to make such files accessible to old 16-bit pro-
grams, files can also have a
MS-DOS
name, which consists of eight alphanumeric
characters optionally followed by a dot and an extension of no more than three
