Ms-Dos Disk Allocation - Tandy 1000 Programmer's Reference Manual

Chapter 4
MS-DOS DISK ALLOCATION
The
MS-DOS
area on a diskette is formatted as follows:
First copy of File Allocation
Table
-
variable size
Second copy of File Allocation
Table
-
variable size (optional)
Additional copies of File Allocation
Table
-
variable size (optional)
I
File data area
I
Space for a file in the data area is not pre-allocated. The space
is
allocated one cluster a t a time, as needed. A cluster consists of
one or more consecutive sectors. All of the clusters for a file are
"chained" together in the File Allocation Table (FAT).
A second copy of the FAT is usually kept for consistency. If the
disk should develop a bad sector in the first FAT, the second can
be used. This prevents loss of data due to an unusable disk.
The clusters are arranged on disk to minimize head movement
for multi-sided media. All of the space on a track (or cylinder) is
allocated before moving on to the next track. This is done by al-
locating all the sectors sequentially on the lowest-numbered
head, then all the sectors on the next head, and so on until all
sectors on all heads of the track are used. The next sector to use
will be sector 1 on head 0 of the next track.
For diskettes, the following table can be used:
Number Sectors FAT size Directory Directory Sectors
of Per in Sectors Entries per
1
8 1 4 64 1
2
8
1
7
112
2
1 9 2
4 64
1
2
9
2 7 112 2
Sides Track Sectors Cluster
165