Tandy TRS-80 - Page 68

TECHNICAL INFORMATION
Sectors
Each
track
contains
18 sectors,
numbered
1-18.
Each
sector contains
338
bytes. 256 of
them are
for
data.
The
remaining
bytes are for
system
controls.
Byte#
Contents
0-55 System
controls
56-311 Data
312-337
System controls
The
hexadecimal
contents of the system
control
bytes
are:
Byte#
Hexadecimal
Contents
0-7
00
8-10
F5
11
FE
12 Track
Number
13 00
14 Sector
Number
15
01
16-17 Cyclic Redundancy
Check
<CRC)
18-39 4E
40-51 00
52-54
F5
55
FB
312-313 Cyclic
Redundancy
Check (CRC)
314-337 4E
HOW THE
DATA IS
ORGANIZED
Each
track
contains
4,608
bytes
which the Com-
puter can
use for data:
18
sectors per
track
x 256 data
bytes
per sector
4,608
data bytes
per
track
The data bytes
in the 17th track
contain the
disk's
directory.
The data bytes in the
remaining 34
tracks
are
for disk files:
Track
#
0-16
17
18-34
Disk Files
Contents of
Track's Data Bytes
Disk Files
Disk Directory
Disk Files
The
Computer
divides the 34 tracks
for
disk
files
into 68
granules. Since
each track
contains two
granules, one granule is 2,304
bytes
long:
9
sectors in V2 track
x 256
data
bytes per
sector
2,304
bytes
in
a
granule
The
Computer uses
granules
to
allocate space
for
disk
files in 2,304-byte
clusters.
Thus, if a file con-
tains 4,700
bytes, the
Computer
allocates 3 gran-
ules
(6,912
bytes)
of
disk space
for it.
The
location of
the 68 granules,
numbered
0-67,
is
as
follows:
Track
0,
Sectors
1-9
Track
0,
Sectors
10-18
Track
1,
Sectors
1-9
Track
16,
Sectors
10-18
Track
17,
Sectors
1-18
Track
18,
Sectors
1-9
Track
34,
Sectors
10-18
Granule
Granule 1
Granule 2
Granule
33
Directory
Granule
34
Granule 67
Note: The minimum size
of
a
disk
file
is one granule
or 2,304 bytes. A di.sk
will
hold
a maximum
of
68 disk
files.
Disk
Directory
The directory track
(track 17)
contains a file allo-
cation
table and directory
entries.
The sectors on
this track which contain
this
information are:
Sector #
Contents
2 File
allocation table
3-11
Directory
entries
The remaining
sectors
in the directory
track are
for
future
use.
Directory Entries
The 9 sectors of
the
directory containing directory
entries
(sectors
3-11)
will hold up to 72
entries.
Each entry is
32 bytes
long and contains:
58