Radio Shack TRS-80 Owner's Manual page 90

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TRS-80
MODEL
III
DISK
SYSTEM
Technical
Information
Contents of This
Section:
Disk Organization
File
Structure
System Routines
for
Assembly
i/o
Data/Device Control Blocks
Physical and Logical Records
Fundamental trsdos
i/o
Calls
Additional
Routines and Storage Addresses
trsdos
Error
Codes/Messages
Disk Organization
Each trsdos
system
diskette contains a
trsdos
system,
a
utility
command
library,
and
a
file
directory.
Each
diskette
is
single-sided
and
has
40
tracks of information.
Each
track
contains 18 sectors of
256
bytes each.
Normally, data read/write operations
may
be
initiated
only
at
sector boundaries,
and must
consist
of
exactly
256
bytes.
However, trsdos
allows
the user
to
have
maximum
flexibility
with minimal
effort
by
automatically blocking and
de-
blocking
all file
accesses
to
user-specified
logical
record
lengths,
even
if this
requires
"spanning"
of
two
sectors.
The
system
disk
file
structure
allows
maximum
use of
disk
file
space by
automatically
segmenting
files
across a diskette in several
small
pieces.
These
pieces are correlated into
one
logically contiguous
file
by
the system without
your needing
to
know
the physical
file
location. This
structure eliminates time-
consuming
disk-packing
operations.
File
Structure
A
trsdos
file is
composed
of
one
or
more
segments
of storage space. Each
segment
consists
of
from one
to
32
physically contiguous granules of
storage.
A
granule
is
the
minimum
allocatable unit of
storage,
and
consists of
three sectors
(768 bytes).
Since
a
file is
always lengthened by
granules, a small
amount
of
free storage
is
generally present
at
the
end of every
file.
This
free storage allows
minor
file
additions
to
be
made
in
space
which
is
physically
contiguous
to the
file.
Every
time
a disk
file is
extended
(either initialized or lengthened), extra
granules
may
be
allocated
to that
file,
depending on
the
file's
accumulated
length, diskette
space,
saturation,
etc.
These
extra granules, along with
all
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