Radio Shack TRS-80 Trsdos & Disk Basic Reference Manual page 75

TRSDOS
Technical Information
files.
Maximum
file
size
is
limited
only
by
the physical
size
of the
diskette, since a
file
must
be wholly contained
on one
diskette.
Each
diskette
is
single-sided
and
has
35
tracks
of information.
Each track contains 10 sectors of
256 bytes
each.
See
Mini Disk
Operation,
"How
a
Diskette
Works".
Normally, data
read/write operations
may
only be
initiated 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
five
sectors
(1.25K
bytes).
(See Figure below).
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.
The
effect
is
to
decrease the
amount
of "thrashing" present
in a file
which
has
had frequent additions
made.
(A wholly sector-mapped
system could not offer
this
benefit.)
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
granules after
the
one containing the
file's
EOF
mark,
are
recovered
and
returned to the
system
when
the
file
is
closed.
6-3