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information
on the diskette
that
allows
DOS
2.5
to
recognize it
as an enhanced-densi ty diskette
.
•
Because DOS
2.5 extended
files
are invisible
to
DOS 2.0S and
will
not
show
up
on a
directory listing
called
up under DOS
2.0S
,
it is
possible
accidentally
to
use
DOS
2.0S to write a file
to an enhanced-density diskette that already contains
an
invis-
ible file with the same
name. For
example
,
working
with the
diskette
discussed
in the
preceding
paragraphs
.
you
might
delete FILE1 .DAT and
write a new
file
called
FILE3.DAT
in its
place.
If you then called
up
a
directory
of
the diskette under
DOS
2.5
,
you would
discover
two
files
named FILE3. DAT.
However,
DOS
2.5 allows you
to rename
just one of a
pair
of
files bearing
the
same
name-see
option
E.
,
RENAME FILE
,
in Section
3.
Technical Notes
Although a
single-density
diskette has 720 formatted
sectors,
both DOS 2.0S and DOS 2.5 utilize only 719
of them. (This is
because DOS marks sector zero -
which
does not appear
on
810
and XF551 drives
-
as
unavailable,
as
if
it were part
of
the
boot sectors.)
Several programs
are available
which count on DOS 's
not
using
sector
720. Generally,
these
programs write
special
information
(often
including
copy
protection
methods)
directly
to sector 720
.
On
a dual-density
diskette.
DOS
2.5
manages
sectors
numbered
from
1 to
1023. Even
though
DOS
2.5 could easily
use
sector 720
for a
file
,
it
purposely
avoids the
sector,
marking
it as already in
use
even
on a newly
formatted diskette. This
is
done
simply to
ensure
compatibility
with any
program
which
uses
sector 720
for
its
own
purposes.
In
the
same vein, a
dual-density diskette actually
consists of
40
tracks of
26
sectors
each, for
a
total
of
1040 sectors.
However,
due to the
inner
workings
of
DOS,
it
is
timited
to using
sectors 1
through 1023 for files.
(This is
because DOS
uses
a 10-bit
sector
address.
and
the
highest number
that
10
bits
can
represent
is
1023.) In addition, DOS
2.5
uses sector
1024 for the
Extended
Volume
Table
of
Contents
(also
known as the
"volume
direc-
11
4
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