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Magnetic
T
ape
Considerations
The
most
common
medium
for
storage
of
a
device
le
is
magnetic
tape
.
This
section
describes
the
matters
that
you
should
keep
in
mind
when
you
work
with
your
magnetic
tape
les
.
Every
standard
reel
of
magnetic
tape
designed
for
digital
computer
use
has
two
re ective
markers
located
on
the
back
side
of
the
tape
(opposite
the
recording
surface).
As
illustrated
in
Figure
7-2,
one
of
these
marks
is
located
behind
the
tape
leader
at
the
beginning-of-tape
(BOT)
position,
and
the
other
is
located
in
front
of
the
tape
trailer
at
the
end-of-tape
(EOT)
position.
These
markers
are
sensed
by
the
tape
drive
itself
and
their
position
on
the
tape
(left
or
right
side)
determines
whether
they
indicate
the
BOT
or
EOT
positions:
Figure
7-2.
Magnetic
T
ape
Markers
As
far
as
the
magnetic
tape
hardware
and
software
are
concerned,
the
BOT
marker
is
much
more
signicant
than
the
EOT
marker
because
BOT
signals
the
start
of
recorded
information;
but
EOT
simply
indicates
that
the
remaining
tape
supply
is
running
low
,
and
the
program
writing
the
tape
should
bring
the
operation
to
an
orderly
conclusion.
The
dierence
in
treatment
of
these
two
physical
tape
markers
is
re ected
by
the
le
system
intrinsics
when
the
le
being
read,
written,
or
controlled
is
a
magnetic
tape
device
le
.
The
following
paragraphs
discuss
the
characteristics
of
each
appropriate
intrinsic
.
FWRITE.
If
the
magnetic
tape
is
unlabeled
(as
specied
in
the
HPFOPEN /FOPEN
intrinsic
or
FILE
command)
and
a
user
program
attempts
to
write
over
or
beyond
the
physical
EOT
marker
,
the
FWRITE
intrinsic
returns
an
error
condition
code
(CCL).
The
actual
data
is
written
to
the
tape
,
and
a
call
to
FCHECK
reveals
a
le
error
indicating
end-of-tape
.
All
writes
to
the
tape
after
the
EOT
marker
has
been
crossed
transfer
the
data
successfully
,
but
return
a
CCL
condition
code
until
the
tape
crosses
the
EOT
marker
again
in
the
reverse
direction
(rewind
or
backspace).
If
the
magnetic
tape
is
labeled
(as
specied
in
the
HPFOPEN /FOPEN
intrinsic
or
FILE
command),
a
CCL
condition
code
is
not
returned
when
the
tape
passes
the
EOT
marker
.
Attempts
to
write
to
the
tape
after
the
EOT
marker
is
encountered
cause
end-of-volume
(EO
V)
labels
to
be
written.
A
message
then
is
printed
on
the
operator's
console
requesting
another
reel
of
tape
to
be
mounted.
FREAD
.
A
user
program
can
read
data
written
over
an
EOT
marker
and
beyond
the
marker
into
the
tape
trailer
.
The
intrinsic
returns
no
error
condition
code
(CCL
or
CCG)
and
does
not
initiate
a
le
system
error
code
when
the
EOT
marker
is
encountered.
DDS
tape
drives
do
not
permit
an
FREAD
past
the
marker
.
With
DDS
drives
,
every
FWRITE
updates
the
EOT
marker
and
does
not
permit
a
rewrite
of
previous
data.
FSP
A
CE.
A
user
program
can
space
records
over
or
beyond
the
EOT
marker
without
receiving
an
error
condition
code
(CCL
or
CCG)
or
a
le
system
error
.
The
intrinsic
does
,
however
,
return
a
CCG
condition
code
when
a
logical
le
mark
is
encountered.
If
the
user
program
attempts
to
backspace
records
over
the
BOT
marker
,
the
intrinsic
returns
a
CCG
condition
code
and
remains
positioned
on
the
BOT
marker
.
Record
Selection
and
Data
T
ransfer
7-7
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