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INPUT/OUTPUT FUNCTIONS
25.3 -- Device Independent I/O Functions
S
(Bit
5) ,--
Sector/record -Flag
This
bit
controls
whether
sector
01'
record
processing
is
performed during an I/O function.
For
non-diskette
devices,
this bit must always be
zero.
For diskette devices;
this
bit
can
be
in
either
state.
A one implies that logical sector I/O will be
performed.
A zero implies that record
I/O
will
be
perTormed;
however,
care
must
be
taken
·that the
corresponding I/O function is called for
the
proper
state
of the bit.
That is,
the record I/O functions
(.GETRC and .PUTRC) cannot be called if US" is set to
one.
Likewise,
the
logical
sector
I/O functions
(.GETLS and. PUTLS) cannot be called if US" is set to
zero.
Open/closed flag
This bit is supplied
by
the system
liD
functions
if
they
are
properly
called
in
their
correct
sequence.
The
"OU bit must not be changed once I/O
transfers have been made.
A one indicates
that
the
file
(or
device)
is
closed.
A zero, on the other
hand"
indicates that the file (or device) is open.
T· (Bi t
3) --
Truncate -Flag
The truncate -Flag is only applicable to liD on a
dis k e t ted e vic e .
No rma 1 1 Y , the use r
UI
ill not h a vet
0
set or change this biti however,
certain
cases
will
arise where changing of the truncate fiag by the user
may
be
necessary
(see
.CLOSE
function,
section
25.3.6).
The truncate flag is used as an indication
that new space was allocated to a
diskette file.
If
it
is
set
to
one,
any
unused parts of the newly
allocated space (space
beyond
the
maximum
logical
sector
number
referenced
in
IOCMLS)
will
be
deallocated
(returned
to
the
available
diskette
space) when the file is closed.
If the truncate flag
is zero, no truncation will occur upon closing.
A special case exists
if
IOCMLS
contains
the
value SFFFF when the truncate flag is set to ·one.
In
addition
to
having
all
of
the
file's
space.
deallocated,
the
directory
entry
belonging to the
rile
is r-emoved rrom the directory.
The file is.
in
effect~
deleted.
Page
25-21
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