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INPUT/OUTPUT FUNCTIONS
25.3 -- Device Independent
liD
Functions
25.3.1.13 IOCLSN -- Next LSN
The IOCLSN field overlays the fourth two bytes of IOCNAM
after the . OPEN function has been called (diskette I/O only).
This rield is never changed by the calling program if
record
IIO
(5
~
0 of IOCDTT) is being used.
If
logical sector
I/O
is being used
(5
=
1
of
IOCDTT)
I
then
rOCLSN
can
be
changed
b~
the
calling program to specify
~hich
logical sectors are
to be read from or written to the file.
This feature
allows
the
calling
program to randomly access the file (by logical
sector number) without having to know
physically
where
the
file
resides
on the diskette.
After
an I/O
access has been
completed,- IOCLSN
lUill
contain the logical sector' number
or
the next
sect~r
on the
diskette to
be
accessed.
When
using a
multiple sector buffer,
IOCLSN may have been
incremented
by
more than one: depending
on
the number
01=
sectors processed.
25.3.1.14 IOCEOF -- LSN of end-of-file
The
IOCEOF
field
overlays
IOCSUF
after
the
.
OPEN
function has been called <diskette
IIO
only).
IOCEOF
is
a
system-maintained
parameter
that
represents
the
logical
sector number Or the logical end-of-Tile.
This
value
must
not be
changed by
the calling program once the . OPEN Tunction
has
been
invoked.
25.3. 1. 15
IOCR
IB -- PSN
of
RIB
This two-byte rield or the IOCB is initialized with
the
physical
sector
number
of
the
file's RIB after the . OPEN
function has been called <diskette I/O
only>.
The
RIB
is
used
to
access the file via its SDWs to allocate additional
space, to deallocate unused spaceJ and to monitor the
LSN
of
the
rile's
logical
end-of-file.
The
IOCRIB entry should
never be changed
by
the calling program.
25.3.1.16 IOCFDF
File descriptor flags
This two-byte field contains the rlags that describe the
inherent and the changeable attributes of a file.
The format
of the IOCFDF entry is
sho~n
below:
..-/
Page
25-26
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