Advertisement
INPUT/OUTPUT FUNCTIONS
25.3 --
Device Independent I/O FunctiDns
25.3.1. 10 IOCMLS --
Max imum LSN referenced
This
two-byte
field of
the
IOCB
overlays the first two
bytes of the IOCNAM after the .
OPEN
function has been
called
(diskette
I/O
only).
It is a system-maintained field that
contains the maximum logical sector number ever referenced by
any of the
I/O
functions.
IOCMLS and the truncate flag
(T of
IOCDTT)
are
used in determining the amount
o¥
newly allocated
diskette
space that is to be deallocated from a
fi~e
when
it
is closed.
Space will only be deallocated
if
the
truncate
flag
is
set
to
a
one.
Since MDOS automatically sets the
truncate flag to a one if new diskette space is allocated
to
a
file,
any
unused
space
will
always be
retur~ed
to the
available space pool.
Normall~1
the user-
never
changes
the
IOCMLS
O~
the
t~uncate
flag
in
the
IOCDTT
since
the
truncate flag is
automatically set whenever
additional
space
allocation
is
performed
or whenever a new file is created.
When accessing
an ex is tin 9 f
i
1
e
us
i
n 9 bot h in put an d
0
u t put (M = 00 or 11
·0
f
IOCDTT),
however,
the truncate flag may have to be set to one
by the
use~
if
the
file
is
to
be
shortened
or
if
the
end-of-file
pointer
in
the
RIB
is
to be updated.
If an
extant
~ile
does not
gro~
in size,
the truncate flag
~ill
be
zero.
In
addition,
when
files
are
to. be
deleted (upon a
subsequent. CLOSE function' cal-l)
I
the IOCMLS must be set to .a
value of SFFFF and the ti'uncate flag must be set to one.
25.3.1.11
IOCSDW -- Current SDW
The IOCSDW field overlays the second two bytes of IOCNAM
after the . OPEN function has been called <diskette I/O only).
This
field
contains
the
segment
descriptor
word
which
identifies the current file segment that can be accessed.
If
another
segment
of
the
file
is
to be accessed,
the disk
driver will automatically reread the file's RIB
and
extract
the
appropriate
SOW
into
IOCSDW.
The contents of IOCSDW
should never be changed by the calling program.
25.3.1.12
IOCSLS -- Starting LSN of SDW
The IOCSLS field overlays the
thi~d
two bytes of
IOCNAM
after the . OPEN function has been called (diskette IIO only).
This field contains the starting logical sector number of the
current
segment
descriptor
word.
The
contents of IOCSLS
should never be changed by the calling program,
Page
25-25
Advertisement
