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INPUT/OUTPUT FUNCTIONS
25.3 -- Device Independent I/O Functions
F
E
. D
.C
B
A
9
8
7
6
5
4
·3
2
1
o
PSN
EN
~--------~---~---------------------------------------------------
<---------
Not Used
(=0)
------->
Position
within
sector
(0-7)
Physical sector number
(~3-$16)
2~.3.
1. 18 IOCSBP --
Sector buffer pOinter
The
IOCSB?
field
only
applies to diskette
I/O.
This
two-byte
~i@ld
or the IOCE serves
a
dual
purpose.
If
an
existing Tile is being openedJ the initial value of IOeSB? is
ignored.
IT
a
file is being created,
this -Field
must
contain
the initial number of sectors that are to be allocated to the
f i
1 e .
ITt
h e va 1 u e
0
f
z
e
r
0
iss
pee i
fie
d
I
MDOS
UI
i 11
~
e
fa
u 1
t
the
initial
file
size
to
a
full
segment descriptor
(32
clusters) and no error will occur during the
file's
initial
space allocation if fewer than
32
clusters are available.
If
a
non-zero (non-default) initial size is specified,
however,
an error will occur if, that initial size cannot
be
allocated.
The
.ALLOC
system
function
description
(section
27.4>
contains
a
more
detailed
explanation
of
the
allocation
mechanism.
After a file has been
opened,
the
IOeSBP
contains
a
pointer into the sector buffer that is used by the record
110
functions.
Therefore}
the contents of
IOeSB?
must
not
be
changed
by
the calling program once a file is open when using
the record I/O functions.
If the sector
110
functions
are
used,
then
IOCSBF
can be altered
by
the calling program in
any way after
a
file is open.
25.3. 1. 19 IDeSBS -- Sector buffer start
This two-byte field of the IOCB only applies to diskette
I/O.
It
must
be initialized by the calling program before
an~
of
the
I/O
functions
are
invoked.
IOCSBS
must
be
configured
to
contain
the
address
of the first byte of a
buffer into which one or more 128-byte sectors can
be
read.
This
sector
buffer
will
be used
Tor
directory searches as
well as for data transfers.
lOeSaS
will not
be
altered
by
an~
of the I/O functions.
25.3.1.20 IOeSBE -- Sector buffer end
This two-byte field of the IOCB only applies to diskette
lID.
It
must be initialized by the
calling
program
before
Page
25-31
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