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INPUT/OUTPUT FUNCTIONS
25.3 -- Device Independent I/O Functions
called.
IOCDBS must be configured to contain the address of
the first b'lte of a buffer intoUlhich a record is to be re-ad,
or
Trom
which
a
record is to be written.
None of the 1/0
functions will
alte~
IOCDBS.
The
data bUTfer
ma~
be used for
FDR
processing
by
the . OPEN function (section 25.3.3)
~hen
dealing with non-diskette devices.
25.3.1.5 !OCDBE -- Data buffer end
This two-byte field of the
IOCB
must be
initialized
by
the
calling
program
before
any
record
I/O functions are
called.
IOCDBE
must be configured to contain the address
of
the
last byte of a buffer into which a record is to be read,
or from which
a
record
is
to
be
written.
During
record
input~
IOCDBS and IOCDBE define the maximum size record that
the buffer can accommodate.
During record outputl
lOCDES and
IOCDSE
describe
the first and last byte of the record to be
written.
None
of
the
I/O
functions will alter
IOCDBE.
The
data
bUTTer
may
be
used
for
FDR
processing
b~
the . OPEN
function (section
25.3.3)
when
dealing
with
non-diskette
devices.
25.3.1.6 IOCGDW -- Generic device word
This
two-byte field of the IOCB serves a dual function.
Before any I/O functions can be invoked,
IOCGDW must
contain
the
MDOS
device
name
that
is to be accessed
(se~
section
25.1>.
The device name consists
Or
two
ASCII
cha-racters.
Once
the
.
RESRV
function (section 25.3.2) has been called,
IOCGDW will contain the address of the controller
descriptor
block
(CDB,
section
26.2.1)
associated
with that device.
After the CDS address has been put into IOCGDW, the
contents
of
this
field
must
net be changed
by
the calling program.
Section
26.2
contains
a
description of how to
configure
the
IOCGDW field for non-supported devices.
25.3.1.7
IOCLUN -- Logical unit number
The
IOCLUN
byte
contains
two
pieces of information.
Initially, the calling program must store
the
logical
unit
number
of
the
device
to
be
accessed
in this byte.
The
logical unit number identifies a
specific
device
within
a
g en e ric
d e vic e
fa
m i 1 y
(
e. g.,
d r
i
ve
z
e roo
r
the
fa mil
y
DK).
If
there is only one device in a
generic
device
family,
a
logical
unit
number
of
zero
must
be
placed
in IOCLUN.
Logical unit numbers should be ASCII
numbers
in
the
range
$30-$39
(0-9>'
Bit
URn
of IOCLUN indicates whether or not
the IOCB has been
reserved
<.
RESRV
function>.·
Initially,
when
the
logical
unit
number is stored in IOCLUN,
bit URI!
wi 11
be set
to
zero.
After
the
.
RESRV
functi on
has
been
successrully
invoked, bit fiR" will be set to onel
indicating
Page
25-23
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