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INPUT/OUTPUT
PROVISIONS
26.2 -- Device Independent I/O
R (Bit 7) -- Reservable device flag
In1s
bit
determines
whether
a
device can be
reserved
by
multiple IOCSs at the same time.
Certain
devices,
like
diskette
devices,
by
nature of their
operation,
can
allow
input/output
accesses
to
be
performed
from
different
callers
(IOCBs).
Other
devices,
like a line
printerl
cannot logicall\j
allow
multiple
output
accesses Trom different IOCSs to be
processed.
IT the
URn
bit is set to
one,
it
means
that
the device is reservable.
In other UJords,
only
one IOCB can communicate with the device at
a
time.
If
the
URII
bit
is
set to
zero,
it means that the
d e vic e
i
s
non
-1'
e s e r va
b 1
e
(L
e.;
the
d
e vic e
can
communicate with multiple IDCBs).
a
(Bit 6) -- Output device flag
This
bit indicates whether a device can be used
by
output functions.
If the
"0"
bit
~s
set
to
one,
then
the
device can be used
Tor
output.
If the
"0"
Tlag is set to zero,
then the device cannot
be
used
Tor
output.
I (Bit 5) -- Input
dev~ce
flag
Th
i s
bit
i n d
i.c ate s
\11
h e the r a
d
e vic
e
can
b
e
use d
b~
input functions.
If the "I" bit is
set
to
one,
then
the
device
can be used for input.
If the "I"
flag is set to zero,
then the device cannot
be
used
for input.
F (Bit 4) -- File-type device Flag
This
bit determines
whethe~
or not
a
device can
open and close
files.
A
file-type
device
(e.
g.
1
diskette
d~ive,
and
cassette
or
paper
tape
reader/punch) will
be
handled
differently
by
the
.OPEN
and
.
CLOSE
functions
than
a
non-file-type
device
(e.
g.
I
console
printer,
line
printer,
ke~board).
In
addition
to
having
FDR
processing
performed
on
them,
file-type
devices
are
also
sensitive
to
end-of-file
records.
Non-file-type
devices are not
sub~ect
to
FDR
processing,
nor
are
end-or-file
records
read
Trom
them
or written to
them.
A file-type device is indicated by the "F" bit
being set to one.
Non-file-type devices have the
"Fit
bit set to zero.
Page
26-05
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