<s>
QIO	B-Operating_System
(	O
Queue	O
I/O	O
)	O
is	O
a	O
term	O
used	O
in	O
several	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
designed	O
by	O
the	O
former	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
of	O
Maynard	O
,	O
Massachusetts	O
.	O
</s>
<s>
I/O	O
operations	O
on	O
these	O
systems	O
are	O
initiated	O
by	O
issuing	O
a	O
QIO	B-Operating_System
call	O
to	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
There	O
are	O
two	O
types	O
of	O
QIO	B-Operating_System
-	O
Queue	O
I/O	O
,	O
and	O
Queue	O
I/O	O
and	O
Wait	O
.	O
</s>
<s>
For	O
QIO	B-Operating_System
without	O
wait	O
,	O
the	O
call	O
returns	O
immediately	O
.	O
</s>
<s>
On	O
completion	O
,	O
status	O
is	O
returned	O
in	O
the	O
QIO	B-Operating_System
status	O
doubleword	O
.	O
</s>
<s>
The	O
QIO	B-Operating_System
request	O
may	O
also	O
specify	O
that	O
completion	O
set	O
an	O
event	B-Operating_System
flag	I-Operating_System
or	O
issue	O
an	O
Asynchronous	B-Operating_System
System	I-Operating_System
Trap	I-Operating_System
(	O
AST	B-Operating_System
)	O
.	O
</s>
<s>
The	O
following	O
operating	B-General_Concept
systems	I-General_Concept
implement	O
QIO(W )	O
:	O
</s>
<s>
Under	O
VMS	B-Operating_System
,	O
the	O
arguments	O
to	O
the	O
QIO	B-Operating_System
call	O
are	O
:	O
</s>
<s>
The	O
event	B-Operating_System
flag	I-Operating_System
to	O
set	O
when	O
the	O
operation	O
completes	O
.	O
</s>
<s>
It	O
is	O
n't	O
possible	O
to	O
not	O
specify	O
an	O
event	B-Operating_System
flag	I-Operating_System
;	O
flag	O
0	O
is	O
valid	O
.	O
</s>
<s>
It	O
is	O
perfectly	O
permissible	O
to	O
have	O
multiple	O
simultaneous	O
operations	O
that	O
set	O
the	O
same	O
event	B-Operating_System
flag	I-Operating_System
on	O
completion	O
.	O
</s>
<s>
It	O
is	O
then	O
up	O
to	O
the	O
application	O
to	O
sort	O
out	O
any	O
confusion	O
this	O
might	O
cause	O
,	O
or	O
just	O
ignore	O
that	O
event	B-Operating_System
flag	I-Operating_System
.	O
</s>
<s>
At	O
this	O
level	O
,	O
all	O
operations	O
on	O
disk	B-Device
files	O
and	O
directories	O
(	O
filename	O
parsing	O
,	O
directory	O
lookup	O
,	O
file	O
opening/closing	O
)	O
are	O
done	O
by	O
appropriate	O
QIO	B-Operating_System
requests	O
.	O
</s>
<s>
The	O
optional	O
I/O	O
status	O
block	O
(	O
IOSB	O
)	O
,	O
which	O
is	O
cleared	O
by	O
the	O
QIO	B-Operating_System
call	O
,	O
and	O
filled	O
in	O
on	O
completion	O
of	O
the	O
I/O	O
operation	O
.	O
</s>
<s>
The	O
optional	O
AST	B-Operating_System
routine	O
to	O
invoke	O
when	O
the	O
operation	O
completes	O
.	O
</s>
<s>
An	O
additional	O
parameter	O
(	O
whose	O
meaning	O
is	O
up	O
to	O
the	O
caller	O
)	O
to	O
be	O
passed	O
to	O
the	O
AST	B-Operating_System
routine	O
.	O
</s>
<s>
For	O
example	O
,	O
for	O
a	O
computer	B-General_Concept
terminal	I-General_Concept
,	O
P3	O
might	O
be	O
the	O
time	O
to	O
allow	O
for	O
the	O
read	O
to	O
complete	O
whereas	O
,	O
for	O
a	O
disk	B-Device
drive	I-Device
,	O
it	O
might	O
be	O
the	O
starting	O
block	O
number	O
of	O
the	O
transfer	O
.	O
</s>
<s>
When	O
the	O
event	B-Operating_System
flag	I-Operating_System
becomes	O
set	O
.	O
</s>
<s>
When	O
the	O
AST	B-Operating_System
routine	O
executes	O
.	O
</s>
<s>
Simple	O
QIOs	B-Operating_System
,	O
such	O
as	O
read	O
or	O
write	O
requests	O
,	O
are	O
either	O
serviced	O
by	O
the	O
kernel	B-Operating_System
itself	O
or	O
by	O
device	O
drivers	O
.	O
</s>
<s>
The	O
Files-11	B-Application
ODS-1	O
file	O
system	O
on	O
RSX-11	B-Application
was	O
implemented	O
by	O
a	O
subroutine	O
library	O
that	O
communicated	O
with	O
a	O
task	O
named	O
F11ACP	O
using	O
a	O
special	O
set	O
of	O
QIOs	B-Operating_System
called	O
the	O
"	O
ACP	O
QIOs.	O
"	O
</s>
<s>
Originally	O
,	O
the	O
Files-11	B-Application
ODS-2	O
file	O
system	O
was	O
provided	O
by	O
F11BACP	O
on	O
VMS	B-Operating_System
,	O
but	O
the	O
functionality	O
of	O
F11BACP	O
was	O
later	O
incorporated	O
into	O
the	O
VMS	B-Operating_System
kernel	B-Operating_System
to	O
save	O
the	O
overhead	O
of	O
process	O
context	O
switches	O
,	O
and	O
is	O
now	O
called	O
an	O
XQP	O
(	O
eXtended	O
Qio	B-Operating_System
Processor	O
)	O
.	O
</s>
<s>
Probably	O
the	O
most	O
complex	O
single	O
QIO	B-Operating_System
request	O
possible	O
is	O
the	O
VMS	B-Operating_System
terminal	O
driver	O
's	O
call	O
with	O
the	O
modifier	O
;	O
this	O
QIO	B-Operating_System
requires	O
all	O
six	O
additional	O
parameters	O
:	O
</s>
<s>
If	O
omitted	O
,	O
this	O
defaults	O
to	O
the	O
usual	O
VMS	B-Operating_System
line	O
delimiters	O
including	O
carriage-return	O
(	O
but	O
not	O
line-feed	O
)	O
.	O
</s>
