<s>
The	O
Input/Output	B-Application
Supervisor	I-Application
(	O
IOS	O
)	O
is	O
that	O
portion	O
of	O
the	O
control	O
program	O
in	O
the	O
IBM	B-Device
mainframe	I-Device
OS/360	B-Application
and	I-Application
successors	I-Application
operating	B-General_Concept
systems	I-General_Concept
which	O
issues	O
the	O
privileged	O
I/O	O
instructions	O
and	O
supervises	O
the	O
resulting	O
I/O	O
interruptions	O
for	O
any	O
program	O
which	O
requests	O
I/O	O
device	O
operations	O
until	O
the	O
normal	O
or	O
abnormal	O
conclusion	O
of	O
those	O
operations	O
.	O
</s>
<s>
In	O
the	O
early	O
instances	O
of	O
the	O
OS	O
,	O
these	O
sections	O
were	O
brought	O
into	O
the	O
Input/Output	B-Application
Supervisor	I-Application
's	O
"	O
transient	O
area	O
"	O
,	O
not	O
unlike	O
the	O
OS/360	B-Application
Control	O
Program	O
's	O
Supervisor	B-Operating_System
Call	I-Operating_System
"	O
transient	O
areas	O
"	O
.	O
</s>
<s>
Essentially	O
,	O
IOS	O
is	O
a	O
hypervising	O
operating	B-General_Concept
system	I-General_Concept
built	O
on	O
top	O
of	O
the	O
OS	O
itself	O
,	O
and	O
entirely	O
within	O
it	O
,	O
not	O
as	O
a	O
separable	O
function	O
.	O
</s>
<s>
A	O
very	O
specialized	O
hypervisor	B-Operating_System
,	O
to	O
be	O
sure	O
,	O
as	O
the	O
hypervisation	O
is	O
restricted	O
to	O
the	O
several	O
I/O	O
instructions	O
and	O
the	O
one	O
I/O	O
interruption	O
.	O
</s>
<s>
In	O
MVS/370	O
and	O
later	O
instances	O
of	O
the	O
OS	O
,	O
IOS	O
is	O
also	O
designed	O
around	O
a	O
multi-processing	B-Operating_System
concept	O
whereby	O
all	O
available	O
processors	O
,	O
as	O
many	O
as	O
two	O
in	O
MVS/370	O
and	O
as	O
many	O
as	O
sixteen	O
in	O
later	O
instances	O
of	O
the	O
OS	O
(	O
twelve	O
were	O
supported	O
by	O
IBM	O
;	O
sixteen	O
were	O
supported	O
by	O
Amdahl	O
)	O
,	O
are	O
effectively	O
and	O
efficiently	O
utilized	O
.	O
</s>
<s>
And	O
,	O
to	O
best	O
utilize	O
this	O
multi-processing	B-Operating_System
capability	O
,	O
IOS	O
's	O
multi-programming	O
implementation	O
was	O
partitioned	O
into	O
smaller	O
executable	O
units	O
,	O
in	O
particular	O
those	O
which	O
may	O
be	O
executed	O
under	O
the	O
control	O
of	O
an	O
SRB	B-Application
.	O
</s>
