<s>
Single-level	O
storage	O
(	O
SLS	O
)	O
or	O
single-level	B-Device
memory	I-Device
is	O
a	O
computer	B-General_Concept
storage	I-General_Concept
term	O
which	O
has	O
had	O
two	O
meanings	O
.	O
</s>
<s>
The	O
two	O
meanings	O
are	O
related	O
in	O
that	O
in	O
both	O
,	O
pages	B-General_Concept
of	O
memory	O
may	O
be	O
in	O
primary	O
storage	O
(	O
RAM	B-Architecture
)	O
or	O
in	O
secondary	O
storage	O
(	O
disk	O
)	O
,	O
and	O
that	O
the	O
physical	O
location	O
of	O
a	O
page	O
is	O
unimportant	O
to	O
a	O
process	O
.	O
</s>
<s>
The	O
term	O
originally	O
referred	O
to	O
what	O
is	O
now	O
usually	O
called	O
virtual	B-Architecture
memory	I-Architecture
,	O
which	O
was	O
introduced	O
in	O
1962	O
by	O
the	O
Atlas	B-Device
system	O
at	O
the	O
University	O
of	O
Manchester	O
.	O
</s>
<s>
In	O
modern	O
usage	O
,	O
the	O
term	O
usually	O
refers	O
to	O
the	O
organization	O
of	O
a	O
computing	O
system	O
in	O
which	O
there	O
are	O
no	O
files	B-Operating_System
,	O
only	O
persistent	O
objects	O
(	O
sometimes	O
called	O
segments	B-General_Concept
)	O
,	O
which	O
are	O
mapped	O
into	O
processes	B-Operating_System
 '	O
address	B-General_Concept
spaces	I-General_Concept
(	O
which	O
consist	O
entirely	O
of	O
a	O
collection	O
of	O
mapped	O
objects	O
)	O
.	O
</s>
<s>
The	O
persistent	O
object	O
concept	O
was	O
first	O
introduced	O
by	O
Multics	B-Application
in	O
the	O
mid-1960s	O
,	O
in	O
a	O
project	O
shared	O
by	O
MIT	O
,	O
General	O
Electric	O
and	O
Bell	O
Labs	O
.	O
</s>
<s>
It	O
also	O
was	O
implemented	O
as	O
virtual	B-Architecture
memory	I-Architecture
,	O
with	O
the	O
actual	O
physical	O
implementation	O
including	O
a	O
number	O
of	O
levels	O
of	O
storage	O
types	O
.	O
</s>
<s>
(	O
Multics	B-Application
,	O
for	O
instance	O
,	O
had	O
three	O
levels	O
originally	O
:	O
main	O
memory	O
,	O
a	O
high-speed	O
drum	B-General_Concept
,	O
and	O
disks	O
.	O
)	O
</s>
<s>
IBM	O
holds	O
patents	O
to	O
single-level	O
storage	O
as	O
implemented	O
in	O
the	O
IBM	B-Application
i	I-Application
operating	O
system	O
on	O
IBM	B-Device
Power	I-Device
Systems	I-Device
and	O
its	O
predecessors	O
as	O
far	O
back	O
as	O
the	O
System/38	B-Device
that	O
was	O
released	O
in	O
1978	O
.	O
</s>
<s>
With	O
a	O
single-level	O
storage	O
the	O
entire	O
storage	O
of	O
a	O
computer	O
is	O
thought	O
of	O
as	O
a	O
single	O
two-dimensional	O
plane	O
of	O
addresses	O
,	O
pointing	O
to	O
pages	B-General_Concept
.	O
</s>
<s>
Pages	B-General_Concept
may	O
be	O
in	O
primary	O
storage	O
(	O
RAM	B-Architecture
)	O
or	O
in	O
secondary	O
storage	O
(	O
disk	O
)	O
;	O
however	O
,	O
the	O
current	O
location	O
of	O
an	O
address	O
is	O
unimportant	O
to	O
a	O
process	O
.	O
</s>
<s>
The	O
operating	O
system	O
takes	O
on	O
the	O
responsibility	O
of	O
locating	O
pages	B-General_Concept
and	O
making	O
them	O
available	O
for	O
processing	O
.	O
</s>
<s>
If	O
a	O
page	O
is	O
on	O
disk	O
,	O
a	O
page	B-General_Concept
fault	I-General_Concept
occurs	O
and	O
the	O
operating	O
system	O
brings	O
the	O
page	O
into	O
primary	O
storage	O
.	O
</s>
<s>
No	O
explicit	O
I/O	O
to	O
secondary	O
storage	O
is	O
done	O
by	O
processes	B-Operating_System
:	O
instead	O
,	O
reads	O
from	O
secondary	O
storage	O
are	O
done	O
as	O
the	O
result	O
of	O
page	B-General_Concept
faults	I-General_Concept
;	O
writes	O
to	O
secondary	O
storage	O
are	O
done	O
when	O
pages	B-General_Concept
that	O
have	O
been	O
modified	O
since	O
being	O
read	O
from	O
secondary	O
storage	O
into	O
primary	O
storage	O
are	O
written	O
back	O
to	O
their	O
location	O
in	O
secondary	O
storage	O
.	O
</s>
<s>
IBM	O
's	O
design	O
of	O
the	O
single-level	O
storage	O
was	O
originally	O
conceived	O
and	O
pioneered	O
by	O
Frank	O
Soltis	O
and	O
Glenn	O
Henry	O
in	O
the	O
late	O
1970s	O
as	O
a	O
way	O
to	O
build	O
a	O
transitional	O
implementation	O
to	O
computers	O
with	O
100%	O
solid	B-Device
state	I-Device
memory	I-Device
.	O
</s>
<s>
The	O
thinking	O
at	O
the	O
time	O
was	O
that	O
disk	O
drives	O
would	O
become	O
obsolete	O
,	O
and	O
would	O
be	O
replaced	O
entirely	O
with	O
some	O
form	O
of	O
solid	B-Device
state	I-Device
memory	I-Device
.	O
</s>
<s>
System/38	B-Device
was	O
designed	O
to	O
be	O
independent	O
of	O
the	O
form	O
of	O
hardware	O
memory	O
used	O
for	O
secondary	O
storage	O
.	O
</s>
<s>
This	O
has	O
not	O
come	O
to	O
be	O
,	O
however	O
,	O
because	O
while	O
solid	B-Device
state	I-Device
memory	I-Device
has	O
become	O
exponentially	O
cheaper	O
,	O
disk	O
drives	O
have	O
also	O
become	O
similarly	O
cheaper	O
;	O
thus	O
,	O
the	O
price	O
ratio	O
in	O
favour	O
of	O
disk	O
drives	O
continues	O
:	O
very	O
much	O
higher	O
capacities	O
than	O
solid	B-Device
state	I-Device
memory	I-Device
,	O
very	O
much	O
slower	O
to	O
access	O
,	O
and	O
much	O
less	O
expensive	O
.	O
</s>
<s>
In	O
System/38	B-Device
and	O
IBM	B-Application
i	I-Application
,	O
all	O
data	O
in	O
storage	O
is	O
assigned	O
a	O
address	O
,	O
accessible	O
by	O
processor	O
instructions	O
,	O
in	O
virtual	B-Architecture
memory	I-Architecture
,	O
and	O
is	O
referred	O
to	O
by	O
that	O
address	O
,	O
regardless	O
of	O
whether	O
it	O
is	O
currently	O
in	O
physical	O
memory	O
or	O
not	O
.	O
</s>
<s>
If	O
the	O
data	O
referred	O
to	O
by	O
that	O
address	O
is	O
not	O
in	O
physical	O
memory	O
,	O
a	O
page	B-General_Concept
fault	I-General_Concept
occurs	O
.	O
</s>
<s>
The	O
page	B-General_Concept
fault	I-General_Concept
is	O
handled	O
by	O
low-level	O
software	O
that	O
reads	O
the	O
page	O
into	O
an	O
available	O
page	B-General_Concept
frame	I-General_Concept
in	O
primary	O
storage	O
.	O
</s>
<s>
With	O
the	O
IBM	B-Application
i	I-Application
implementation	O
of	O
single-level	O
storage	O
,	O
page	B-General_Concept
faults	I-General_Concept
are	O
divided	O
into	O
two	O
categories	O
.	O
</s>
