<s>
The	O
THE	B-Operating_System
multiprogramming	I-Operating_System
system	I-Operating_System
or	O
THE	B-Operating_System
OS	I-Operating_System
was	O
a	O
computer	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
designed	O
by	O
a	O
team	O
led	O
by	O
Edsger	O
W	O
.	O
Dijkstra	O
,	O
described	O
in	O
monographs	O
in	O
1965-66	O
and	O
published	O
in	O
1968	O
.	O
</s>
<s>
The	O
THE	O
system	O
was	O
primarily	O
a	O
batch	B-General_Concept
system	I-General_Concept
that	O
supported	O
multitasking	B-Operating_System
;	O
it	O
was	O
not	O
designed	O
as	O
a	O
multi-user	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
It	O
was	O
much	O
like	O
the	O
SDS	B-Device
940	I-Device
,	O
but	O
"	O
the	O
set	O
of	O
processes	O
in	O
the	O
THE	O
system	O
was	O
static	O
"	O
.	O
</s>
<s>
The	O
THE	O
system	O
apparently	O
introduced	O
the	O
first	O
forms	O
of	O
software-based	O
paged	B-General_Concept
virtual	B-Architecture
memory	I-Architecture
(	O
the	O
Electrologica	B-Device
X8	I-Device
did	O
not	O
support	O
hardware-based	B-General_Concept
memory	B-General_Concept
management	I-General_Concept
)	O
,	O
freeing	O
programs	O
from	O
being	O
forced	O
to	O
use	O
physical	O
locations	O
on	O
the	O
drum	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
It	O
did	O
this	O
by	O
using	O
a	O
modified	O
ALGOL	B-Language
compiler	B-Language
(	O
the	O
only	O
programming	O
language	O
supported	O
by	O
Dijkstra	O
's	O
system	O
)	O
to	O
"	O
automatically	O
generate	O
calls	B-Operating_System
to	I-Operating_System
system	I-Operating_System
routines	I-Operating_System
,	O
which	O
made	O
sure	O
the	O
requested	O
information	O
was	O
in	O
memory	O
,	O
swapping	B-Architecture
if	O
necessary	O
"	O
.	O
</s>
<s>
Paged	B-General_Concept
virtual	B-Architecture
memory	I-Architecture
was	O
also	O
used	O
for	O
buffering	B-General_Concept
input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
device	O
data	O
,	O
and	O
for	O
a	O
significant	O
portion	O
of	O
the	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
code	O
,	O
and	O
nearly	O
all	O
the	O
ALGOL	B-Language
60	I-Language
compiler	B-Language
.	O
</s>
<s>
In	O
this	O
system	O
,	O
semaphores	B-Operating_System
were	O
used	O
as	O
a	O
programming	O
construct	O
for	O
the	O
first	O
time	O
.	O
</s>
<s>
The	O
design	O
of	O
the	O
THE	B-Operating_System
multiprogramming	I-Operating_System
system	I-Operating_System
is	O
significant	O
for	O
its	O
use	O
of	O
a	O
layered	B-Application
structure	I-Application
,	O
in	O
which	O
"	O
higher	O
"	O
layers	O
depend	O
on	O
"	O
lower	O
"	O
layers	O
only	O
:	O
</s>
<s>
Layer	O
0	O
was	O
responsible	O
for	O
the	O
multiprogramming	O
aspects	O
of	O
the	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
It	O
decided	O
which	O
process	B-Operating_System
was	O
allocated	O
to	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
,	O
and	O
accounted	O
for	O
processes	O
that	O
were	O
blocked	O
on	O
semaphores	B-Operating_System
.	O
</s>
<s>
It	O
dealt	O
with	O
interrupts	B-Application
and	O
performed	O
the	O
context	B-Operating_System
switches	I-Operating_System
when	O
a	O
process	B-Operating_System
change	O
was	O
needed	O
.	O
</s>
<s>
In	O
modern	O
terms	O
,	O
this	O
was	O
the	O
scheduler	B-Algorithm
.	O
</s>
<s>
Layer	O
1	O
was	O
concerned	O
with	O
allocating	B-General_Concept
memory	I-General_Concept
to	O
processes	O
.	O
</s>
<s>
Layer	O
2	O
dealt	O
with	O
communication	O
between	O
the	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
and	O
the	O
system	B-Device
console	I-Device
.	O
</s>
<s>
Layer	O
3	O
managed	O
all	O
I/O	B-General_Concept
between	O
the	O
devices	O
attached	O
to	O
the	O
computer	O
.	O
</s>
<s>
This	O
included	O
buffering	B-General_Concept
information	O
from	O
the	O
various	O
devices	O
.	O
</s>
<s>
Layer	O
4	O
consisted	O
of	O
user	B-Operating_System
programs	I-Operating_System
.	O
</s>
<s>
There	O
were	O
5	O
processes	O
:	O
in	O
total	O
,	O
they	O
handled	O
the	O
compiling	B-Language
,	O
executing	B-General_Concept
,	O
and	O
printing	O
of	O
user	B-Operating_System
programs	I-Operating_System
.	O
</s>
<s>
When	O
finished	O
,	O
they	O
passed	O
control	O
back	O
to	O
the	O
schedule	O
queue	B-Application
,	O
which	O
was	O
priority-based	B-Application
,	O
favoring	O
recently	O
started	O
processes	O
and	O
ones	O
that	O
blocked	O
because	O
of	O
I/O	B-General_Concept
.	O
</s>
<s>
The	O
layers	O
were	O
implemented	O
in	O
order	O
,	O
layer	O
0	O
first	O
,	O
with	O
thorough	O
testing	O
of	O
the	O
abstractions	B-Application
provided	O
by	O
each	O
layer	O
in	O
turn	O
.	O
</s>
<s>
This	O
division	O
of	O
the	O
kernel	B-Operating_System
into	O
layers	O
was	O
similar	O
in	O
some	O
ways	O
to	O
Multics	B-Application
 '	O
later	O
ring-segmentation	B-Operating_System
model	O
.	O
</s>
<s>
Several	O
subsequent	O
operating	B-General_Concept
systems	I-General_Concept
have	O
used	O
layering	O
to	O
some	O
extent	O
,	O
including	O
Windows	B-Device
NT	I-Device
and	O
macOS	B-Application
,	O
although	O
usually	O
with	O
fewer	O
layers	O
.	O
</s>
<s>
The	O
code	O
of	O
the	O
system	O
was	O
written	O
in	O
assembly	B-Language
language	I-Language
for	O
the	O
Dutch	O
Electrologica	B-Device
X8	I-Device
computer	O
.	O
</s>
<s>
This	O
computer	O
had	O
a	O
word	O
size	O
of	O
27	O
bits	O
,	O
48	O
kilowords	O
of	O
core	B-General_Concept
memory	I-General_Concept
,	O
512	O
kilowords	O
of	O
drum	B-General_Concept
memory	I-General_Concept
providing	O
backing	B-General_Concept
store	I-General_Concept
for	O
the	O
LRU	B-General_Concept
cache	I-General_Concept
algorithm	I-General_Concept
,	O
paper	O
tape	O
readers	O
,	O
paper	O
tape	O
punches	O
,	O
plotters	O
,	O
and	O
printers	O
.	O
</s>
