<s>
The	O
RC	B-Operating_System
4000	I-Operating_System
Multiprogramming	I-Operating_System
System	I-Operating_System
(	O
also	O
termed	O
Monitor	O
or	O
RC	O
4000	O
depending	O
on	O
reference	O
)	O
is	O
a	O
discontinued	O
operating	B-General_Concept
system	I-General_Concept
developed	O
for	O
the	O
RC4000	O
minicomputer	B-Architecture
in	O
1969	O
.	O
</s>
<s>
The	O
RC	B-Operating_System
4000	I-Operating_System
Multiprogramming	I-Operating_System
System	I-Operating_System
is	O
historically	O
notable	O
for	O
being	O
the	O
first	O
attempt	O
to	O
break	O
down	O
an	O
operating	B-General_Concept
system	I-General_Concept
into	O
a	O
group	O
of	O
interacting	O
programs	O
communicating	O
via	O
a	O
message	B-Architecture
passing	I-Architecture
kernel	B-Operating_System
.	O
</s>
<s>
RC4000	O
was	O
not	O
widely	O
used	O
,	O
but	O
was	O
highly	O
influential	O
,	O
sparking	O
the	O
microkernel	B-Operating_System
concept	O
that	O
dominated	O
operating	B-General_Concept
system	I-General_Concept
research	O
through	O
the	O
1970s	O
and	O
1980s	O
.	O
</s>
<s>
Brinch	O
Hansen	O
found	O
that	O
no	O
existing	O
operating	B-General_Concept
system	I-General_Concept
was	O
suited	O
to	O
the	O
new	O
machine	O
,	O
and	O
was	O
tired	O
of	O
having	O
to	O
adapt	O
existing	O
systems	O
.	O
</s>
<s>
He	O
felt	O
that	O
a	O
better	O
solution	O
was	O
to	O
build	O
an	O
underlying	O
kernel	B-Operating_System
,	O
which	O
he	O
referred	O
to	O
as	O
the	O
nucleus	B-Operating_System
,	O
that	O
could	O
be	O
used	O
to	O
build	O
up	O
an	O
operating	B-General_Concept
system	I-General_Concept
from	O
interacting	O
programs	O
.	O
</s>
<s>
Unix	B-Application
,	O
for	O
instance	O
,	O
uses	O
small	O
interacting	O
programs	O
for	O
many	O
tasks	O
,	O
transferring	O
data	O
through	O
a	O
system	O
called	O
pipelines	B-Operating_System
or	I-Operating_System
pipes	I-Operating_System
.	O
</s>
<s>
However	O
,	O
a	O
large	O
amount	O
of	O
fundamental	O
code	O
is	O
integrated	O
into	O
the	O
kernel	B-Operating_System
,	O
notably	O
things	O
like	O
file	B-Application
systems	I-Application
and	O
program	O
control	O
.	O
</s>
<s>
Monitor	O
would	O
relocate	O
such	O
code	O
also	O
,	O
making	O
almost	O
the	O
entire	O
system	O
a	O
set	O
of	O
interacting	O
programs	O
,	O
reducing	O
the	O
kernel	B-Operating_System
(	O
nucleus	B-Operating_System
)	O
to	O
a	O
communications	O
and	O
support	O
system	O
only	O
.	O
</s>
<s>
Monitor	O
used	O
a	O
pipe-like	O
system	O
of	O
shared	O
memory	O
as	O
the	O
basis	O
of	O
its	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
.	O
</s>
<s>
Data	O
to	O
be	O
sent	O
from	O
one	O
process	O
to	O
another	O
was	O
copied	O
into	O
an	O
empty	O
memory	O
data	B-General_Concept
buffer	I-General_Concept
,	O
and	O
when	O
the	O
receiving	O
program	O
was	O
ready	O
,	O
back	O
out	O
again	O
.	O
</s>
<s>
The	O
buffer	B-General_Concept
was	O
then	O
returned	O
to	O
the	O
pool	O
.	O
</s>
<s>
Programs	O
had	O
a	O
very	O
simple	O
application	O
programming	O
interface	O
(	O
API	B-General_Concept
)	O
for	O
passing	O
data	O
,	O
using	O
an	O
asynchronous	O
set	O
of	O
four	O
methods	O
.	O
</s>
<s>
Note	O
that	O
messages	O
had	O
an	O
implicit	O
"	O
return	O
path	O
"	O
for	O
every	O
message	O
sent	O
,	O
making	O
the	O
semantics	O
more	O
like	O
a	O
remote	B-Operating_System
procedure	I-Operating_System
call	I-Operating_System
than	O
Mach	B-Operating_System
's	I-Operating_System
completely	O
input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
based	O
system	O
.	O
</s>
<s>
External	O
processes	O
were	O
handled	O
outside	O
of	O
user	O
space	O
by	O
the	O
nucleus	B-Operating_System
,	O
although	O
they	O
could	O
be	O
started	O
and	O
stopped	O
just	O
like	O
any	O
other	O
program	O
.	O
</s>
<s>
Internal	O
processes	O
were	O
started	O
in	O
the	O
context	O
of	O
the	O
parent	O
that	O
launched	O
them	O
,	O
so	O
each	O
user	O
could	O
effectively	O
build	O
up	O
their	O
own	O
operating	B-General_Concept
system	I-General_Concept
by	O
starting	O
and	O
stopping	O
programs	O
in	O
their	O
own	O
context	O
.	O
</s>
<s>
Scheduling	O
was	O
left	O
entirely	O
to	O
the	O
programs	O
,	O
if	O
required	O
at	O
all	O
(	O
in	O
the	O
1960s	O
,	O
computer	B-Operating_System
multitasking	I-Operating_System
was	O
a	O
feature	O
of	O
debatable	O
value	O
)	O
.	O
</s>
<s>
One	O
user	O
could	O
start	O
a	O
session	O
in	O
a	O
pre-emptive	O
multitasking	B-Operating_System
environment	O
,	O
while	O
another	O
might	O
start	O
in	O
a	O
single-user	O
mode	O
to	O
run	O
batch	B-General_Concept
processing	I-General_Concept
at	O
higher	O
speed	O
.	O
</s>
<s>
Real-time	B-General_Concept
scheduling	O
could	O
be	O
supported	O
by	O
sending	O
messages	O
to	O
a	O
timer	O
process	O
that	O
would	O
only	O
return	O
at	O
the	O
appropriate	O
time	O
.	O
</s>
<s>
For	O
instance	O
,	O
Mach	B-Operating_System
required	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
to	O
improve	O
messaging	O
by	O
using	O
the	O
copy-on-write	B-Language
protocol	O
and	O
mapping	O
(	O
instead	O
of	O
copying	O
)	O
data	O
from	O
process	O
to	O
process	O
.	O
</s>
<s>
Mach	B-Operating_System
also	O
used	O
threading	O
extensively	O
,	O
allowing	O
the	O
external	O
programs	O
,	O
or	O
servers	O
in	O
more	O
modern	O
terms	O
,	O
to	O
easily	O
start	O
up	O
new	O
handlers	O
for	O
incoming	O
requests	O
.	O
</s>
<s>
Still	O
,	O
Mach	B-Operating_System
IPC	O
was	O
too	O
slow	O
to	O
make	O
the	O
microkernel	B-Operating_System
approach	O
practically	O
useful	O
.	O
</s>
<s>
This	O
only	O
changed	O
when	O
Jochen	O
Liedtke	O
's	O
L4	B-Operating_System
microkernel	I-Operating_System
demonstrated	O
IPC	O
overheads	O
reduced	O
by	O
an	O
order-of-magnitude	O
.	O
</s>
