<s>
CP-40	B-Application
was	O
a	O
research	O
precursor	O
to	O
CP-67	B-Application
,	O
which	O
in	O
turn	O
was	O
part	O
of	O
IBM	O
's	O
then-revolutionary	O
CP[-67]/CMS	B-Application
–	O
a	O
virtual	O
machine/virtual	O
memory	O
time-sharing	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
for	O
the	O
IBM	B-Device
System/360	I-Device
Model	I-Device
67	I-Device
,	O
and	O
the	O
parent	O
of	O
IBM	O
's	O
VM	B-Application
family	I-Application
.	O
</s>
<s>
CP-40	B-Application
ran	O
multiple	O
instances	O
of	O
client	O
operating	B-General_Concept
systems	I-General_Concept
–	O
particularly	O
CMS	B-Operating_System
,	O
the	O
Cambridge	B-Operating_System
Monitor	I-Operating_System
System	I-Operating_System
,	O
built	O
as	O
part	O
of	O
the	O
same	O
effort	O
.	O
</s>
<s>
Like	O
CP-67	B-Application
,	O
CP-40	B-Application
and	O
the	O
first	O
version	O
of	O
CMS	B-Operating_System
were	O
developed	O
by	O
IBM	O
's	O
Cambridge	O
Scientific	O
Center	O
(	O
CSC	O
)	O
staff	O
,	O
working	O
closely	O
with	O
MIT	O
researchers	O
at	O
Project	O
MAC	O
and	O
Lincoln	O
Laboratory	O
.	O
</s>
<s>
CP-40/CMS	O
production	O
use	O
began	O
in	O
January	O
1967	O
.	O
</s>
<s>
CP-40	B-Application
ran	O
on	O
a	O
unique	O
,	O
specially	O
modified	O
IBM	B-Device
System/360	I-Device
Model	I-Device
40	I-Device
.	O
</s>
<s>
CP-40	B-Application
was	O
a	O
one-off	O
research	O
system	O
.	O
</s>
<s>
Provide	O
research	O
input	O
to	O
the	O
System/360	B-Device
Model	I-Device
67	I-Device
team	O
working	O
in	O
Poughkeepsie	O
,	O
who	O
were	O
breaking	O
new	O
ground	O
with	O
the	O
as-yet-unproven	O
concept	O
of	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Support	O
CSC	O
's	O
time-sharing	B-General_Concept
requirements	O
in	O
Cambridge	O
.	O
</s>
<s>
However	O
,	O
there	O
was	O
also	O
an	O
important	O
unofficial	O
mission	O
:	O
To	O
demonstrate	O
IBM	O
's	O
commitment	O
to	O
and	O
capability	O
for	O
supporting	O
time-sharing	B-General_Concept
users	O
like	O
MIT	O
.	O
</s>
<s>
CP-40	B-Application
(	O
and	O
its	O
successor	O
)	O
achieved	O
its	O
goals	O
from	O
technical	O
and	O
social	O
standpoints	O
–	O
they	O
helped	O
to	O
prove	O
the	O
viability	O
of	O
virtual	B-Architecture
machines	I-Architecture
,	O
to	O
establish	O
a	O
culture	O
of	O
time-sharing	B-General_Concept
users	O
,	O
and	O
to	O
launch	O
a	O
remote	O
computer	O
services	O
industry	O
.	O
</s>
<s>
However	O
,	O
the	O
project	O
became	O
embroiled	O
in	O
an	O
internal	O
IBM	O
political	O
war	O
over	O
time-sharing	B-General_Concept
versus	O
batch	O
processing	O
;	O
and	O
it	O
failed	O
to	O
win	O
the	O
hearts	O
and	O
minds	O
of	O
the	O
academic	O
computer	O
science	O
community	O
,	O
which	O
ultimately	O
turned	O
away	O
from	O
IBM	O
to	O
systems	O
like	O
Multics	B-Application
,	O
UNIX	B-Application
,	O
TENEX	B-Operating_System
,	O
and	O
various	O
DEC	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Ultimately	O
,	O
however	O
,	O
the	O
virtualization	O
concepts	O
developed	O
in	O
the	O
CP-40	B-Application
project	O
bore	O
fruit	O
in	O
diverse	O
areas	O
,	O
and	O
remain	O
important	O
today	O
.	O
</s>
<s>
CP-40	B-Application
was	O
the	O
first	O
operating	B-General_Concept
system	I-General_Concept
that	O
implemented	O
complete	O
virtualization	O
,	O
i.e.	O
</s>
<s>
it	O
provided	O
a	O
virtual	B-Architecture
machine	I-Architecture
environment	O
supporting	O
all	O
aspects	O
of	O
its	O
target	O
computer	O
system	O
(	O
a	O
S/360	O
-40	O
)	O
,	O
such	O
that	O
other	O
S/360	O
operating	B-General_Concept
systems	I-General_Concept
could	O
be	O
installed	O
,	O
tested	O
,	O
and	O
used	O
as	O
if	O
on	O
a	O
stand-alone	O
machine	O
.	O
</s>
<s>
CP-40	B-Application
supported	O
fourteen	O
simultaneous	O
virtual	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
Each	O
virtual	B-Architecture
machine	I-Architecture
ran	O
in	O
"	O
problem	O
state	O
"	O
–	O
privileged	O
instructions	O
such	O
as	O
I/O	O
operations	O
caused	O
exceptions	O
,	O
which	O
were	O
then	O
caught	O
by	O
the	O
control	O
program	O
and	O
simulated	O
.	O
</s>
<s>
Similarly	O
,	O
references	O
to	O
virtual	B-Architecture
memory	I-Architecture
locations	O
not	O
present	O
in	O
main	O
memory	O
cause	O
page	B-General_Concept
faults	I-General_Concept
,	O
which	O
again	O
were	O
handled	O
by	O
control	O
program	O
rather	O
than	O
reflected	O
to	O
the	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Further	O
details	O
on	O
this	O
implementation	O
are	O
found	O
in	O
CP/CMS	B-Application
(	O
architecture	O
)	O
.	O
</s>
<s>
The	O
basic	O
architecture	O
and	O
user	O
interface	O
of	O
CP-40	B-Application
were	O
carried	O
forward	O
into	O
CP-67/CMS	B-Application
,	O
which	O
evolved	O
to	O
become	O
IBM	O
's	O
current	O
VM	B-Application
product	O
line	O
.	O
</s>
<s>
A	O
Model	B-Device
67	I-Device
was	O
not	O
available	O
for	O
building	O
CP-40	B-Application
,	O
so	O
a	O
custom	O
virtual	B-Architecture
memory	I-Architecture
device	O
based	O
on	O
associative	O
memory	O
(	O
the	O
"	O
CAT	O
box	O
"	O
)	O
was	O
designed	O
and	O
built	O
for	O
CSC	O
.	O
</s>
<s>
These	O
changes	O
gave	O
the	O
unit	O
the	O
technology	O
needed	O
for	O
full	B-Architecture
virtualization	I-Architecture
of	O
the	O
System/360	O
hardware	O
.	O
</s>
<s>
This	O
modified	O
Model	O
40	O
influenced	O
the	O
design	O
of	O
the	O
forthcoming	O
Model	B-Device
67	I-Device
,	O
which	O
was	O
intended	O
to	O
meet	O
the	O
needs	O
of	O
the	O
same	O
community	O
of	O
time-sharing	B-General_Concept
users	O
(	O
notably	O
MIT	O
's	O
Project	O
MAC	O
and	O
Bell	O
Laboratories	O
–	O
though	O
both	O
of	O
these	O
sites	O
became	O
notable	O
IBM	O
sales	O
failures	O
)	O
.	O
</s>
<s>
Three	O
distinct	O
virtual	B-Architecture
memory	I-Architecture
systems	O
were	O
implemented	O
by	O
IBM	O
during	O
this	O
period	O
:	O
</s>
<s>
CP-40	B-Application
'	O
s	O
CAT	O
box	O
was	O
a	O
key	O
milestone	O
.	O
</s>
<s>
cite	O
an	O
IEEE	O
paper	O
about	O
the	O
CP-40	B-Application
virtual	B-Architecture
memory	I-Architecture
hardware	O
,	O
and	O
states	O
that	O
it	O
was	O
"	O
unique	O
in	O
that	O
it	O
included	O
a	O
parallel-search	O
register	O
bank	O
to	O
speed	O
dynamic	O
address	O
translation	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
note	O
that	O
,	O
although	O
virtualization	O
support	O
was	O
an	O
explicit	O
goal	O
for	O
CSC	O
's	O
modified	O
Model	O
40	O
,	O
this	O
was	O
not	O
apparently	O
the	O
case	O
for	O
the	O
original	O
Model	B-Device
67	I-Device
design	O
.	O
</s>
<s>
The	O
fact	O
that	O
virtualization	O
capabilities	O
were	O
ultimately	O
implemented	O
in	O
the	O
-67	O
,	O
and	O
thus	O
enabled	O
the	O
success	O
of	O
CP-67/CMS	B-Application
,	O
speaks	O
to	O
the	O
tenacity	O
and	O
persuasiveness	O
of	O
the	O
CSC	O
team	O
.	O
</s>
<s>
CMS	B-Operating_System
was	O
first	O
built	O
in	O
1964	O
at	O
CSC	O
to	O
run	O
as	O
a	O
'	O
client	O
 '	O
operating	B-General_Concept
system	I-General_Concept
under	O
CP-40	B-Application
.	O
</s>
<s>
The	O
CMS	B-Operating_System
project	O
leader	O
was	O
John	O
Harmon	O
.	O
</s>
<s>
Although	O
any	O
S/360	O
operating	B-General_Concept
system	I-General_Concept
could	O
be	O
run	O
in	O
a	O
CP-40	B-Application
virtual	B-Architecture
machine	I-Architecture
,	O
it	O
was	O
decided	O
that	O
a	O
new	O
,	O
simple	O
,	O
single-user	O
interactive	O
operating	B-General_Concept
system	I-General_Concept
would	O
be	O
best	O
for	O
supporting	O
interactive	O
time-sharing	B-General_Concept
users	O
.	O
</s>
<s>
This	O
would	O
avoid	O
the	O
complexity	O
and	O
overhead	O
of	O
running	O
a	O
multi-user	O
system	O
like	O
CTSS	B-Operating_System
.	O
</s>
<s>
(	O
Contrast	O
this	O
with	O
IBM	O
's	O
OS/MVT	B-Application
-TSO	I-Application
and	O
its	O
successors	O
–	O
essentially	O
a	O
time-sharing	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
running	O
as	O
a	O
single	O
task	O
under	O
an	O
IBM	O
batch	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
With	O
CMS	B-Operating_System
,	O
each	O
interactive	O
user	O
gets	O
a	O
private	O
virtual	B-Architecture
machine	I-Architecture
.	O
)	O
</s>
<s>
By	O
September	O
1965	O
,	O
many	O
important	O
CMS	B-Operating_System
design	O
decisions	O
had	O
already	O
been	O
made	O
:	O
</s>
<s>
These	O
were	O
radical	O
departures	O
from	O
the	O
difficult	O
file	O
naming	O
,	O
job	O
control	O
(	O
via	O
JCL	O
)	O
,	O
and	O
other	O
requirements	O
of	O
IBM	O
's	O
"	O
real	O
"	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
(	O
Some	O
of	O
these	O
concepts	O
had	O
been	O
goals	O
for	O
operating	B-General_Concept
systems	I-General_Concept
from	O
other	O
vendors	O
,	O
such	O
as	O
Control	O
Data	O
Corporation	O
and	O
DEC	O
.	O
)	O
</s>
<s>
The	O
CMS	B-Operating_System
file	O
system	O
design	O
,	O
with	O
its	O
flat	O
directory	O
structure	O
,	O
was	O
kept	O
deliberately	O
simple	O
.	O
</s>
<s>
We	O
simplified	O
the	O
design	O
of	O
this	O
and	O
other	O
components	O
of	O
CMS	B-Operating_System
to	O
reduce	O
implementation	O
complexity.	O
"	O
</s>
<s>
Application	O
programs	O
running	O
under	O
CMS	B-Operating_System
executed	O
within	O
the	O
same	O
address	O
space	O
.	O
</s>
<s>
They	O
accessed	O
system	O
services	O
,	O
such	O
as	O
the	O
CMS	B-Operating_System
file	O
system	O
,	O
through	O
a	O
simple	O
programming	O
interface	O
to	O
the	O
CMS	B-Operating_System
nucleus	I-Operating_System
,	O
which	O
resided	O
in	O
low	O
memory	O
within	O
the	O
CMS	B-Operating_System
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
A	O
variety	O
of	O
system	O
calls	O
were	O
provided	O
,	O
most	O
of	O
which	O
would	O
be	O
familiar	O
to	O
current	O
CMS	B-Operating_System
programmers	O
.	O
</s>
<s>
(	O
Since	O
applications	O
ran	O
in	O
the	O
CMS	B-Operating_System
virtual	B-Architecture
machine	I-Architecture
,	O
they	O
could	O
potentially	O
misbehave	O
,	O
by	O
overwriting	O
CMS	B-Operating_System
data	O
,	O
using	O
privileged	O
instructions	O
,	O
or	O
taking	O
other	O
actions	O
that	O
could	O
take	O
over	O
or	O
crash	O
the	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Of	O
course	O
,	O
doing	O
so	O
could	O
not	O
affect	O
other	O
virtual	B-Architecture
machines	I-Architecture
,	O
which	O
were	O
all	O
mutually	O
isolated	O
;	O
nor	O
could	O
it	O
damage	O
the	O
underlying	O
control	O
program	O
.	O
</s>
<s>
Unlike	O
most	O
operating	B-General_Concept
systems	I-General_Concept
,	O
CP	O
crashes	O
rarely	O
stemmed	O
from	O
application	O
errors	O
–	O
and	O
were	O
thus	O
themselves	O
relatively	O
rare	O
.	O
)	O
</s>
<s>
The	O
following	O
notes	O
provide	O
brief	O
quotes	O
,	O
primarily	O
from	O
Pugh	O
,	O
Varian	O
,	O
and	O
Creasy	O
[	O
see	O
references ]	O
,	O
illustrating	O
the	O
development	O
context	O
of	O
CP-40	B-Application
.	O
</s>
<s>
Also	O
see	O
History	B-Application
of	I-Application
CP/CMS	I-Application
for	O
additional	O
context	O
.	O
</s>
<s>
Genesis	O
of	O
the	O
CP-40	B-Application
project	O
:	O
</s>
<s>
CSC	O
's	O
Rasmussen	O
felt	O
"	O
very	O
dubious	O
"	O
about	O
TSS/360	B-Application
and	O
decided	O
that	O
his	O
idle	O
CSC	O
resources	O
should	O
be	O
put	O
to	O
use	O
creating	O
a	O
"	O
credible	O
time-sharing	B-General_Concept
system	I-General_Concept
for	O
the	O
S/360	O
"	O
,	O
which	O
became	O
known	O
as	O
CP-40	B-Application
.	O
</s>
<s>
Project	O
leader	O
was	O
Robert	O
Creasy	O
,	O
who	O
had	O
been	O
a	O
CTSS	B-Operating_System
programmer	O
.	O
</s>
<s>
CP-40	B-Application
goals	O
included	O
both	O
conducting	O
research	O
(	O
obtaining	O
and	O
analyzing	O
data	O
about	O
systems	O
and	O
software	O
,	O
including	O
the	O
use	O
of	O
associative	O
memory	O
)	O
and	O
meeting	O
CSC	O
's	O
own	O
computing	O
requirements	O
via	O
time-sharing	B-General_Concept
.	O
</s>
<s>
Varian	O
adds	O
:	O
"	O
The	O
project	O
’s	O
real	O
purpose	O
was	O
to	O
build	O
a	O
time-sharing	B-General_Concept
system	I-General_Concept
,	O
but	O
the	O
other	O
objectives	O
were	O
genuine	O
,	O
too	O
,	O
and	O
they	O
were	O
always	O
emphasized	O
in	O
order	O
to	O
disguise	O
the	O
project	O
’s	O
“	O
counter-strategic	O
”	O
aspects.	O
"	O
</s>
<s>
Creasy	O
describes	O
CP/CMS	B-Application
goals	O
this	O
way	O
:	O
It	O
was	O
to	O
be	O
"	O
a	O
second	O
generation	O
time-sharing	B-General_Concept
system	I-General_Concept
for	O
the	O
newly	O
announced	O
IBM	O
360	O
...	O
[	O
that	O
would ]	O
support	O
all	O
the	O
activities	O
of	O
the	O
Cambridge	O
center	O
,	O
including	O
such	O
diverse	O
activities	O
as	O
operating	B-General_Concept
system	I-General_Concept
research	O
,	O
application	O
development	O
,	O
and	O
report	O
preparation	O
by	O
programmers	O
,	O
scientists	O
,	O
secretaries	O
,	O
and	O
managers	O
.	O
</s>
<s>
CSC	O
's	O
research	O
was	O
important	O
to	O
IBM	O
,	O
because	O
at	O
the	O
time	O
,	O
"	O
not	O
much	O
was	O
really	O
known	O
about	O
virtual	B-Architecture
memory	I-Architecture
systems.	O
"	O
</s>
<s>
Comeau	O
:	O
"[IBM's]	O
commitment	O
to	O
virtual	B-Architecture
memory	I-Architecture
was	O
backed	O
with	O
no	O
successful	O
experience	O
....	O
What	O
was	O
frightening	O
is	O
that	O
nobody	O
who	O
was	O
setting	O
this	O
virtual	B-Architecture
memory	I-Architecture
direction	O
at	O
IBM	O
knew	O
why	O
[	O
the	O
contemporary	O
virtual	B-Architecture
memory	I-Architecture
system	O
Ferranti ]	O
Atlas	O
did	O
n’t	O
work.	O
"	O
</s>
<s>
(	O
Comeau	O
later	O
concluded	O
that	O
the	O
Atlas	O
had	O
suffered	O
from	O
thrashing	B-General_Concept
,	O
which	O
was	O
not	O
studied	O
until	O
it	O
was	O
observed	O
on	O
the	O
IBM	B-Device
M44/44X	I-Device
and	O
on	O
CP-40	B-Application
.	O
)	O
</s>
<s>
CP-40	B-Application
design	O
:	O
Pugh	O
et	O
al	O
.	O
</s>
<s>
write	O
that	O
:	O
"	O
In	O
1964	O
...	O
IBM	O
Research	O
were	O
recommending	O
the	O
use	O
of	O
virtual-machine	O
principles	O
to	O
time-sharing	B-General_Concept
planners	O
...	O
[	O
which	O
were ]	O
picked	O
up	O
by	O
the	O
Cambridge	O
 [ CSC ] 	O
team	O
,	O
who	O
wanted	O
,	O
among	O
other	O
things	O
,	O
a	O
system	O
capable	O
of	O
testing	O
operating	O
systems.	O
"	O
</s>
<s>
A	O
key	O
design	O
decision	O
,	O
made	O
by	O
Creasy	O
and	O
Comeau	O
at	O
the	O
end	O
of	O
1964	O
,	O
was	O
to	O
base	O
CP-40	B-Application
not	O
just	O
on	O
virtual	B-Architecture
memory	I-Architecture
but	O
on	O
virtual	B-Architecture
machines	I-Architecture
(	O
at	O
first	O
called	O
pseudo-machines	O
,	O
until	O
the	O
later	O
term	O
was	O
appropriated	O
from	O
the	O
IBM	B-Device
M44/44X	I-Device
project	O
–	O
which	O
Creasy	O
describes	O
as	O
having	O
"	O
similar	O
but	O
independent	O
ideas.	O
"	O
</s>
<s>
Creasy	O
provides	O
a	O
clear	O
description	O
of	O
CP	O
's	O
virtualization	O
strategy	O
,	O
based	O
on	O
the	O
S/360	O
instruction	B-General_Concept
set	I-General_Concept
,	O
which	O
consisted	O
of	O
privileged	O
'	O
supervisor	O
state	O
 '	O
instructions	O
distinct	O
from	O
normal	O
'	O
problem	O
state	O
 '	O
instructions	O
:	O
"	O
Each	O
 [ CP ] 	O
virtual	B-Architecture
machine	I-Architecture
program	O
is	O
actually	O
executed	O
 [ entirely ] 	O
in	O
problem	O
state	O
....	O
Privileged	O
instructions	O
...	O
reproduced	O
by	O
CP	O
within	O
the	O
virtual	B-Architecture
machines	I-Architecture
....	O
"	O
By	O
running	O
an	O
OS	O
in	O
problem	O
state	O
,	O
all	O
the	O
'	O
tricky	O
 '	O
instructions	O
would	O
automatically	O
be	O
intercepted	O
by	O
the	O
hardware	O
.	O
</s>
<s>
"	O
Experience	O
[	O
with	O
CTSS ]	O
suggested	O
...	O
the	O
need	O
for	O
dynamic	O
program	O
relocation	O
...	O
to	O
break	O
programs	O
into	O
pieces	O
which	O
could	O
be	O
moved	O
into	O
,	O
out	O
of	O
,	O
and	O
within	O
the	O
memory	O
independently	O
of	O
each	O
other.	O
"	O
</s>
<s>
CP-40	B-Application
virtual	B-Architecture
machines	I-Architecture
:	O
</s>
<s>
Earlier	O
research	O
projects	O
involving	O
the	O
virtual	B-Architecture
machine	I-Architecture
concept	O
,	O
such	O
as	O
the	O
IBM	B-Device
M44/44X	I-Device
,	O
had	O
not	O
attempted	O
to	O
create	O
an	O
exact	O
virtual	O
duplicate	O
of	O
a	O
real	O
machine	O
.	O
</s>
<s>
CP-40	B-Application
took	O
the	O
bold	O
step	O
of	O
full	B-Architecture
virtualization	I-Architecture
,	O
creating	O
fourteen	O
virtual	O
S/360	O
environments	O
,	O
each	O
with	O
a	O
fixed	O
virtual	B-Architecture
memory	I-Architecture
size	O
of	O
256K	O
,	O
mapped	O
access	O
to	O
disk	O
partitions	O
,	O
and	O
spooled	O
access	O
to	O
unit	O
record	O
devices	O
(	O
e.g.	O
</s>
<s>
Comeau	O
:	O
"	O
[	O
Creating	O
full	O
virtualization ]	O
permitted	O
simultaneous	O
development	O
of	O
CP	O
and	O
CMS	B-Operating_System
;	O
it	O
allowed	O
us	O
to	O
measure	O
non-virtual	O
systems	O
,	O
OS	O
and	O
DOS	O
,	O
in	O
a	O
virtual	B-Architecture
memory	I-Architecture
environment	O
,	O
and	O
it	O
also	O
provided	O
a	O
high	O
level	O
of	O
integrity	O
and	O
security.	O
"	O
</s>
<s>
In	O
addition	O
to	O
revealing	O
the	O
value	O
of	O
full	B-Architecture
virtualization	I-Architecture
,	O
the	O
experimental	O
IBM	B-Device
M44/44X	I-Device
"	O
implanted	O
the	O
idea	O
that	O
the	O
virtual	B-Architecture
machine	I-Architecture
concept	O
is	O
not	O
necessarily	O
less	O
efficient	O
than	O
more	O
conventional	O
approaches	O
"	O
–	O
a	O
core	O
assumption	O
in	O
the	O
CP-40	B-Application
architecture	O
,	O
and	O
one	O
that	O
ultimately	O
proved	O
very	O
successful	O
.	O
</s>
<s>
CP-40	B-Application
would	O
soon	O
support	O
"	O
up	O
to	O
a	O
dozen	O
virtual	O
System/360	O
machines	O
"	O
under	O
terminal	O
control	O
[	O
most	O
sources	O
state	O
fourteen ]	O
.	O
</s>
<s>
(	O
CP-67	B-Application
later	O
"	O
exploited	O
...	O
address	O
translation	O
...	O
and	O
increased	O
speed	O
...	O
to	O
double	O
the	O
capacity	O
"	O
of	O
CP-40	B-Application
.	O
)	O
</s>
<s>
CMS	B-Operating_System
under	O
CP-40	B-Application
:	O
</s>
<s>
About	O
the	O
decision	O
to	O
separate	O
CMS	B-Operating_System
and	O
CP	O
,	O
Creasy	O
writes	O
:	O
"	O
The	O
implementation	O
of	O
CTSS	B-Operating_System
illustrated	O
the	O
necessity	O
of	O
modular	O
design	O
for	O
system	O
evolution	O
.	O
</s>
<s>
A	O
key	O
concept	O
of	O
the	O
CP/CMS	B-Application
design	O
was	O
the	O
bifurcation	O
of	O
computer	O
resource	O
management	O
and	O
user	O
support	O
.	O
</s>
<s>
In	O
effect	O
,	O
the	O
integrated	O
design	O
[	O
of	O
CTSS ]	O
was	O
split	O
into	O
CP	O
and	O
CMS.	O
"	O
</s>
<s>
The	O
value	O
of	O
experience	O
gained	O
on	O
the	O
CTSS	B-Operating_System
project	O
cannot	O
be	O
overstated	O
.	O
</s>
<s>
About	O
early	O
CMS	B-Operating_System
,	O
Creasy	O
writes	O
:	O
CMS	B-Operating_System
"	O
provided	O
single	O
user	O
service	O
unencumbered	O
by	O
the	O
problems	O
of	O
sharing	O
,	O
allocation	O
,	O
and	O
protection.	O
"	O
</s>
<s>
Early	O
CMS	B-Operating_System
development	O
involved	O
booting	O
CMS	B-Operating_System
under	O
BPS	B-Application
,	O
an	O
early	O
S/360	O
support	O
system	O
,	O
until	O
CMS	B-Operating_System
was	O
far	O
enough	O
along	O
to	O
boot	O
stand-alone	O
.	O
</s>
<s>
Eventually	O
development	O
moved	O
to	O
virtual	B-Architecture
machines	I-Architecture
under	O
CP	O
.	O
</s>
