<s>
VP/CSS	B-Application
was	O
a	O
time-sharing	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
developed	O
by	O
National	O
CSS	O
.	O
</s>
<s>
It	O
began	O
life	O
in	O
1968	O
as	O
a	O
copy	O
of	O
IBM	O
's	O
CP/CMS	B-Application
,	O
which	O
at	O
the	O
time	O
was	O
distributed	O
to	O
IBM	O
customers	O
at	O
no	O
charge	O
,	O
in	O
source	O
code	O
form	O
,	O
without	O
support	O
,	O
as	O
part	O
of	O
the	O
IBM	B-Application
Type-III	I-Application
Library	I-Application
.	O
</s>
<s>
Through	O
extensive	O
in-house	O
development	O
,	O
in	O
what	O
today	O
would	O
be	O
termed	O
a	O
software	B-Application
fork	I-Application
,	O
National	O
CSS	O
took	O
VP/CSS	B-Application
in	O
a	O
different	O
direction	O
from	O
CP/CMS	B-Application
.	O
</s>
<s>
VP/CSS	B-Application
ran	O
on	O
IBM	O
and	O
IBM	O
plug	B-Device
compatible	I-Device
hardware	O
owned	O
by	O
NCSS	O
(	O
and	O
by	O
a	O
few	O
customers	O
with	O
site	O
licenses	O
,	O
including	O
Bank	O
of	O
America	O
and	O
Standard	O
Oil	O
of	O
California	O
)	O
.	O
</s>
<s>
After	O
an	O
initial	O
period	O
running	O
on	O
the	O
IBM	B-Device
System/360	I-Device
-67	I-Device
platform	O
used	O
by	O
CP/CMS	B-Application
,	O
VP/CSS	B-Application
was	O
ported	O
to	O
the	O
System/370	B-Device
series	O
,	O
made	O
possible	O
when	O
IBM	O
added	O
virtual	B-Architecture
memory	I-Architecture
capabilities	O
to	O
the	O
S/370	B-Device
series	O
in	O
1972	O
.	O
</s>
<s>
VP/CSS	B-Application
was	O
notable	O
for	O
supporting	O
very	O
large	O
numbers	O
of	O
interactive	O
users	O
per	O
machine	O
,	O
when	O
compared	O
with	O
other	O
IBM	O
mainframe	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
VP/CSS	B-Application
shared	O
the	O
basic	O
architecture	O
and	O
concepts	O
of	O
CP/CMS	B-Application
,	O
which	O
were	O
revolutionary	O
for	O
their	O
time	O
.	O
</s>
<s>
A	O
control	O
program	O
(	O
called	O
CP	O
in	O
CP/CMS	B-Application
,	O
VP	O
in	O
VP/CSS	B-Application
)	O
created	O
multiple	O
independent	O
virtual	B-Architecture
machines	I-Architecture
(	O
VMs	B-Application
)	O
,	O
implementing	O
a	O
full	B-Architecture
virtualization	I-Architecture
of	O
the	O
underlying	O
hardware	O
–	O
meaning	O
that	O
each	O
time-sharing	B-General_Concept
user	O
was	O
provided	O
with	O
a	O
private	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Each	O
appeared	O
to	O
be	O
an	O
entire	O
,	O
stand-alone	O
computer	O
,	O
capable	O
of	O
running	O
any	O
software	O
that	O
could	O
run	O
on	O
the	O
bare	O
machine	O
,	O
including	O
other	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
(	O
This	O
concept	O
was	O
pioneered	O
with	O
IBM	O
's	O
research	O
system	O
CP-40	B-Application
in	O
the	O
first	O
version	O
of	O
CP/CMS	B-Application
.	O
)	O
</s>
<s>
This	O
design	O
was	O
a	O
departure	O
from	O
IBM	O
's	O
other	O
monolithic	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Isolating	O
users	O
from	O
each	O
other	O
improved	O
system	O
stability	O
:	O
a	O
bug	O
in	O
one	O
user	O
's	O
software	O
could	O
not	O
crash	O
another	O
user	O
's	O
virtual	B-Architecture
machine	I-Architecture
,	O
nor	O
the	O
underlying	O
control	O
program	O
.	O
</s>
<s>
This	O
approach	O
made	O
CP/CMS	B-Application
a	O
superior	O
choice	O
for	O
commercial	O
time-sharing	B-General_Concept
,	O
and	O
thus	O
a	O
strong	O
foundation	O
for	O
National	O
CSS	O
as	O
it	O
broke	O
new	O
ground	O
in	O
what	O
would	O
become	O
an	O
important	O
new	O
industry	O
.	O
</s>
<s>
Each	O
VM	B-Application
created	O
by	O
VP	O
ran	O
a	O
simple	O
,	O
single-user	O
operating	B-General_Concept
system	I-General_Concept
called	O
CSS	O
,	O
derived	O
from	O
IBM	O
's	O
Cambridge	B-Operating_System
Monitor	I-Operating_System
System	I-Operating_System
.	O
</s>
<s>
Continuous	O
development	O
and	O
introduction	O
of	O
new	O
features	O
in	O
both	O
VP/CSS	B-Application
and	O
IBM	O
's	O
VM	B-Application
led	O
to	O
significant	O
differences	O
over	O
time	O
;	O
but	O
the	O
systems	O
retained	O
an	O
obvious	O
family	O
resemblance	O
.	O
</s>
<s>
IBM	O
's	O
decision	O
to	O
add	O
virtualization	O
and	O
virtual	B-Architecture
memory	I-Architecture
features	O
to	O
the	O
S/370	B-Device
reflects	O
the	O
success	O
of	O
the	O
virtual	B-Architecture
machine	I-Architecture
approach	O
to	O
time-sharing	B-General_Concept
.	O
</s>
<s>
In	O
1968	O
,	O
the	O
founders	O
of	O
National	O
CSS	O
saw	O
that	O
the	O
CP/CMS	B-Application
operating	B-General_Concept
system	I-General_Concept
would	O
be	O
a	O
good	O
foundation	O
for	O
a	O
time-sharing	B-General_Concept
business	O
–	O
because	O
of	O
the	O
system	O
's	O
technical	O
merits	O
,	O
its	O
ability	O
to	O
share	O
mainframe	O
resources	O
among	O
many	O
interactive	O
users	O
,	O
and	O
its	O
availability	O
in	O
source	O
code	O
form	O
at	O
no	O
cost	O
.	O
</s>
<s>
Each	O
firm	O
lured	O
away	O
key	O
CP/CMS	B-Application
technical	O
personnel	O
from	O
CSC	O
,	O
MIT	O
,	O
and	O
Union	O
Carbide	O
.	O
</s>
<s>
National	O
CSS	O
quickly	O
discovered	O
,	O
however	O
,	O
that	O
CP/CMS	B-Application
initial	O
performance	O
was	O
not	O
adequate	O
to	O
sustain	O
profitable	O
operations	O
–	O
that	O
,	O
literally	O
,	O
selling	O
every	O
available	O
minute	O
of	O
interactive	O
time	O
would	O
only	O
pay	O
for	O
the	O
$	O
50K/month	O
equipment	O
lease	O
.	O
</s>
<s>
At	O
the	O
end	O
of	O
its	O
lifespan	O
,	O
VP/CSS	B-Application
had	O
diverged	O
a	O
long	O
way	O
from	O
its	O
CP/CMS	B-Application
roots	O
,	O
and	O
boasted	O
a	O
surprising	O
array	O
of	O
features	O
,	O
some	O
of	O
which	O
would	O
be	O
considered	O
quite	O
modern	O
even	O
today	O
.	O
</s>
<s>
Key	O
enhancements	O
to	O
the	O
original	O
CP/CMS	B-Application
system	O
included	O
changes	O
in	O
the	O
dispatching	O
algorithm	O
and	O
the	O
paging	O
system	O
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
was	O
of	O
course	O
a	O
new	O
concept	O
at	O
the	O
time	O
,	O
and	O
the	O
IBM	B-Device
System/360	I-Device
-67	I-Device
address	O
translation	O
technology	O
enabled	O
various	O
technical	O
approaches	O
.	O
</s>
<s>
Ultimately	O
,	O
the	O
VP/CSS	B-Application
page	O
migration	O
algorithm	O
and	O
three-queue	O
dispatcher	O
became	O
well-known	O
,	O
and	O
some	O
NCSS	O
personnel	O
eventually	O
joined	O
IBM	O
's	O
Thomas	O
J	O
.	O
Watson	O
Research	O
Center	O
to	O
work	O
on	O
VM	B-Application
technologies	O
.	O
</s>
<s>
Another	O
area	O
for	O
throughput	O
improvement	O
was	O
in	O
the	O
performance	O
of	O
the	O
CSS	O
single-user	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
One	O
important	O
change	O
was	O
replacing	O
Channel	O
Command	O
Words	O
(	O
CCWs	O
)	O
and	O
other	O
expensive	O
simulated	O
instructions	O
with	O
something	O
like	O
what	O
today	O
are	O
termed	O
BIOS	B-Device
calls	I-Device
.	O
</s>
<s>
Simulating	O
the	O
complex	O
S/360	O
I/O	O
architecture	O
through	O
virtualization	O
was	O
an	O
amazing	O
feat	O
–	O
done	O
in	O
CP	O
's	O
complex	O
innermost	O
core	O
,	O
in	O
a	O
routine	O
called	O
"	O
CCWTRANS	O
,	O
"	O
as	O
I/O	O
operations	O
were	O
trapped	O
within	O
each	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
However	O
,	O
it	O
proved	O
enormously	O
cheaper	O
to	O
make	O
direct	O
hypervisor	B-Operating_System
calls	O
for	O
targeted	O
functions	O
,	O
rather	O
than	O
simulating	O
the	O
operation	O
of	O
low-level	O
I/O	O
commands	O
.	O
</s>
<s>
In	O
VP/CSS	B-Application
,	O
this	O
was	O
done	O
using	O
paravirtualization	O
via	O
the	O
non-virtualized	O
DIAG	O
(	O
diagnose	O
)	O
instruction	O
.	O
</s>
<s>
The	O
same	O
technique	O
was	O
used	O
by	O
IBM	O
in	O
CP/CMS	B-Application
release	O
3.1	O
,	O
and	O
carried	O
forward	O
into	O
VM/370	B-Application
.	O
</s>
<s>
Early	O
National	O
CSS	O
technical	O
efforts	O
quickly	O
established	O
VP/CSS	B-Application
as	O
a	O
commercially	O
viable	O
version	O
of	O
CP/CMS	B-Application
.	O
</s>
<s>
VP/CSS	B-Application
was	O
reputed	O
to	O
have	O
much	O
better	O
performance	O
than	O
IBM	O
's	O
reimplementation	O
of	O
CP/CMS	B-Application
,	O
VM/370	B-Application
–	O
which	O
in	O
turn	O
was	O
reputed	O
to	O
have	O
a	O
substantial	O
performance	O
advantage	O
over	O
IBM	O
's	O
"	O
preferred	O
"	O
timesharing	B-General_Concept
solution	O
,	O
TSO	O
.	O
</s>
<s>
Regarding	O
VM/CMS	B-Application
performance	O
relative	O
to	O
OS/TSO	O
:	O
</s>
<s>
Numerous	O
VM	B-Application
documents	O
,	O
such	O
as	O
Varian	O
's	O
famous	O
paper	O
,	O
cite	O
"	O
CP	O
's	O
performance	O
advantages	O
over	O
TSO	O
"	O
.	O
</s>
<s>
From	O
structural	O
arguments	O
,	O
it	O
is	O
reasonable	O
that	O
CMS	O
under	O
VM	B-Application
should	O
consistently	O
outperform	O
TSO	O
under	O
OS	O
.	O
</s>
<s>
VM	B-Application
was	O
designed	O
as	O
a	O
time-sharing	B-General_Concept
system	I-General_Concept
,	O
and	O
had	O
a	O
substantial	O
technical	O
edge	O
in	O
running	O
interactive	O
applications	O
.	O
</s>
<s>
(	O
The	O
MVS	B-Application
performance	O
group	O
famously	O
adopted	O
the	O
turkey	O
as	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
mascot	O
.	O
)	O
</s>
<s>
CP/CMS	B-Application
started	O
with	O
good	O
relative	O
performance	O
;	O
and	O
then	O
a	O
broad	O
range	O
of	O
performance	O
improvements	O
followed	O
.	O
</s>
<s>
Regarding	O
VP/CSS	B-Application
performance	O
relative	O
to	O
VM/CMS	B-Application
:	O
</s>
<s>
NCSS	O
succeeded	O
in	O
selling	O
large	O
VP/CSS	B-Application
site	O
license	O
installations	O
to	O
the	O
likes	O
of	O
Bank	O
of	O
America	O
and	O
Standard	O
Oil	O
of	O
California	O
for	O
time-sharing	B-General_Concept
use	O
–	O
in	O
spite	O
of	O
the	O
manifest	O
problems	O
that	O
such	O
large	O
IBM	O
customers	O
would	O
face	O
by	O
going	O
against	O
the	O
IBM	O
mainstream	O
.	O
</s>
<s>
As	O
described	O
in	O
History	B-Application
of	I-Application
CP/CMS	I-Application
,	O
IBM	O
's	O
primary	O
emphasis	O
on	O
MVS	B-Application
and	O
its	O
successors	O
as	O
its	O
core	O
mainframe	O
operating	B-General_Concept
system	I-General_Concept
led	O
IBM	O
to	O
waste	O
the	O
substantial	O
technical	O
advantage	O
represented	O
by	O
VM/370	B-Application
.	O
</s>
<s>
Ultimately	O
,	O
after	O
Amdahl	O
Corporation	O
publicized	O
its	O
sales	O
wins	O
at	O
several	O
large	O
VP/CSS	B-Application
data	O
centers	O
,	O
IBM	O
began	O
to	O
pay	O
more	O
public	O
attention	O
to	O
NCSS	O
and	O
its	O
technical	O
innovations	O
.	O
</s>
<s>
Not	O
long	O
thereafter	O
,	O
as	O
the	O
time-sharing	B-General_Concept
industry	O
began	O
to	O
feel	O
pressure	O
from	O
the	O
personal	B-Device
computer	I-Device
revolution	O
,	O
the	O
need	O
for	O
a	O
super-optimized	O
multiuser	O
mainframe	O
operating	B-General_Concept
system	I-General_Concept
waned	O
.	O
</s>
<s>
NCSS	O
moved	O
away	O
from	O
VP/CSS	B-Application
in	O
the	O
mid	O
80s	O
,	O
retargeting	O
Nomad	O
for	O
VM	B-Application
and	O
other	O
platforms	O
.	O
</s>
<s>
It	O
is	O
unknown	O
how	O
long	O
any	O
NCSS	O
site-license	O
customers	O
continued	O
to	O
use	O
VP/CSS	B-Application
on	O
their	O
own	O
equipment	O
(	O
though	O
it	O
seems	O
unlikely	O
any	O
could	O
continue	O
for	O
long	O
without	O
ongoing	O
NCSS	O
operating	B-General_Concept
system	I-General_Concept
support	O
)	O
.	O
</s>
