<s>
VAX	B-Device
(	O
an	O
acronym	O
for	O
Virtual	O
Address	O
eXtension	O
)	O
is	O
a	O
series	O
of	O
computers	O
featuring	O
a	O
32-bit	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
(	O
ISA	O
)	O
and	O
virtual	B-Architecture
memory	I-Architecture
that	O
was	O
developed	O
and	O
sold	O
by	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
in	O
the	O
late	O
20th	O
century	O
.	O
</s>
<s>
The	O
VAX-11/780	O
,	O
introduced	O
October	O
25	O
,	O
1977	O
,	O
was	O
the	O
first	O
of	O
a	O
range	O
of	O
popular	O
and	O
influential	O
computers	O
implementing	O
the	O
VAX	B-Device
ISA	O
.	O
</s>
<s>
The	B-Device
VAX	I-Device
family	I-Device
was	O
a	O
huge	O
success	O
for	O
DEC	O
–	O
over	O
100	O
models	O
were	O
introduced	O
over	O
the	O
lifetime	O
of	O
the	O
design	O
,	O
with	O
the	O
last	O
members	O
arriving	O
in	O
the	O
early	O
1990s	O
.	O
</s>
<s>
The	O
VAX	B-Device
was	O
succeeded	O
by	O
the	O
DEC	B-Device
Alpha	I-Device
,	O
which	O
included	O
several	O
features	O
from	O
VAX	B-Device
machines	O
to	O
make	O
porting	O
from	O
the	O
VAX	B-Device
easier	O
.	O
</s>
<s>
VAX	B-Device
was	O
designed	O
as	O
a	O
successor	O
to	O
the	O
16-bit	B-Device
PDP-11	B-Device
,	O
one	O
of	O
the	O
most	O
successful	O
minicomputers	B-Architecture
in	O
history	O
with	O
approximately	O
600,000	O
examples	O
sold	O
.	O
</s>
<s>
The	O
system	O
was	O
designed	O
to	O
offer	O
backward	B-General_Concept
compatibility	I-General_Concept
with	O
the	O
PDP-11	B-Device
while	O
extending	O
the	O
memory	O
to	O
a	O
full	O
32-bit	O
implementation	O
and	O
adding	O
demand	B-Architecture
paged	I-Architecture
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
The	O
name	O
VAX	B-Device
refers	O
to	O
its	O
Virtual	O
Address	O
eXtension	O
concept	O
that	O
allowed	O
programs	O
to	O
make	O
use	O
of	O
this	O
newly	O
available	O
memory	O
while	O
still	O
being	O
compatible	O
with	O
unmodified	O
user	O
mode	O
PDP-11	B-Device
code	O
.	O
</s>
<s>
The	O
name	O
"	O
VAX-11	O
"	O
,	O
used	O
on	O
early	O
models	O
,	O
was	O
chosen	O
to	O
highlight	O
this	O
capability	O
.	O
</s>
<s>
The	O
VAX	B-Device
ISA	O
is	O
considered	O
a	O
complex	B-Architecture
instruction	I-Architecture
set	I-Architecture
computer	I-Architecture
(	O
CISC	B-Architecture
)	O
design	O
.	O
</s>
<s>
Later	O
models	O
in	O
the	O
series	O
dropped	O
the	O
−11	O
branding	O
as	O
PDP-11	B-Device
compatibility	O
was	O
no	O
longer	O
a	O
major	O
concern	O
.	O
</s>
<s>
The	O
line	O
expanded	O
to	O
both	O
high-end	O
machines	O
like	O
the	O
VAX	B-Device
9000	I-Device
as	O
well	O
as	O
to	O
the	O
workstation-scale	O
systems	O
like	O
the	O
VAXstation	B-Device
series	O
.	O
</s>
<s>
The	B-Device
VAX	I-Device
family	I-Device
ultimately	O
contained	O
ten	O
distinct	O
designs	O
and	O
over	O
100	O
individual	O
models	O
in	O
total	O
.	O
</s>
<s>
All	O
of	O
these	O
were	O
compatible	O
with	O
each	O
other	O
and	O
normally	O
ran	O
the	O
VAX/VMS	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
VAX	B-Device
has	O
been	O
perceived	O
as	O
the	O
quintessential	O
CISC	B-Architecture
ISA	O
,	O
with	O
its	O
very	O
large	O
number	O
of	O
assembly	B-Language
language	I-Language
programmer-friendly	O
addressing	B-Language
modes	I-Language
and	O
machine	O
instructions	O
,	O
highly	O
orthogonal	B-General_Concept
instruction	I-General_Concept
set	I-General_Concept
architecture	O
,	O
and	O
instructions	O
for	O
complex	O
operations	O
such	O
as	O
queue	B-Application
insertion	O
or	O
deletion	O
,	O
number	O
formatting	O
,	O
and	O
polynomial	O
evaluation	O
.	O
</s>
<s>
The	O
name	O
"	O
VAX	B-Device
"	O
originated	O
as	O
an	O
acronym	O
for	O
Virtual	O
Address	O
eXtension	O
,	O
both	O
because	O
the	O
VAX	B-Device
was	O
seen	O
as	O
a	O
32-bit	O
extension	O
of	O
the	O
older	O
16-bit	B-Device
PDP-11	B-Device
and	O
because	O
it	O
was	O
(	O
after	O
Prime	O
Computer	O
)	O
an	O
early	O
adopter	O
of	O
virtual	B-Architecture
memory	I-Architecture
to	O
manage	O
this	O
larger	O
address	O
space	O
.	O
</s>
<s>
Early	O
versions	O
of	O
the	O
VAX	B-Device
processor	O
implement	O
a	O
"	O
compatibility	O
mode	O
"	O
that	O
emulates	O
many	O
of	O
the	O
PDP-11	B-Device
'	O
s	O
instructions	O
,	O
giving	O
it	O
the	O
11	O
in	O
VAX-11	O
to	O
highlight	O
this	O
compatibility	O
.	O
</s>
<s>
Later	O
versions	O
offloaded	O
the	O
compatibility	O
mode	O
and	O
some	O
of	O
the	O
less	O
used	O
CISC	B-Architecture
instructions	O
to	O
emulation	O
in	O
the	O
operating	B-General_Concept
system	I-General_Concept
software	O
.	O
</s>
<s>
The	O
VAX	B-Device
instruction	B-General_Concept
set	I-General_Concept
was	O
designed	O
to	O
be	O
powerful	O
and	O
orthogonal	B-General_Concept
.	O
</s>
<s>
When	O
it	O
was	O
introduced	O
,	O
many	O
programs	O
were	O
written	O
in	O
assembly	B-Language
language	I-Language
,	O
so	O
having	O
a	O
"	O
programmer-friendly	O
"	O
instruction	B-General_Concept
set	I-General_Concept
was	O
important	O
.	O
</s>
<s>
In	O
time	O
,	O
as	O
more	O
programs	O
were	O
written	O
in	O
high-level	B-Language
programming	I-Language
languages	I-Language
,	O
the	O
instruction	B-General_Concept
set	I-General_Concept
became	O
less	O
visible	O
,	O
and	O
the	O
only	O
ones	O
much	O
concerned	O
about	O
it	O
were	O
compiler	O
writers	O
.	O
</s>
<s>
One	O
unusual	O
aspect	O
of	O
the	O
VAX	B-Device
instruction	B-General_Concept
set	I-General_Concept
is	O
the	O
presence	O
of	O
register	O
masks	O
at	O
the	O
start	O
of	O
each	O
subprogram	O
.	O
</s>
<s>
The	O
native	O
VAX	B-Device
operating	B-General_Concept
system	I-General_Concept
is	O
Digital	O
's	O
VAX/VMS	B-Operating_System
(	O
renamed	O
to	O
OpenVMS	B-Operating_System
in	O
1991	O
or	O
early	O
1992	O
when	O
it	O
was	O
ported	O
to	O
Alpha	B-Device
,	O
modified	O
to	O
comply	O
with	O
POSIX	O
standards	O
,	O
and	O
branded	O
as	O
compliant	O
with	O
XPG4	O
by	O
the	O
X/Open	O
consortium	O
)	O
.	O
</s>
<s>
The	O
VAX	B-Device
architecture	O
and	O
VMS	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
were	O
"	O
engineered	O
concurrently	O
"	O
to	O
take	O
maximum	O
advantage	O
of	O
each	O
other	O
,	O
as	O
was	O
the	O
initial	O
implementation	O
of	O
the	O
VAXcluster	B-Operating_System
facility	O
.	O
</s>
<s>
During	O
the	O
1980s	O
,	O
a	O
hypervisor	B-Operating_System
for	O
the	O
VAX	B-Device
architecture	O
named	O
VMM	O
(	O
Virtual	B-Operating_System
Machine	I-Operating_System
Monitor	I-Operating_System
)	O
,	O
also	O
known	O
as	O
the	O
VAX	B-Device
Security	O
Kernel	O
,	O
was	O
developed	O
at	O
Digital	O
with	O
the	O
aim	O
of	O
allowing	O
multiple	O
isolated	O
instances	O
of	O
VMS	B-Operating_System
and	O
ULTRIX	B-Operating_System
to	O
be	O
run	O
on	O
the	O
same	O
hardware	O
.	O
</s>
<s>
By	O
the	O
late	O
1980s	O
,	O
it	O
was	O
operational	O
on	O
VAX	B-Device
8000	I-Device
series	O
hardware	O
,	O
but	O
was	O
abandoned	O
before	O
release	O
to	O
customers	O
.	O
</s>
<s>
Other	O
VAX	B-Device
operating	B-General_Concept
systems	I-General_Concept
have	O
included	O
various	O
releases	O
of	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
(	O
BSD	B-Operating_System
)	O
UNIX	B-Application
up	O
to	O
4.3BSD	B-Operating_System
,	O
Ultrix-32	B-Operating_System
,	O
VAXELN	B-Operating_System
,	O
and	O
Xinu	B-Operating_System
.	O
</s>
<s>
More	O
recently	O
,	O
NetBSD	B-Device
and	O
OpenBSD	B-Operating_System
have	O
supported	O
various	O
VAX	B-Device
models	O
and	O
some	O
work	O
has	O
been	O
done	O
on	O
porting	O
Linux	B-Application
to	O
the	O
VAX	B-Device
architecture	O
.	O
</s>
<s>
OpenBSD	B-Operating_System
discontinued	O
support	O
for	O
the	O
architecture	O
in	O
September	O
2016	O
.	O
</s>
<s>
The	O
first	O
VAX	B-Device
model	O
sold	O
was	O
the	O
VAX-11/780	O
,	O
which	O
was	O
introduced	O
on	O
October	O
25	O
,	O
1977	O
,	O
at	O
the	O
Digital	O
Equipment	O
Corporation	O
's	O
Annual	O
Meeting	O
of	O
Shareholders	O
.	O
</s>
<s>
VAX	B-Device
superminicomputers	B-Device
were	O
very	O
popular	O
in	O
the	O
early	O
1980s	O
.	O
</s>
<s>
For	O
a	O
while	O
the	O
VAX-11/780	O
was	O
used	O
as	O
a	O
standard	O
in	O
CPU	B-General_Concept
benchmarks	O
.	O
</s>
<s>
It	O
was	O
initially	O
described	O
as	O
a	O
one-MIPS	O
machine	O
,	O
because	O
its	O
performance	O
was	O
equivalent	O
to	O
an	O
IBM	B-Application
System/360	I-Application
that	O
ran	O
at	O
one	O
MIPS	O
,	O
and	O
the	O
System/360	B-Application
implementations	O
had	O
previously	O
been	O
de	O
facto	O
performance	O
standards	O
.	O
</s>
<s>
The	O
result	O
was	O
the	O
definition	O
of	O
a	O
"	O
VAX	B-Device
MIPS	O
,	O
"	O
the	O
speed	O
of	O
a	O
VAX-11/780	O
;	O
a	O
computer	O
performing	O
at	O
27	O
VAX	B-Device
MIPS	O
would	O
run	O
the	O
same	O
program	O
roughly	O
27	O
times	O
faster	O
than	O
the	O
VAX-11/780	O
.	O
</s>
<s>
Within	O
the	O
Digital	O
community	O
the	O
term	O
VUP	O
(	O
VAX	B-Device
Unit	O
of	O
Performance	O
)	O
was	O
the	O
more	O
common	O
term	O
,	O
because	O
MIPS	O
do	O
not	O
compare	O
well	O
across	O
different	O
architectures	O
.	O
</s>
<s>
The	O
related	O
term	O
cluster	O
VUPs	O
was	O
informally	O
used	O
to	O
describe	O
the	O
aggregate	O
performance	O
of	O
a	O
VAXcluster	B-Operating_System
.	O
</s>
<s>
(	O
The	O
performance	O
of	O
the	O
VAX-11/780	O
still	O
serves	O
as	O
the	O
baseline	O
metric	O
in	O
the	O
BRL-CAD	B-Language
Benchmark	O
,	O
a	O
performance	O
analysis	O
suite	O
included	O
in	O
the	O
BRL-CAD	B-Language
solid	O
modeling	O
software	O
distribution	O
.	O
)	O
</s>
<s>
The	O
VAX-11/780	O
included	O
a	O
subordinate	O
stand-alone	O
LSI-11	B-Device
computer	O
that	O
performed	O
microcode	B-Device
load	O
,	O
booting	O
,	O
and	O
diagnostic	O
functions	O
for	O
the	O
parent	O
computer	O
.	O
</s>
<s>
This	O
was	O
dropped	O
from	O
subsequent	O
VAX	B-Device
models	O
.	O
</s>
<s>
Enterprising	O
VAX-11/780	O
users	O
could	O
therefore	O
run	O
three	O
different	O
Digital	O
Equipment	O
Corporation	O
operating	B-General_Concept
systems	I-General_Concept
:	O
VMS	B-Operating_System
on	O
the	O
VAX	B-Device
processor	O
(	O
from	O
the	O
hard	O
drives	O
)	O
,	O
and	O
either	O
RSX-11S	O
or	O
RT-11	O
on	O
the	O
LSI-11	B-Device
(	O
from	O
the	O
single	O
density	O
single	O
drive	O
floppy	O
disk	O
)	O
.	O
</s>
<s>
The	O
VAX	B-Device
went	O
through	O
many	O
different	O
implementations	O
.	O
</s>
<s>
The	O
original	O
VAX	B-Device
11/780	O
was	O
implemented	O
in	O
TTL	B-General_Concept
and	O
filled	O
a	O
four-by-five-foot	O
cabinet	O
with	O
a	O
single	O
CPU	B-General_Concept
.	O
</s>
<s>
Through	O
the	O
1980s	O
,	O
the	O
high-end	O
of	O
the	O
family	O
was	O
continually	O
improved	O
using	O
ever-faster	O
discrete	O
components	O
,	O
an	O
evolution	O
that	O
ended	O
with	O
the	O
introduction	O
of	O
the	O
VAX	B-Device
9000	I-Device
in	O
October	O
1989	O
.	O
</s>
<s>
CPU	B-General_Concept
implementations	O
that	O
consisted	O
of	O
multiple	O
emitter-coupled	B-General_Concept
logic	I-General_Concept
(	O
ECL	O
)	O
gate	O
array	O
or	O
macrocell	O
array	O
chips	O
included	O
the	O
VAX	B-Device
8600	I-Device
and	I-Device
8800	I-Device
superminis	O
and	O
finally	O
the	O
VAX	B-Device
9000	I-Device
mainframe	B-Architecture
class	O
machines	O
.	O
</s>
<s>
CPU	B-General_Concept
implementations	O
that	O
consisted	O
of	O
multiple	O
MOSFET	B-Architecture
custom	O
chips	O
included	O
the	O
8100	O
and	O
8200	O
class	O
machines	O
.	O
</s>
<s>
The	O
VAX	B-Device
11-730	O
and	O
725	O
low-end	O
machines	O
were	O
built	O
using	O
AMD	B-General_Concept
Am2901	I-General_Concept
bit-slice	B-General_Concept
components	O
for	O
the	O
ALU	B-General_Concept
.	O
</s>
<s>
The	O
MicroVAX	B-Device
I	O
represented	O
a	O
major	O
transition	O
within	O
the	B-Device
VAX	I-Device
family	I-Device
.	O
</s>
<s>
At	O
the	O
time	O
of	O
its	O
design	O
,	O
it	O
was	O
not	O
yet	O
possible	O
to	O
implement	O
the	O
full	O
VAX	B-Device
architecture	O
as	O
a	O
single	O
VLSI	O
chip	O
(	O
or	O
even	O
a	O
few	O
VLSI	O
chips	O
as	O
was	O
later	O
done	O
with	O
the	O
V-11	B-Device
CPU	B-General_Concept
of	O
the	O
VAX	B-Device
8200/8300	O
)	O
.	O
</s>
<s>
Instead	O
,	O
the	O
MicroVAX	B-Device
I	O
was	O
the	O
first	O
VAX	B-Device
implementation	O
to	O
move	O
some	O
of	O
the	O
more	O
complex	O
VAX	B-Device
instructions	O
(	O
such	O
as	O
the	O
packed	O
decimal	O
and	O
related	O
opcodes	O
)	O
into	O
emulation	O
software	O
.	O
</s>
<s>
This	O
partitioning	O
substantially	O
reduced	O
the	O
amount	O
of	O
microcode	B-Device
required	O
and	O
was	O
referred	O
to	O
as	O
the	O
"	O
MicroVAX	B-Device
"	O
architecture	O
.	O
</s>
<s>
In	O
the	O
MicroVAX	B-Device
I	O
,	O
the	O
ALU	B-General_Concept
and	O
registers	O
were	O
implemented	O
as	O
a	O
single	O
gate-array	O
chip	O
while	O
the	O
rest	O
of	O
the	O
machine	O
control	O
was	O
conventional	O
logic	O
.	O
</s>
<s>
A	O
full	O
VLSI	O
(	O
microprocessor	B-Architecture
)	O
implementation	O
of	O
the	O
MicroVAX	B-Device
architecture	O
arrived	O
with	O
the	O
MicroVAX	B-Device
II	I-Device
's	I-Device
78032	I-Device
(	O
or	O
DC333	O
)	O
CPU	B-General_Concept
and	O
78132	O
(	O
DC335	O
)	O
FPU	O
.	O
</s>
<s>
The	O
78032	O
was	O
the	O
first	O
microprocessor	B-Architecture
with	O
an	O
on-board	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
The	O
MicroVAX	B-Device
II	O
was	O
based	O
on	O
a	O
single	O
,	O
quad-sized	O
processor	O
board	O
which	O
carried	O
the	O
processor	O
chips	O
and	O
ran	O
the	O
MicroVMS	O
or	O
Ultrix-32	B-Operating_System
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
The	O
machine	O
featured	O
1	O
MB	O
of	O
on-board	O
memory	O
and	O
a	O
Q22-bus	B-Architecture
interface	O
with	O
DMA	B-General_Concept
transfers	O
.	O
</s>
<s>
The	O
MicroVAX	B-Device
II	O
was	O
succeeded	O
by	O
many	O
further	O
MicroVAX	B-Device
models	O
with	O
much	O
improved	O
performance	O
and	O
memory	O
.	O
</s>
<s>
Further	O
VLSI	O
VAX	B-Device
processors	O
followed	O
in	O
the	O
form	O
of	O
the	O
V-11	B-Device
,	O
CVAX	B-Device
,	O
CVAX	B-Device
SOC	O
(	O
"	O
System	O
On	O
Chip	O
"	O
,	O
a	O
single-chip	O
CVAX	B-Device
)	O
,	O
Rigel	B-Device
,	O
Mariah	O
and	O
NVAX	B-Device
implementations	O
.	O
</s>
<s>
The	O
VAX	B-Device
microprocessors	B-Architecture
extended	O
the	O
architecture	O
to	O
inexpensive	O
workstations	B-Device
and	O
later	O
also	O
supplanted	O
the	O
high-end	O
VAX	B-Device
models	O
.	O
</s>
<s>
This	O
wide	O
range	O
of	O
platforms	O
(	O
mainframe	B-Architecture
to	O
workstation	B-Device
)	O
using	O
one	O
architecture	O
was	O
unique	O
in	O
the	O
computer	O
industry	O
at	O
that	O
time	O
.	O
</s>
<s>
Sundry	O
graphics	O
were	O
etched	O
onto	O
the	O
CVAX	B-Device
microprocessor	B-Architecture
die	O
.	O
</s>
<s>
The	O
phrase	O
CVAX	B-Device
...	O
when	O
you	O
care	O
enough	O
to	O
steal	O
the	O
very	O
best	O
was	O
etched	O
in	O
broken	O
Russian	O
as	O
a	O
play	O
on	O
a	O
Hallmark	O
Cards	O
slogan	O
,	O
intended	O
as	O
a	O
message	O
to	O
Soviet	O
engineers	O
who	O
were	O
known	O
to	O
be	O
both	O
purloining	O
DEC	O
computers	O
for	O
military	O
applications	O
and	O
reverse	O
engineering	O
their	O
chip	O
design	O
.	O
</s>
<s>
By	O
the	O
late	O
1980s	O
,	O
the	O
VAX	B-Device
microprocessors	B-Architecture
had	O
grown	O
in	O
power	O
to	O
be	O
competitive	O
with	O
discrete	O
designs	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
abandonment	O
of	O
the	O
8000	O
and	O
9000	O
series	O
and	O
their	O
replacement	O
by	O
Rigel-powered	O
models	O
of	O
the	O
VAX	B-Device
6000	I-Device
,	O
and	O
later	O
by	O
NVAX-powered	O
VAX	B-Device
7000	I-Device
systems	O
.	O
</s>
<s>
In	O
DEC	O
's	O
product	O
offerings	O
,	O
the	O
VAX	B-Device
architecture	O
was	O
eventually	O
superseded	O
by	O
RISC	B-Architecture
technology	O
.	O
</s>
<s>
In	O
1989	O
DEC	O
introduced	O
a	O
range	O
of	O
workstations	B-Device
and	O
servers	O
that	O
ran	O
Ultrix	B-Operating_System
,	O
the	O
DECstation	B-Device
and	O
DECsystem	B-Device
respectively	O
,	O
based	O
on	O
processors	O
that	O
implemented	O
the	O
MIPS	B-Device
architecture	I-Device
.	O
</s>
<s>
In	O
1992	O
DEC	O
introduced	O
their	O
own	O
RISC	B-Architecture
instruction	I-Architecture
set	I-Architecture
architecture	O
,	O
the	O
Alpha	B-Device
AXP	I-Device
(	O
later	O
renamed	O
Alpha	B-Device
)	O
,	O
and	O
their	O
own	O
Alpha-based	O
microprocessor	B-Architecture
,	O
the	O
DECchip	B-General_Concept
21064	I-General_Concept
,	O
a	O
high	O
performance	O
64-bit	B-Device
design	O
capable	O
of	O
running	O
OpenVMS	B-Operating_System
.	O
</s>
<s>
In	O
August	O
2000	O
,	O
Compaq	O
announced	O
that	O
the	O
remaining	O
VAX	B-Device
models	O
would	O
be	O
discontinued	O
by	O
the	O
end	O
of	O
the	O
year	O
,	O
but	O
old	O
systems	O
remain	O
in	O
widespread	O
use	O
.	O
</s>
<s>
The	O
Stromasys	O
CHARON-VAX	O
and	O
SIMH	B-Application
software-based	O
VAX	B-Device
emulators	O
remain	O
available	O
.	O
</s>
<s>
VMS	B-Operating_System
is	O
now	O
developed	O
by	O
VMS	B-Operating_System
Software	O
Incorporated	O
,	O
albeit	O
only	O
for	O
the	O
Alpha	B-Device
,	O
HPE	B-General_Concept
Integrity	I-General_Concept
,	O
and	O
x86-64	B-Device
platforms	O
.	O
</s>
<s>
The	O
VAX	B-Device
virtual	B-Architecture
memory	I-Architecture
is	O
divided	O
into	O
four	O
sections	O
.	O
</s>
<s>
For	O
VMS	B-Operating_System
,	O
P0	O
was	O
used	O
for	O
user	O
process	O
space	O
,	O
P1	O
for	O
process	O
stack	O
,	O
S0	O
for	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
and	O
S1	O
was	O
reserved	O
.	O
</s>
<s>
The	O
VAX	B-Device
has	O
four	O
hardware	O
implemented	O
privilege	O
modes	O
:	O
</s>
<s>
The	O
first	O
VAX-based	O
system	O
was	O
the	O
VAX-11/780	O
,	O
a	O
member	O
of	O
the	O
VAX-11	O
family	O
.	O
</s>
<s>
The	O
high-end	O
VAX	B-Device
8600	I-Device
replaced	O
the	O
VAX-11/780	O
in	O
October	O
1984	O
and	O
was	O
joined	O
by	O
the	O
entry-level	O
MicroVAX	B-Device
minicomputers	B-Architecture
and	O
the	O
VAXstation	B-Device
workstations	B-Device
in	O
the	O
mid-1980s	O
.	O
</s>
<s>
The	O
MicroVAX	B-Device
was	O
superseded	O
by	O
the	O
VAX	B-Device
4000	I-Device
,	O
the	O
VAX	B-Device
8000	I-Device
was	O
superseded	O
by	O
the	O
VAX	B-Device
6000	I-Device
in	O
the	O
late	O
1980s	O
and	O
the	O
mainframe-class	O
VAX	B-Device
9000	I-Device
was	O
introduced	O
.	O
</s>
<s>
In	O
the	O
early	O
1990s	O
,	O
the	O
fault-tolerant	B-General_Concept
VAXft	B-Device
was	O
introduced	O
,	O
as	O
were	O
the	O
Alpha	B-Device
compatible	O
VAX	B-Device
7000/10000	I-Device
.	O
</s>
<s>
A	O
variant	O
of	O
various	O
VAX-based	O
systems	O
were	O
sold	O
as	O
the	O
VAXserver	B-Device
.	O
</s>
<s>
System	O
Industries	O
developed	O
an	O
ability	O
to	O
give	O
more	O
than	O
one	O
DEC	O
CPU	B-General_Concept
,	O
but	O
not	O
at	O
the	O
same	O
time	O
,	O
write	O
access	O
to	O
a	O
shared	O
disk	O
.	O
</s>
<s>
They	O
implemented	O
an	O
enhancement	O
named	O
SImultaneous	O
Machine	O
ACceSs	O
(	O
SIMACS	O
)	O
,	O
which	O
allowed	O
their	O
special	O
disk	O
controller	O
to	O
set	O
a	O
semaphore	B-Operating_System
flag	I-Operating_System
for	O
disk	O
access	O
,	O
allowing	O
multiple	O
WRITES	O
to	O
the	O
same	O
files	O
;	O
the	O
disk	O
is	O
shared	O
by	O
multiple	O
DEC	O
systems	O
.	O
</s>
<s>
SIMACS	O
also	O
existed	O
on	O
PDP-11	B-Device
RSTS	B-Operating_System
systems	O
.	O
</s>
<s>
Canceled	O
systems	O
include	O
the	O
BVAX	O
,	O
a	O
high-end	O
emitter-coupled	B-General_Concept
logic	I-General_Concept
(	O
ECL	O
)	O
based	O
VAX	B-Device
,	O
and	O
two	O
other	O
ECL-based	O
VAX	B-Device
models	O
:	O
Argonaut	O
and	O
Raven	O
.	O
</s>
<s>
A	O
VAX	B-Device
named	O
Gemini	O
was	O
also	O
canceled	O
,	O
which	O
was	O
a	O
fall-back	O
in	O
case	O
the	O
LSI-based	O
Scorpio	O
failed	O
.	O
</s>
<s>
A	O
number	O
of	O
VAX	B-Device
clones	O
,	O
both	O
authorized	O
and	O
unauthorized	O
,	O
were	O
produced	O
.	O
</s>
<s>
Systime	O
Computers	O
Ltd	O
of	O
the	O
United	O
Kingdom	O
produced	O
clones	O
of	O
early	O
VAX	B-Device
models	O
such	O
as	O
the	O
Systime	O
8750	O
(	O
equivalent	O
to	O
the	O
VAX	B-Device
11/750	O
)	O
.	O
</s>
<s>
Norden	O
Systems	O
produced	O
the	O
ruggedized	O
,	O
Military-specification	O
MIL	O
VAX	B-Device
series	O
.	O
</s>
<s>
The	O
Hungarian	O
Central	O
Research	O
Institute	O
for	O
Physics	O
(	O
KFKI	O
)	O
produced	O
a	O
series	O
of	O
clones	O
of	O
early	O
VAX	B-Device
models	O
,	O
the	O
TPA-11/540	O
,	O
560	O
and	O
580	O
.	O
</s>
<s>
The	O
East	O
German	O
VEB	O
Robotron	O
K	O
1840	O
(	O
SM	O
1710	O
)	O
is	O
a	O
clone	O
of	O
the	O
VAX-11/780	O
and	O
Robotron	B-Device
K	I-Device
1820	I-Device
(	O
SM	O
1720	O
)	O
is	O
a	O
copy	O
of	O
the	O
MicroVAX	B-Device
II	O
.	O
</s>
<s>
The	O
SM-1700	O
is	O
a	O
Soviet	O
clone	O
of	O
the	O
VAX-11/730	O
,	O
SM-1702	O
was	O
a	O
clone	O
of	O
MicroVAX	B-Device
II	O
and	O
SM-1705	O
was	O
a	O
clone	O
of	O
VAX-11/785	O
.	O
</s>
<s>
These	O
systems	O
ran	O
a	O
variety	O
of	O
clone	O
operating	B-General_Concept
systems	I-General_Concept
-	O
DEMOS	B-Language
(	O
based	O
on	O
BSD	B-Operating_System
Unix	I-Operating_System
)	O
,	O
MOS	O
VP	O
(	O
based	O
on	O
VAX/VMS	B-Operating_System
)	O
or	O
MOS	O
VP	O
RV	O
(	O
based	O
on	O
VAXELN	B-Operating_System
)	O
.	O
</s>
<s>
The	O
NCI-2780	O
Super-mini	O
,	O
also	O
sold	O
as	O
Taiji-2780	O
,	O
is	O
a	O
clone	O
of	O
the	O
VAX-11/780	O
developed	O
by	O
North	O
China	O
Institute	O
of	O
Computing	O
Technology	O
in	O
Beijing	O
.	O
</s>
