<s>
Xenix	B-Application
is	O
a	O
discontinued	O
version	O
of	O
the	O
Unix	B-Application
operating	I-Application
system	I-Application
for	O
various	O
microcomputer	B-Architecture
platforms	O
,	O
licensed	O
by	O
Microsoft	O
from	O
AT&T	O
Corporation	O
in	O
the	O
late	O
1970s	O
.	O
</s>
<s>
The	O
Santa	O
Cruz	O
Operation	O
(	O
SCO	O
)	O
later	O
acquired	O
exclusive	O
rights	O
to	O
the	O
software	O
,	O
and	O
eventually	O
replaced	O
it	O
with	O
SCO	B-Operating_System
UNIX	I-Operating_System
(	O
now	O
known	O
as	O
SCO	B-Operating_System
OpenServer	I-Operating_System
)	O
.	O
</s>
<s>
In	O
the	O
mid-to-late	O
1980s	O
,	O
Xenix	B-Application
was	O
the	O
most	O
common	O
Unix	B-Application
variant	O
,	O
measured	O
according	O
to	O
the	O
number	O
of	O
machines	O
on	O
which	O
it	O
was	O
installed	O
.	O
</s>
<s>
Microsoft	O
chairman	O
Bill	O
Gates	O
said	O
at	O
Unix	B-Application
Expo	O
in	O
1996	O
that	O
,	O
for	O
a	O
long	O
time	O
,	O
Microsoft	O
had	O
the	O
highest-volume	O
AT&T	B-Operating_System
Unix	I-Operating_System
license	O
.	O
</s>
<s>
Bell	O
Labs	O
,	O
the	O
developer	O
of	O
Unix	B-Application
,	O
was	O
part	O
of	O
the	O
regulated	O
Bell	O
System	O
and	O
could	O
not	O
sell	O
Unix	B-Application
directly	O
to	O
most	O
end	O
users	O
(	O
academic	O
and	O
research	O
institutions	O
excepted	O
)	O
;	O
it	O
could	O
,	O
however	O
,	O
license	O
it	O
to	O
software	O
vendors	O
who	O
would	O
then	O
resell	O
it	O
to	O
end	O
users	O
(	O
or	O
their	O
own	O
resellers	O
)	O
,	O
combined	O
with	O
their	O
own	O
added	O
features	O
.	O
</s>
<s>
Microsoft	O
,	O
which	O
expected	O
that	O
Unix	B-Application
would	O
be	O
its	O
operating	B-General_Concept
system	I-General_Concept
of	O
the	O
future	O
when	O
personal	O
computers	O
became	O
powerful	O
enough	O
,	O
purchased	O
a	O
license	O
for	O
Version	B-Operating_System
7	I-Operating_System
Unix	I-Operating_System
from	O
AT&T	O
in	O
1978	O
,	O
and	O
announced	O
on	O
August	O
25	O
,	O
1980	O
,	O
that	O
it	O
would	O
make	O
the	O
software	O
available	O
for	O
the	O
16-bit	B-Device
microcomputer	B-Architecture
market	O
.	O
</s>
<s>
Because	O
Microsoft	O
was	O
not	O
able	O
to	O
license	O
the	O
"	O
Unix	B-Application
"	O
name	O
itself	O
,	O
the	O
company	O
gave	O
it	O
an	O
original	O
name	O
.	O
</s>
<s>
Microsoft	O
called	O
Xenix	B-Application
"	O
a	O
universal	O
operating	O
environment	O
"	O
.	O
</s>
<s>
It	O
did	O
not	O
sell	O
Xenix	B-Application
directly	O
to	O
end	O
users	O
,	O
but	O
licensed	O
the	O
software	O
to	O
OEMs	O
such	O
as	O
IBM	O
,	O
Intel	O
,	O
Management	O
Systems	O
Development	O
,	O
Tandy	O
,	O
Altos	O
Computer	O
,	O
SCO	O
,	O
and	O
Siemens	O
(	O
SINIX	B-Operating_System
)	O
which	O
then	O
ported	O
it	O
to	O
their	O
own	O
proprietary	B-Application
computer	B-General_Concept
architectures	I-General_Concept
.	O
</s>
<s>
In	O
1981	O
,	O
Microsoft	O
said	O
the	O
first	O
version	O
of	O
Xenix	B-Application
was	O
"	O
very	O
close	O
to	O
the	O
original	O
Unix	B-Operating_System
version	I-Operating_System
7	I-Operating_System
source	O
"	O
on	O
the	O
PDP-11	B-Device
,	O
and	O
later	O
versions	O
were	O
to	O
incorporate	O
its	O
own	O
fixes	O
and	O
improvements	O
.	O
</s>
<s>
The	O
company	O
stated	O
that	O
it	O
intended	O
to	O
port	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
the	O
Zilog	B-Device
Z8000	I-Device
series	O
,	O
Digital	O
LSI-11	O
,	O
Intel	B-General_Concept
8086	I-General_Concept
and	O
80286	B-General_Concept
,	O
Motorola	B-Device
68000	I-Device
,	O
and	O
possibly	O
"	O
numerous	O
other	O
processors	O
"	O
,	O
and	O
provide	O
Microsoft	O
's	O
"	O
full	O
line	O
of	O
system	O
software	O
products	O
"	O
,	O
including	O
BASIC	B-Language
and	O
other	O
languages	O
.	O
</s>
<s>
The	O
first	O
port	O
was	O
for	O
the	O
Z8001	B-Device
16-bit	B-Device
processor	I-Device
:	O
the	O
first	O
customer	O
ship	O
was	O
January	O
1981	O
for	O
Central	O
Data	O
Corporation	O
of	O
Illinois	O
,	O
followed	O
in	O
March	O
1981	O
by	O
Paradyne	O
Corporation	O
's	O
Z8001	B-Device
product	O
.	O
</s>
<s>
The	O
first	O
8086	B-General_Concept
port	O
was	O
for	O
the	O
Altos	O
Computer	O
Systems	O
 '	O
non-PC-compatible	O
8600-series	O
computers	O
(	O
first	O
customer	O
ship	O
date	O
Q1	O
1982	O
)	O
.	O
</s>
<s>
Intel	O
sold	O
complete	O
computers	O
with	O
Xenix	B-Application
under	O
their	O
Intel	O
System	O
86	O
brand	O
(	O
with	O
specific	O
models	O
such	O
as	O
86/330	O
or	O
86/380X	O
)	O
;	O
they	O
also	O
offered	O
the	O
individual	O
boards	O
that	O
made	O
these	O
computers	O
under	O
their	O
iSBC	B-Device
brand	O
.	O
</s>
<s>
This	O
included	O
processor	O
boards	O
like	O
iSBC	B-Device
86/12	O
and	O
also	O
MMU	O
boards	O
such	O
as	O
the	O
iSBC	B-Device
309	O
.	O
</s>
<s>
The	O
first	O
Intel	O
Xenix	B-Application
systems	O
shipped	O
in	O
July	O
1982	O
.	O
</s>
<s>
Tandy	O
more	O
than	O
doubled	O
the	O
Xenix	B-Application
installed	O
base	O
when	O
it	O
made	O
TRS-Xenix	B-Application
the	O
default	O
operating	B-General_Concept
system	I-General_Concept
for	O
its	O
TRS-80	O
Model	O
16	O
68000-based	O
computer	O
in	O
early	O
1983	O
,	O
and	O
was	O
the	O
largest	O
Unix	B-Application
vendor	O
in	O
1984	O
.	O
</s>
<s>
Seattle	O
Computer	O
Products	O
also	O
made	O
(	O
PC-incompatible	O
)	O
8086	B-General_Concept
computers	O
bundled	O
with	O
Xenix	B-Application
,	O
like	O
their	O
Gazelle	O
II	O
,	O
which	O
used	O
the	O
S-100	B-Architecture
bus	I-Architecture
and	O
was	O
available	O
in	O
late	O
1983	O
or	O
early	O
1984	O
.	O
</s>
<s>
There	O
was	O
also	O
a	O
port	O
for	O
IBM	B-Device
System	I-Device
9000	I-Device
.	O
</s>
<s>
SCO	O
had	O
initially	O
worked	O
on	O
its	O
own	O
PDP-11	B-Device
port	O
of	O
V7	O
,	O
called	O
Dynix	B-Library
,	O
but	O
then	O
struck	O
an	O
agreement	O
with	O
Microsoft	O
for	O
joint	O
development	O
and	O
technology	O
exchange	O
on	O
Xenix	B-Application
in	O
1982	O
.	O
</s>
<s>
Microsoft	O
and	O
SCO	O
then	O
further	O
engaged	O
Human	O
Computing	O
Resources	O
Corporation	O
(	O
HCR	O
)	O
in	O
Canada	O
,	O
and	O
a	O
software	O
products	O
group	O
within	O
Logica	O
plc	O
in	O
the	O
United	O
Kingdom	O
,	O
as	O
part	O
of	O
making	O
further	O
improvements	O
to	O
Xenix	B-Application
and	O
porting	O
Xenix	B-Application
to	O
other	O
platforms	O
.	O
</s>
<s>
In	O
doing	O
so	O
,	O
Microsoft	O
gave	O
HCR	O
and	O
Logica	O
the	O
rights	O
to	O
do	O
Xenix	B-Application
ports	O
and	O
to	O
license	O
Xenix	B-Application
binary	O
distributions	O
in	O
those	O
territories	O
.	O
</s>
<s>
In	O
1984	O
,	O
a	O
port	O
to	O
the	O
68000-based	O
Apple	B-Device
Lisa	I-Device
2	O
was	O
jointly	O
developed	O
by	O
SCO	O
and	O
Microsoft	O
and	O
it	O
was	O
the	O
first	O
shrink-wrapped	O
binary	O
product	O
sold	O
by	O
SCO	O
.	O
</s>
<s>
The	O
Multiplan	B-Device
spreadsheet	O
was	O
released	O
for	O
it	O
.	O
</s>
<s>
In	O
its	O
1983	O
OEM	O
directory	O
,	O
Microsoft	O
said	O
the	O
difficulty	O
in	O
porting	O
to	O
the	O
various	O
8086	B-General_Concept
and	O
Z8000-based	O
machines	O
had	O
been	O
the	O
lack	O
of	O
a	O
standardized	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
and	O
protection	O
facilities	O
.	O
</s>
<s>
Hardware	O
manufacturers	O
compensated	O
by	O
designing	O
their	O
own	O
hardware	O
,	O
but	O
the	O
ensuing	O
complexity	O
made	O
it	O
"	O
extremely	O
difficult	O
if	O
not	O
impossible	O
for	O
the	O
very	O
small	O
manufacturer	O
to	O
develop	O
a	O
computer	O
capable	O
of	O
supporting	O
a	O
system	O
such	O
as	O
Xenix	B-Application
from	O
scratch	O
,	O
"	O
and	O
"	O
the	O
Xenix	B-Application
kernel	O
must	O
be	O
custom-tailored	O
to	O
each	O
new	O
hardware	O
environment.	O
"	O
</s>
<s>
A	O
generally	O
available	O
port	O
to	O
the	O
unmapped	O
Intel	B-Operating_System
8086/8088	I-Operating_System
architecture	I-Operating_System
was	O
done	O
by	O
The	O
Santa	O
Cruz	O
Operation	O
around	O
1983	O
.	O
</s>
<s>
SCO	B-Application
Xenix	I-Application
for	O
the	O
PC	B-Device
XT	I-Device
shipped	O
sometime	O
in	O
1984	O
and	O
contained	O
some	O
enhancement	O
from	O
4.2BSD	O
;	O
it	O
also	O
supported	O
the	O
Micnet	O
local	O
area	O
networking	O
.	O
</s>
<s>
The	O
later	O
286	B-General_Concept
version	O
of	O
Xenix	B-Application
used	O
the	O
integrated	O
MMU	O
present	O
on	O
this	O
chip	O
,	O
by	O
running	O
in	O
286	B-General_Concept
protected	O
mode	O
.	O
</s>
<s>
The	O
286	B-General_Concept
Xenix	B-Application
was	O
accompanied	O
by	O
new	O
hardware	O
from	O
Xenix	B-Application
OEMs	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Sperry	O
PC/IT	O
,	O
an	O
IBM	B-Operating_System
PC	I-Operating_System
AT	I-Operating_System
clone	O
,	O
was	O
advertised	O
as	O
capable	O
of	O
supporting	O
eight	O
simultaneous	O
dumb	O
terminal	O
users	O
under	O
this	O
version	O
.	O
</s>
<s>
While	O
Xenix	B-Application
2.0	I-Application
was	O
still	O
based	O
on	O
Version	B-Operating_System
7	I-Operating_System
Unix	I-Operating_System
,	O
version	O
3.0	O
was	O
upgraded	O
to	O
a	O
Unix	B-Operating_System
System	I-Operating_System
III	I-Operating_System
code	O
base	O
,	O
a	O
1984	O
Intel	O
manual	O
for	O
Xenix	B-Application
286	I-Application
noted	O
that	O
the	O
Xenix	B-Application
kernel	O
had	O
about	O
10,000	O
lines	O
at	O
this	O
time	O
.	O
</s>
<s>
It	O
was	O
followed	O
by	O
a	O
System	B-Operating_System
V	I-Operating_System
R2	I-Operating_System
codebase	O
in	O
Xenix	B-Application
5.0	I-Application
(	O
a.k.a.	O
</s>
<s>
Xenix	B-Application
System	I-Application
V	I-Application
)	O
.	O
</s>
<s>
"	O
Microsoft	O
hopes	O
that	O
Xenix	B-Application
will	O
become	O
the	O
preferred	O
choice	O
for	O
software	O
production	O
and	O
exchange	O
"	O
,	O
the	O
company	O
stated	O
in	O
1981	O
.	O
</s>
<s>
Microsoft	O
referred	O
to	O
its	O
own	O
MS-DOS	B-Application
as	O
its	O
"	O
single-user	B-Operating_System
,	O
single-tasking	O
operating	B-General_Concept
system	I-General_Concept
"	O
,	O
and	O
advised	O
customers	O
that	O
wanted	O
multiuser	B-Operating_System
or	O
multitasking	B-Operating_System
support	O
to	O
buy	O
Xenix	B-Application
.	O
</s>
<s>
It	O
planned	O
to	O
over	O
time	O
improve	O
MS-DOS	B-Application
so	O
it	O
would	O
be	O
almost	O
indistinguishable	O
from	O
single-user	B-Operating_System
Xenix	B-Application
,	O
or	O
XEDOS	B-Application
,	O
which	O
would	O
also	O
run	O
on	O
the	O
68000	B-Device
,	O
Z8000	B-Device
,	O
and	O
LSI-11	O
;	O
they	O
would	O
be	O
upwardly	B-General_Concept
compatible	I-General_Concept
with	O
Xenix	B-Application
,	O
which	O
Byte	B-Application
in	O
1983	O
described	O
as	O
"	O
the	O
multi-user	B-Operating_System
MS-DOS	B-Application
of	O
the	O
future	O
"	O
.	O
</s>
<s>
Microsoft	O
's	O
Chris	O
Larson	O
described	O
MS-DOS	B-Application
2.0	O
'	O
s	O
Xenix	B-Application
compatibility	O
as	O
"	O
the	O
second	O
most	O
important	O
feature	O
"	O
.	O
</s>
<s>
His	O
company	O
advertised	O
DOS	O
and	O
Xenix	B-Application
together	O
,	O
describing	O
MS-DOS	B-Application
2.0	O
(	O
its	O
"	O
single-user	B-Operating_System
OS	O
"	O
)	O
as	O
sharing	O
features	O
and	O
system	B-Operating_System
calls	I-Operating_System
with	O
Xenix	B-Application
(	O
"	O
the	O
multi-user	B-Operating_System
,	O
multi-tasking	O
,	O
Unix-derived	O
operating	B-General_Concept
system	I-General_Concept
"	O
)	O
,	O
and	O
promising	O
easy	O
porting	O
between	O
them	O
.	O
</s>
<s>
After	O
the	O
breakup	O
of	O
the	O
Bell	O
System	O
in	O
1982	O
,	O
AT&T	O
started	O
selling	O
System	B-Operating_System
V	I-Operating_System
.	O
Microsoft	O
,	O
believing	O
that	O
it	O
could	O
not	O
compete	O
with	O
Unix	B-Application
's	O
developer	O
,	O
decided	O
to	O
abandon	O
Xenix	B-Application
.	O
</s>
<s>
The	O
decision	O
was	O
not	O
immediately	O
transparent	O
,	O
which	O
led	O
to	O
the	O
term	O
vaporware	B-Application
.	O
</s>
<s>
It	O
agreed	O
with	O
IBM	O
to	O
develop	O
OS/2	B-Application
,	O
and	O
the	O
Xenix	B-Application
team	O
(	O
together	O
with	O
the	O
best	O
MS-DOS	B-Application
developers	O
)	O
was	O
assigned	O
to	O
that	O
project	O
.	O
</s>
<s>
In	O
1987	O
,	O
Microsoft	O
transferred	O
ownership	O
of	O
Xenix	B-Application
to	O
SCO	O
in	O
an	O
agreement	O
that	O
left	O
Microsoft	O
owning	O
slightly	O
less	O
than	O
20%	O
of	O
SCO	O
(	O
this	O
amount	O
prevented	O
both	O
companies	O
from	O
having	O
to	O
disclose	O
the	O
exact	O
amount	O
in	O
the	O
event	O
of	O
an	O
SCO	O
IPO	O
)	O
.	O
</s>
<s>
And	O
SCO	O
would	O
acquire	O
both	O
of	O
the	O
other	O
companies	O
that	O
had	O
Xenix	B-Application
rights	O
,	O
Logica	O
's	O
software	O
products	O
group	O
in	O
1986	O
and	O
HCR	O
in	O
1990	O
.	O
</s>
<s>
When	O
Microsoft	O
eventually	O
lost	O
interest	O
in	O
OS/2	B-Application
as	O
well	O
,	O
the	O
company	O
based	O
its	O
further	O
high-end	O
strategy	O
on	O
Windows	B-Device
NT	I-Device
.	O
</s>
<s>
In	O
1987	O
,	O
SCO	O
ported	O
Xenix	B-Application
to	O
the	O
386	B-General_Concept
processor	O
,	O
a	O
32-bit	O
chip	O
,	O
after	O
securing	O
knowledge	O
from	O
Microsoft	O
insiders	O
that	O
Microsoft	O
was	O
no	O
longer	O
developing	O
Xenix	B-Application
.	O
</s>
<s>
Xenix	B-Application
System	I-Application
V	I-Application
Release	O
2.3.1	O
introduced	O
support	O
for	O
i386	B-General_Concept
,	O
SCSI	B-Architecture
and	O
TCP/IP	B-Protocol
.	O
</s>
<s>
SCO	O
's	O
Xenix	O
System	O
V/386	O
was	O
the	O
first	O
32-bit	O
operating	B-General_Concept
system	I-General_Concept
available	O
on	O
the	O
market	O
for	O
the	O
x86	B-Operating_System
CPU	O
architecture	O
.	O
</s>
<s>
Microsoft	O
continued	O
to	O
use	O
Xenix	B-Application
internally	O
,	O
submitting	O
a	O
patch	O
to	O
support	O
functionality	O
in	O
Unix	B-Application
to	O
AT&T	O
in	O
1987	O
,	O
which	O
trickled	O
down	O
to	O
the	O
code	O
base	O
of	O
both	O
Xenix	B-Application
and	O
SCO	B-Operating_System
Unix	I-Operating_System
.	O
</s>
<s>
Microsoft	O
is	O
said	O
to	O
have	O
used	O
Xenix	B-Application
on	O
Sun	O
workstations	O
and	O
VAX	B-Device
minicomputers	B-Architecture
extensively	O
within	O
their	O
company	O
as	O
late	O
as	O
1988	O
.	O
</s>
<s>
All	O
internal	O
Microsoft	O
email	O
transport	O
was	O
done	O
on	O
Xenix-based	O
68000	B-Device
systems	O
until	O
1995	O
–	O
1996	O
,	O
when	O
the	O
company	O
moved	O
to	O
its	O
own	O
Exchange	B-Operating_System
Server	I-Operating_System
product	O
.	O
</s>
<s>
SCO	O
released	O
its	O
SCO	B-Operating_System
Unix	I-Operating_System
as	O
a	O
higher-end	O
product	O
,	O
based	O
on	O
System	B-Operating_System
V	I-Operating_System
R3	O
and	O
offering	O
a	O
number	O
of	O
technical	O
advances	O
over	O
Xenix	B-Application
;	O
Xenix	B-Application
remained	O
in	O
the	O
product	O
line	O
.	O
</s>
<s>
In	O
the	O
meantime	O
,	O
AT&T	O
and	O
Sun	O
Microsystems	O
completed	O
the	O
merge	O
of	O
Xenix	B-Application
,	O
BSD	B-Operating_System
,	O
SunOS	B-Operating_System
and	O
System	B-Operating_System
V	I-Operating_System
R3	O
into	O
System	B-Operating_System
V	I-Operating_System
R4	O
.	O
</s>
<s>
The	O
last	O
version	O
of	O
SCO	O
Xenix/386	O
itself	O
was	O
System	O
V	O
R2.3.4	O
,	O
released	O
in	O
1991	O
.	O
</s>
<s>
Aside	O
from	O
its	O
AT&T	B-Operating_System
Unix	I-Operating_System
base	O
,	O
Xenix	B-Application
incorporated	O
elements	O
from	O
BSD	B-Operating_System
,	O
notably	O
the	O
vi	B-Application
text	O
editor	O
and	O
its	O
supporting	O
libraries	O
(	O
termcap	B-Application
and	O
curses	B-Device
)	O
.	O
</s>
<s>
Its	O
kernel	O
featured	O
some	O
original	O
extensions	O
by	O
Microsoft	O
,	O
notably	O
file	B-Application
locking	I-Application
and	O
semaphores	B-Operating_System
,	O
while	O
to	O
the	O
userland	B-Operating_System
Microsoft	O
added	O
a	O
"	O
visual	O
shell	O
"	O
for	O
menu-driven	O
operation	O
instead	O
of	O
the	O
traditional	B-Application
UNIX	I-Application
shell	O
.	O
</s>
<s>
A	O
limited	O
form	O
of	O
local	O
networking	O
over	O
serial	O
lines	O
(	O
RS-232	O
ports	O
)	O
was	O
possible	O
through	O
the	O
"	O
micnet	O
"	O
software	O
,	O
which	O
supported	O
file	O
transfer	O
and	O
electronic	O
mail	O
,	O
although	O
UUCP	B-Protocol
was	O
still	O
used	O
for	O
networking	O
via	O
modems	B-Application
.	O
</s>
<s>
OEMs	O
often	O
added	O
further	O
modifications	O
to	O
the	O
Xenix	B-Application
system	O
.	O
</s>
<s>
Trusted	O
Xenix	B-Application
was	O
a	O
variant	O
initially	O
developed	O
by	O
IBM	O
,	O
under	O
the	O
name	O
Secure	O
XENIX	B-Application
;	O
later	O
versions	O
,	O
under	O
the	O
Trusted	O
Xenix	B-Application
name	O
,	O
were	O
developed	O
by	O
Trusted	O
Information	O
Systems	O
.	O
</s>
<s>
It	O
was	O
evaluated	O
by	O
formal	B-Architecture
methods	I-Architecture
and	O
achieved	O
a	O
B2	O
security	O
rating	O
under	O
the	O
DoD	O
's	O
Trusted	O
Computer	O
System	O
Evaluation	O
Criteria	O
—	O
the	O
second	O
highest	O
rating	O
ever	O
achieved	O
by	O
an	O
evaluated	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
