<s>
Incompatible	B-Application
Timesharing	I-Application
System	I-Application
(	O
ITS	O
)	O
is	O
a	O
time-sharing	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
developed	O
principally	O
by	O
the	O
MIT	O
Artificial	O
Intelligence	O
Laboratory	O
,	O
with	O
help	O
from	O
Project	O
MAC	O
.	O
</s>
<s>
The	O
name	O
is	O
the	O
jocular	O
complement	O
of	O
the	O
MIT	O
Compatible	B-Operating_System
Time-Sharing	I-Operating_System
System	I-Operating_System
(	O
CTSS	O
)	O
.	O
</s>
<s>
Remote	O
"	O
guest	O
"	O
or	O
"	O
tourist	O
"	O
access	O
was	O
easily	O
available	O
via	O
the	O
early	O
ARPAnet	O
,	O
allowing	O
many	O
interested	O
parties	O
to	O
informally	O
try	O
out	O
features	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
application	O
programs	O
.	O
</s>
<s>
The	O
wide-open	O
ITS	O
philosophy	O
and	O
collaborative	O
online	O
community	O
were	O
a	O
major	O
influence	O
on	O
the	O
hacker	O
culture	O
,	O
as	O
described	O
in	O
Steven	O
Levy	O
's	O
book	O
Hackers	O
,	O
and	O
were	O
the	O
direct	O
forerunners	O
of	O
the	O
free	B-License
and	I-License
open-source	I-License
software	I-License
,	O
open-design	O
,	O
and	O
Wiki	B-Protocol
movements	O
.	O
</s>
<s>
ITS	O
development	O
was	O
initiated	O
in	O
the	O
late	O
1960s	O
by	O
those	O
(	O
the	O
majority	O
of	O
the	O
MIT	O
AI	O
Lab	O
staff	O
at	O
that	O
time	O
)	O
who	O
disagreed	O
with	O
the	O
direction	O
taken	O
by	O
Project	O
MAC	O
's	O
Multics	B-Application
project	O
(	O
which	O
had	O
started	O
in	O
the	O
mid-1960s	O
)	O
,	O
particularly	O
such	O
decisions	O
as	O
the	O
inclusion	O
of	O
powerful	O
system	O
security	O
.	O
</s>
<s>
The	O
name	O
was	O
chosen	O
by	O
Tom	O
Knight	O
as	O
a	O
joke	O
on	O
the	O
name	O
of	O
the	O
earliest	O
MIT	O
time-sharing	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
,	O
the	O
Compatible	B-Operating_System
Time-Sharing	I-Operating_System
System	I-Operating_System
,	O
which	O
dated	O
from	O
the	O
early	O
1960s	O
.	O
</s>
<s>
By	O
simplifying	O
their	O
system	O
compared	O
to	O
Multics	B-Application
,	O
ITS	O
's	O
authors	O
were	O
able	O
to	O
quickly	O
produce	O
a	O
functional	O
operating	B-General_Concept
system	I-General_Concept
for	O
their	O
lab	O
.	O
</s>
<s>
ITS	O
was	O
written	O
in	O
assembly	B-Language
language	I-Language
,	O
originally	O
for	O
the	O
Digital	O
Equipment	O
Corporation	O
PDP-6	B-Device
computer	O
,	O
but	O
the	O
majority	O
of	O
ITS	O
development	O
and	O
use	O
was	O
on	O
the	O
later	O
,	O
largely	O
compatible	O
,	O
PDP-10	B-Device
.	O
</s>
<s>
Today	O
,	O
some	O
ITS	O
implementations	O
continue	O
to	O
be	O
remotely	O
accessible	O
,	O
via	O
emulation	B-Application
of	O
PDP-10	B-Device
hardware	O
running	O
on	O
modern	O
,	O
low-cost	O
computers	O
supported	O
by	O
interested	O
hackers	O
.	O
</s>
<s>
The	O
first	O
device-independent	B-Operating_System
graphics	O
terminal	O
output	O
;	O
programs	O
generated	O
generic	O
commands	O
to	O
control	O
screen	O
content	O
,	O
which	O
the	O
system	O
automatically	O
translated	O
into	O
the	O
appropriate	O
character	O
sequences	O
for	O
the	O
particular	O
type	O
of	O
terminal	O
operated	O
by	O
the	O
user	O
.	O
</s>
<s>
A	O
general	O
mechanism	O
for	O
implementing	O
virtual	B-Application
devices	I-Application
in	O
software	O
running	O
in	O
user	O
processes	O
(	O
which	O
were	O
called	O
"	O
jobs	O
"	O
in	O
ITS	O
)	O
.	O
</s>
<s>
Using	O
the	O
virtual-device	O
mechanism	O
,	O
ITS	O
provided	O
transparent	O
inter-machine	B-Protocol
filesystem	I-Protocol
access	I-Protocol
.	O
</s>
<s>
Sophisticated	O
process	B-Operating_System
management	I-Operating_System
;	O
user	O
processes	O
were	O
organized	O
in	O
a	O
tree	B-Application
,	O
and	O
a	O
superior	O
process	O
could	O
control	O
a	O
large	O
number	O
of	O
inferior	O
processes	O
.	O
</s>
<s>
An	O
advanced	O
software	O
interrupt	B-Application
facility	O
that	O
allowed	O
user	O
processes	O
to	O
operate	O
asynchronously	O
,	O
using	O
complex	O
interrupt	B-Application
handling	I-Application
mechanisms	O
.	O
</s>
<s>
PCLSRing	B-Operating_System
,	O
a	O
mechanism	O
providing	O
what	O
appeared	O
(	O
to	O
user	O
processes	O
)	O
to	O
be	O
quasi-atomic	O
,	O
safely-interruptible	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
No	O
process	O
could	O
ever	O
observe	O
any	O
process	O
(	O
including	O
itself	O
)	O
in	O
the	O
middle	O
of	O
executing	O
any	O
system	B-Operating_System
call	I-Operating_System
.	O
</s>
<s>
In	O
support	O
of	O
the	O
AI	O
Lab	O
's	O
robotics	O
work	O
,	O
ITS	O
also	O
supported	O
simultaneous	O
real-time	B-General_Concept
and	O
time-sharing	B-General_Concept
operation	O
.	O
</s>
<s>
The	O
environment	O
seen	O
by	O
ITS	O
users	O
was	O
philosophically	O
significantly	O
different	O
from	O
that	O
provided	O
by	O
most	O
operating	B-General_Concept
systems	I-General_Concept
at	O
the	O
time	O
.	O
</s>
<s>
To	O
deal	O
with	O
a	O
rash	O
of	O
incidents	O
where	O
users	O
sought	O
out	O
flaws	O
in	O
the	O
system	O
in	O
order	O
to	O
crash	B-General_Concept
it	O
,	O
a	O
novel	O
approach	O
was	O
taken	O
.	O
</s>
<s>
A	O
command	O
that	O
caused	O
the	O
system	O
to	O
crash	B-General_Concept
was	O
implemented	O
and	O
could	O
be	O
run	O
by	O
anyone	O
,	O
which	O
took	O
away	O
all	O
the	O
fun	O
and	O
challenge	O
of	O
doing	O
so	O
.	O
</s>
<s>
All	O
users	O
could	O
talk	O
with	O
instant	B-Application
messaging	I-Application
on	O
another	O
's	O
terminal	O
,	O
or	O
they	O
could	O
use	O
a	O
command	O
(	O
SHOUT	O
)	O
to	O
ask	O
all	O
active	O
users	O
for	O
help	O
.	O
</s>
<s>
Users	O
could	O
see	B-Protocol
what	I-Protocol
was	I-Protocol
happening	I-Protocol
on	O
another	O
's	O
terminal	O
(	O
using	O
a	O
command	O
called	O
OS	O
for	O
"	O
output	O
spy	O
"	O
)	O
.	O
</s>
<s>
Ease	O
of	O
access	O
,	O
with	O
or	O
without	O
a	O
guest	O
account	O
,	O
allowed	O
interested	O
parties	O
to	O
informally	O
explore	O
and	O
experiment	O
with	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
application	O
programs	O
,	O
and	O
"	O
hacker	O
"	O
culture	O
.	O
</s>
<s>
The	O
wide-open	O
ITS	O
philosophy	O
and	O
collaborative	O
community	O
were	O
the	O
direct	O
forerunner	O
of	O
the	O
free	B-License
and	I-License
open-source	I-License
software	I-License
,	O
open-design	O
,	O
and	O
Wiki	B-Protocol
movements	O
.	O
</s>
<s>
The	O
EMACS	B-Application
(	O
"	O
Editor	B-Application
MACroS	I-Application
"	O
)	O
editor	O
was	O
originally	O
written	O
on	O
ITS	O
.	O
</s>
<s>
In	O
its	O
ITS	O
instantiation	O
it	O
was	O
a	O
collection	O
of	O
TECO	B-Application
programs	O
(	O
called	O
"	O
macros	O
"	O
)	O
.	O
</s>
<s>
On	O
later	O
operating	B-General_Concept
systems	I-General_Concept
,	O
it	O
was	O
written	O
in	O
the	O
common	O
language	O
of	O
those	O
systems	O
–	O
for	O
example	O
,	O
the	O
C	O
language	O
under	O
Unix	O
,	O
and	O
Zetalisp	B-Language
under	O
the	O
Lisp	B-Operating_System
Machine	I-Operating_System
system	O
.	O
</s>
<s>
GNU	B-Operating_System
‘	O
s	O
info	B-Protocol
help	O
system	O
was	O
originally	O
an	O
EMACS	B-Application
subsystem	O
,	O
and	O
then	O
was	O
later	O
written	O
as	O
a	O
complete	O
standalone	O
system	O
for	O
Unix-like	O
machines	O
.	O
</s>
<s>
Several	O
important	O
programming	O
languages	O
and	O
systems	O
were	O
developed	O
on	O
ITS	O
,	O
including	O
MacLisp	B-Language
(	O
the	O
precursor	O
of	O
Zetalisp	B-Language
and	O
Common	B-Language
Lisp	I-Language
)	O
,	O
Microplanner	O
(	O
implemented	O
in	O
MacLisp	B-Language
)	O
,	O
MDL	B-Language
(	O
which	O
became	O
the	O
basis	O
of	O
Infocom	O
's	O
programming	O
environment	O
)	O
,	O
and	O
Scheme	B-Language
.	O
</s>
<s>
Among	O
other	O
significant	O
and	O
influential	O
software	O
subsystems	O
developed	O
on	O
ITS	O
,	O
the	O
Macsyma	B-Language
symbolic	O
algebra	O
system	O
,	O
started	O
in	O
1968	O
,	O
was	O
the	O
first	O
widely-known	O
mathematical	O
computing	O
environment	O
.	O
</s>
<s>
It	O
was	O
a	O
forerunner	O
of	O
Maxima	B-Language
,	O
MATLAB	B-Language
,	O
Wolfram	B-Application
Mathematica	I-Application
,	O
and	O
many	O
other	O
computer	O
algebra	O
systems	O
.	O
</s>
<s>
The	O
computer	O
game	O
Zork	B-Application
was	O
also	O
originally	O
written	O
on	O
ITS	O
.	O
</s>
<s>
The	O
default	O
ITS	O
top-level	O
command	B-Application
interpreter	I-Application
was	O
the	O
PDP-10	B-Device
machine	O
language	O
debugger	O
(	O
DDT	B-Operating_System
)	O
.	O
</s>
<s>
The	O
usual	O
text	B-Application
editor	I-Application
on	O
ITS	O
was	O
TECO	B-Application
and	O
later	O
Emacs	B-Application
,	O
which	O
was	O
written	O
in	O
TECO	B-Application
.	O
</s>
<s>
Both	O
DDT	B-Operating_System
and	O
TECO	B-Application
were	O
implemented	O
through	O
simple	O
dispatch	O
tables	O
on	O
single-letter	O
commands	O
,	O
and	O
thus	O
had	O
no	O
true	O
syntax	O
.	O
</s>
<s>
The	O
ITS	O
task	B-General_Concept
manager	I-General_Concept
was	O
called	O
PEEK	O
.	O
</s>
<s>
The	O
local	O
spelling	O
"	O
TURIST	O
"	O
is	O
an	O
artifact	O
of	O
six-character	O
filename	O
(	O
and	O
other	O
identifier	O
)	O
limitations	O
,	O
which	O
is	O
traceable	O
to	O
six	O
SIXBIT	O
encoded	O
characters	O
fitting	O
into	O
a	O
single	O
36-bit	O
PDP-10	B-Device
word	O
.	O
</s>
<s>
"	O
TURIST	O
"	O
may	O
also	O
have	O
been	O
a	O
pun	B-Application
on	O
Alan	O
Turing	O
,	O
a	O
pioneer	O
of	O
theoretical	O
computer	B-General_Concept
science	I-General_Concept
.	O
</s>
<s>
The	O
document	O
described	O
much	O
of	O
the	O
terminology	O
,	O
puns	B-Application
,	O
and	O
culture	O
of	O
the	O
two	O
AI	O
Labs	O
and	O
related	O
research	O
groups	O
,	O
and	O
is	O
the	O
direct	O
predecessor	O
of	O
the	O
Hacker	O
's	O
Dictionary	O
(	O
1983	O
)	O
,	O
the	O
first	O
compendium	O
of	O
hacker	O
jargon	O
to	O
be	O
issued	O
by	O
a	O
major	O
publisher	O
(	O
MIT	O
Press	O
)	O
.	O
</s>
<s>
The	O
device	O
used	O
a	O
stepper	B-Algorithm
motor	I-Algorithm
and	O
a	O
formerly	O
hand-held	O
wire	O
stripper	O
tool	O
and	O
cutter	O
,	O
operated	O
by	O
solenoid	B-Algorithm
,	O
all	O
under	O
computer	O
control	O
from	O
ITS	O
software	O
.	O
</s>
