<s>
The	O
GE	B-Device
645	I-Device
mainframe	B-Architecture
computer	I-Architecture
was	O
a	O
development	O
of	O
the	O
GE	B-Device
635	I-Device
for	O
use	O
in	O
the	O
Multics	B-Application
project	O
.	O
</s>
<s>
The	O
original	O
CTSS	B-Operating_System
was	O
implemented	O
on	O
a	O
modified	O
IBM	O
7094	O
with	O
two	O
banks	O
of	O
memory	O
and	O
bank-switching	O
between	O
user	O
and	O
supervisor	O
mode	O
,	O
i.e.	O
</s>
<s>
The	O
Multics	B-Application
operating	I-Application
system	I-Application
implemented	O
multilevel	O
security	O
(	O
MLS	O
)	O
on	O
a	O
GE	B-Device
635	I-Device
by	O
running	O
a	O
simulator	O
of	O
the	O
645	O
starting	O
on	O
October	O
18	O
,	O
1965	O
,	O
in	O
the	O
MIT	O
Tech	O
Center	O
.	O
</s>
<s>
The	O
GECOS	B-Application
operating	O
system	O
was	O
fully	O
replaced	O
by	O
Multics	B-Application
in	O
1969	O
with	O
the	O
Multics	B-Application
supervisor	O
(	O
master	O
mode	O
now	O
known	O
as	O
kernel	O
mode	O
)	O
separated	O
by	O
protection	B-Operating_System
rings	I-Operating_System
with	O
"	O
gates	O
"	O
allowing	O
access	O
from	O
user	O
mode	O
.	O
</s>
<s>
A	O
later	O
generation	O
in	O
the	O
form	O
of	O
the	O
645F	O
(	O
F	O
for	O
follow-on	O
)	O
was	O
n't	O
completed	O
by	O
the	O
time	O
the	O
division	O
was	O
sold	O
to	O
Honeywell	O
,	O
and	O
became	O
known	O
as	O
the	O
Honeywell	B-Device
6180	I-Device
.	O
</s>
<s>
The	O
bulk	O
of	O
these	O
computers	O
running	O
time-sharing	B-General_Concept
on	O
Multics	B-Application
were	O
installed	O
at	O
the	O
NSA	O
and	O
similar	O
governmental	O
sites	O
.	O
</s>
<s>
Their	O
usage	O
was	O
limited	O
by	O
the	O
extreme	O
security	O
measures	O
and	O
had	O
limited	O
impact	O
on	O
subsequent	O
systems	O
,	O
other	O
than	O
the	O
protection	B-Operating_System
ring	I-Operating_System
.	O
</s>
<s>
The	O
hardware	O
protection	O
introduced	O
on	O
this	O
computer	O
and	O
modified	O
on	O
the	O
6180	O
was	O
later	O
implemented	O
in	O
the	O
Intel	B-General_Concept
286	I-General_Concept
computer	O
processor	O
as	O
a	O
four-layer	O
protection	B-Operating_System
ring	I-Operating_System
,	O
but	O
four	O
rings	O
was	O
found	O
to	O
be	O
too	O
cumbersome	O
to	O
program	O
and	O
too	O
slow	O
to	O
operate	O
.	O
</s>
<s>
Protection	B-Operating_System
ring	I-Operating_System
architecture	O
is	O
now	O
used	O
only	O
to	O
protect	O
kernel	O
mode	O
from	O
user	O
mode	O
code	O
just	O
as	O
it	O
was	O
in	O
the	O
original	O
use	O
of	O
the	O
645	O
.	O
</s>
