<s>
Safe	B-Operating_System
semantics	I-Operating_System
is	O
a	O
computer	B-Architecture
hardware	I-Architecture
consistency	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
It	O
describes	O
one	O
type	O
of	O
guarantee	O
that	O
a	O
data	B-General_Concept
register	I-General_Concept
provides	O
when	O
it	O
is	O
shared	O
by	O
several	O
processors	B-General_Concept
in	O
a	O
parallel	B-Operating_System
computer	I-Operating_System
or	O
in	O
a	O
network	O
of	O
computers	O
working	O
together	O
.	O
</s>
<s>
Safe	B-Operating_System
semantics	I-Operating_System
was	O
first	O
defined	O
by	O
Leslie	O
Lamport	O
in	O
1985	O
.	O
</s>
<s>
Safe	B-Operating_System
register	I-Operating_System
has	O
been	O
implemented	O
in	O
many	O
distributed	O
systems	O
.	O
</s>
<s>
Safe	B-Operating_System
semantics	I-Operating_System
are	O
defined	O
for	O
a	O
variable	O
with	O
a	O
single	O
writer	O
but	O
multiple	O
readers	O
(	O
SWMR	O
)	O
.	O
</s>
<s>
A	O
read	O
operation	O
not	O
concurrent	B-Operating_System
with	O
any	O
write	O
operation	O
returns	O
the	O
value	O
written	O
by	O
the	O
latest	O
write	O
operation	O
.	O
</s>
<s>
A	O
read	O
operation	O
that	O
is	O
concurrent	B-Operating_System
with	O
a	O
write	O
operation	O
may	O
return	O
any	O
value	O
within	O
the	O
register	O
's	O
allowed	O
range	O
of	O
values	O
(	O
for	O
example	O
,	O
0	O
,	O
1	O
,	O
2	O
,...	O
)	O
.	O
</s>
<s>
A	O
binary	O
safe	B-Operating_System
register	I-Operating_System
can	O
be	O
seen	O
as	O
modeling	O
a	O
bit	O
flickering	O
.	O
</s>
<s>
show	O
that	O
no	O
register	O
can	O
have	O
the	O
stronger	O
property	O
of	O
regular	B-Operating_System
semantics	I-Operating_System
in	O
a	O
synchronous	B-Application
system	I-Application
under	O
continuous	O
churn	O
.	O
</s>
<s>
However	O
,	O
a	O
safe	B-Operating_System
register	I-Operating_System
can	O
be	O
implemented	O
under	O
continuous	O
churn	O
in	O
a	O
non-synchronous	O
system	O
.	O
</s>
<s>
Modeling	O
and	O
implementing	O
a	O
type	O
of	O
storage	O
memory	O
(	O
Safe	B-Operating_System
Register	I-Operating_System
)	O
under	O
non-quiescent	O
churn	O
requires	O
some	O
system	O
models	O
such	O
as	O
client	O
and	O
server	O
systems	O
.	O
</s>
<s>
Safe	B-Operating_System
register	I-Operating_System
implementation	O
involves	O
:	O
</s>
<s>
Safe	B-Operating_System
register	I-Operating_System
is	O
maintained	O
by	O
the	O
set	O
of	O
active	O
servers	O
.	O
</s>
<s>
Size	O
of	O
the	O
Read	O
and	O
Write	O
operation	O
executed	O
on	O
quora	O
=	O
n	O
–	O
f	O
–	O
J	O
(	O
n	O
is	O
the	O
number	O
of	O
servers	O
,	O
J	O
is	O
the	O
number	O
of	O
servers	O
that	O
enter	O
and	O
exit	O
,	O
and	O
f	O
is	O
the	O
number	O
of	O
Byzantine	B-Operating_System
failures	I-Operating_System
.	O
</s>
<s>
The	O
validity	O
of	O
the	O
safe	B-Operating_System
register	I-Operating_System
(	O
If	O
a	O
read	O
is	O
not	O
concurrent	B-Operating_System
with	O
any	O
write	O
,	O
return	O
the	O
last	O
value	O
written	O
)	O
was	O
proved	O
based	O
on	O
the	O
quorum	O
system	O
.	O
</s>
<s>
were	O
B	O
is	O
the	O
number	O
of	O
Byzantine	B-Operating_System
failures	I-Operating_System
.	O
</s>
