<s>
A	O
dirty	B-Operating_System
bit	I-Operating_System
or	O
modified	O
bit	O
is	O
a	O
bit	O
that	O
is	O
associated	O
with	O
a	O
block	O
of	O
computer	O
memory	B-Architecture
and	O
indicates	O
whether	O
the	O
corresponding	O
block	O
of	O
memory	B-Architecture
has	O
been	O
modified	O
.	O
</s>
<s>
The	O
dirty	B-Operating_System
bit	I-Operating_System
is	O
set	O
when	O
the	O
processor	B-General_Concept
writes	O
to	O
(	O
modifies	O
)	O
this	O
memory	B-Architecture
.	O
</s>
<s>
The	O
bit	O
indicates	O
that	O
its	O
associated	O
block	O
of	O
memory	B-Architecture
has	O
been	O
modified	O
and	O
has	O
not	O
been	O
saved	O
to	O
storage	O
yet	O
.	O
</s>
<s>
When	O
a	O
block	O
of	O
memory	B-Architecture
is	O
to	O
be	O
replaced	O
,	O
its	O
corresponding	O
dirty	B-Operating_System
bit	I-Operating_System
is	O
checked	O
to	O
see	O
if	O
the	O
block	O
needs	O
to	O
be	O
written	O
back	O
to	O
secondary	O
memory	B-Architecture
before	O
being	O
replaced	O
or	O
if	O
it	O
can	O
simply	O
be	O
removed	O
.	O
</s>
<s>
Dirty	B-Operating_System
bits	I-Operating_System
are	O
used	O
by	O
the	O
CPU	B-General_Concept
cache	I-General_Concept
and	O
in	O
the	O
page	B-General_Concept
replacement	I-General_Concept
algorithms	I-General_Concept
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Dirty	B-Operating_System
bits	I-Operating_System
can	O
also	O
be	O
used	O
in	O
Incremental	O
computing	O
by	O
marking	O
segments	O
of	O
data	O
that	O
need	O
to	O
be	O
processed	O
or	O
have	O
yet	O
to	O
be	O
processed	O
.	O
</s>
<s>
This	O
ensures	O
the	O
unchanged	O
segments	O
are	O
not	O
recalculated	O
and	O
saves	O
processor	B-General_Concept
time	O
.	O
</s>
<s>
When	O
speaking	O
about	O
page	B-General_Concept
replacement	I-General_Concept
,	O
each	O
page	B-General_Concept
may	O
have	O
a	O
modify	O
bit	O
associated	O
with	O
it	O
in	O
the	O
hardware	O
.	O
</s>
<s>
The	O
dirty	B-Operating_System
bit	I-Operating_System
for	O
a	O
page	B-General_Concept
is	O
set	O
by	O
the	O
hardware	O
whenever	O
any	O
word	O
or	O
byte	B-Application
in	O
the	O
page	B-General_Concept
is	O
written	O
into	O
,	O
indicating	O
that	O
the	O
page	B-General_Concept
has	O
been	O
modified	O
.	O
</s>
<s>
When	O
a	O
page	B-General_Concept
is	O
selected	O
for	O
replacement	O
,	O
the	O
modify	O
bit	O
is	O
examined	O
.	O
</s>
<s>
If	O
the	O
bit	O
is	O
set	O
,	O
the	O
page	B-General_Concept
has	O
been	O
modified	O
since	O
it	O
was	O
read	O
in	O
from	O
the	O
disk	B-Device
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
page	B-General_Concept
must	O
be	O
written	O
to	O
the	O
disk	B-Device
.	O
</s>
<s>
If	O
the	O
dirty	B-Operating_System
bit	I-Operating_System
is	O
not	O
set	O
,	O
however	O
,	O
the	O
page	B-General_Concept
has	O
not	O
been	O
modified	O
since	O
it	O
was	O
read	O
into	O
memory	B-Architecture
.	O
</s>
<s>
Therefore	O
,	O
if	O
the	O
copy	O
of	O
the	O
page	B-General_Concept
on	O
the	O
disk	B-Device
has	O
not	O
been	O
overwritten	O
(	O
by	O
some	O
other	O
page	B-General_Concept
,	O
for	O
example	O
)	O
,	O
then	O
there	O
is	O
no	O
need	O
to	O
write	O
the	O
memory	B-General_Concept
page	I-General_Concept
to	O
the	O
disk	B-Device
:	O
it	O
is	O
already	O
there	O
.	O
</s>
