<s>
Pseudo-LRU	B-General_Concept
or	O
PLRU	B-General_Concept
is	O
a	O
family	O
of	O
cache	B-General_Concept
algorithms	I-General_Concept
which	O
improve	O
on	O
the	O
performance	O
of	O
the	O
Least	O
Recently	O
Used	O
(	O
LRU	O
)	O
algorithm	O
by	O
replacing	O
values	O
using	O
approximate	O
measures	O
of	O
age	O
rather	O
than	O
maintaining	O
the	O
exact	O
age	O
of	O
every	O
value	O
in	O
the	O
cache	O
.	O
</s>
<s>
PLRU	B-General_Concept
usually	O
refers	O
to	O
two	O
cache	B-General_Concept
replacement	I-General_Concept
algorithms	I-General_Concept
:	O
tree-PLRU	O
and	O
bit-PLRU	O
.	O
</s>
<s>
Tree-PLRU	O
is	O
an	O
efficient	O
algorithm	O
to	O
select	O
an	O
item	O
that	O
most	O
likely	O
has	O
not	O
been	O
accessed	O
very	O
recently	O
,	O
given	O
a	O
set	O
of	O
items	O
and	O
a	O
sequence	O
of	O
access	O
events	O
to	O
the	O
items	O
.	O
</s>
<s>
This	O
technique	O
is	O
used	O
in	O
the	O
CPU	B-General_Concept
cache	I-General_Concept
of	O
the	O
Intel	B-General_Concept
486	I-General_Concept
and	O
in	O
many	O
processors	O
in	O
the	O
PowerPC	B-Architecture
family	I-Architecture
,	O
such	O
as	O
Freescale	O
's	O
PowerPC	B-General_Concept
G4	I-General_Concept
used	O
by	O
Apple	O
Computer	O
.	O
</s>
<s>
The	O
algorithm	O
works	O
as	O
follows	O
:	O
consider	O
a	O
binary	B-Language
search	I-Language
tree	I-Language
for	O
the	O
items	O
in	O
question	O
.	O
</s>
<s>
Each	O
node	O
of	O
the	O
tree	O
has	O
a	O
one-bit	O
flag	O
denoting	O
"	O
go	O
left	O
to	O
insert	O
a	O
pseudo-LRU	B-General_Concept
element	O
"	O
or	O
"	O
go	O
right	O
to	O
insert	O
a	O
pseudo-LRU	B-General_Concept
element	O
"	O
.	O
</s>
<s>
To	O
find	O
a	O
pseudo-LRU	B-General_Concept
element	O
,	O
traverse	O
the	O
tree	O
according	O
to	O
the	O
values	O
of	O
the	O
flags	O
.	O
</s>
<s>
Bit-PLRU	O
stores	O
one	O
status	O
bit	O
for	O
each	O
cache	O
line	O
.	O
</s>
