<s>
The	O
zero	B-General_Concept
page	I-General_Concept
or	O
base	B-General_Concept
page	I-General_Concept
is	O
the	O
block	O
of	O
memory	B-General_Concept
at	O
the	O
very	O
beginning	O
of	O
a	O
computer	O
's	O
address	B-General_Concept
space	I-General_Concept
;	O
that	O
is	O
,	O
the	O
page	B-General_Concept
whose	O
starting	O
address	O
is	O
zero	O
.	O
</s>
<s>
The	O
size	O
of	O
a	O
page	B-General_Concept
depends	O
on	O
the	O
context	O
,	O
and	O
the	O
significance	O
of	O
zero	B-General_Concept
page	I-General_Concept
memory	B-General_Concept
versus	O
higher	O
addressed	O
memory	B-General_Concept
is	O
highly	O
dependent	O
on	O
machine	O
architecture	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Motorola	B-Device
6800	I-Device
and	O
MOS	B-General_Concept
Technology	I-General_Concept
6502	I-General_Concept
processor	O
families	O
treat	O
the	O
first	O
256	O
bytes	B-Application
of	O
memory	B-General_Concept
specially	O
,	O
whereas	O
many	O
other	O
processors	O
do	O
not	O
.	O
</s>
<s>
Unlike	O
more	O
modern	O
hardware	O
,	O
in	O
the	O
1970s	O
computer	O
RAM	B-Architecture
was	O
as	O
fast	O
as	O
or	O
faster	O
than	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
Thus	O
it	O
made	O
sense	O
to	O
have	O
few	O
registers	O
and	O
use	O
the	O
main	O
memory	B-General_Concept
as	O
an	O
extended	O
pool	O
of	O
extra	O
registers	O
.	O
</s>
<s>
In	O
machines	O
with	O
a	O
relatively	O
wide	O
16-bit	B-Device
address	B-Architecture
bus	I-Architecture
and	O
comparatively	O
narrow	O
8-bit	O
data	B-General_Concept
bus	I-General_Concept
,	O
accessing	O
zero	B-General_Concept
page	I-General_Concept
locations	O
could	O
be	O
faster	O
than	O
accessing	O
other	O
locations	O
.	O
</s>
<s>
Zero	B-General_Concept
page	I-General_Concept
addressing	O
now	O
has	O
mostly	O
historical	O
significance	O
,	O
since	O
the	O
developments	O
in	O
integrated	O
circuit	O
technology	O
have	O
made	O
adding	O
more	O
registers	O
to	O
a	O
CPU	B-General_Concept
less	O
expensive	O
and	O
CPU	B-General_Concept
operations	O
much	O
faster	O
than	O
RAM	B-Architecture
accesses	O
.	O
</s>
<s>
The	O
actual	O
size	O
of	O
the	O
zero	B-General_Concept
page	I-General_Concept
in	O
bytes	B-Application
is	O
determined	O
by	O
the	O
microprocessor	B-Architecture
design	O
and	O
in	O
older	O
designs	O
,	O
is	O
often	O
equal	O
to	O
the	O
largest	O
value	O
that	O
can	O
be	O
referenced	O
by	O
the	O
processor	O
's	O
indexing	O
registers	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
aforementioned	O
8-bit	O
processors	O
have	O
8-bit	O
index	O
registers	O
and	O
a	O
page	B-General_Concept
size	I-General_Concept
of	O
256	O
bytes	B-Application
.	O
</s>
<s>
Therefore	O
,	O
their	O
zero	B-General_Concept
page	I-General_Concept
extends	O
from	O
address	O
0	O
to	O
address	O
255	O
.	O
</s>
<s>
In	O
early	O
computers	O
,	O
including	O
the	O
PDP-8	B-Device
,	O
the	O
zero	B-General_Concept
page	I-General_Concept
had	O
a	O
special	O
fast	O
addressing	B-Language
mode	I-Language
,	O
which	O
facilitated	O
its	O
use	O
for	O
temporary	O
storage	B-General_Concept
of	I-General_Concept
data	I-General_Concept
and	O
compensated	O
for	O
the	O
relative	O
shortage	O
of	O
CPU	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
The	O
PDP-8	B-Device
had	O
only	O
one	O
register	B-General_Concept
,	O
so	O
zero	B-General_Concept
page	I-General_Concept
addressing	O
was	O
essential	O
.	O
</s>
<s>
In	O
the	O
original	O
PDP-10	O
KA-10	O
models	O
,	O
the	O
registers	O
it	O
has	O
are	O
simply	O
the	O
first	O
16	O
words	O
,	O
36-bits	O
long	O
,	O
of	O
main	O
memory	B-General_Concept
.	O
</s>
<s>
Those	O
locations	O
can	O
be	O
accessed	O
as	O
both	O
registers	O
and	O
memory	B-General_Concept
locations	O
.	O
</s>
<s>
Unlike	O
more	O
modern	O
hardware	O
,	O
1970s	O
computer	O
RAM	B-Architecture
was	O
as	O
fast	O
as	O
or	O
faster	O
than	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
Thus	O
,	O
it	O
made	O
sense	O
to	O
have	O
few	O
registers	O
and	O
use	O
the	O
main	O
memory	B-General_Concept
as	O
an	O
extended	O
pool	O
of	O
extra	O
registers	O
.	O
</s>
<s>
In	O
machines	O
with	O
a	O
relatively	O
wide	O
16-bit	B-Device
address	B-Architecture
bus	I-Architecture
and	O
comparatively	O
narrow	O
8-bit	O
data	B-General_Concept
bus	I-General_Concept
,	O
accessing	O
zero	B-General_Concept
page	I-General_Concept
locations	O
could	O
be	O
faster	O
than	O
accessing	O
other	O
locations	O
.	O
</s>
<s>
Since	O
zero	B-General_Concept
page	I-General_Concept
locations	O
could	O
be	O
addressed	O
by	O
a	O
single	O
byte	B-Application
,	O
the	O
instructions	B-General_Concept
accessing	O
them	O
could	O
be	O
smaller	O
and	O
faster	O
loading	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
MOS	B-General_Concept
Technology	I-General_Concept
6502	I-General_Concept
has	O
only	O
one	O
general	O
purpose	O
register	B-General_Concept
(	O
the	O
accumulator	O
)	O
.	O
</s>
<s>
To	O
offset	O
this	O
weakness	O
and	O
gain	O
a	O
performance	O
advantage	O
,	O
it	O
was	O
designed	O
to	O
make	O
special	O
use	O
of	O
the	O
zero	B-General_Concept
page	I-General_Concept
,	O
providing	O
special	O
instructions	B-General_Concept
that	O
are	O
smaller	O
,	O
thus	O
requiring	O
fewer	O
memory	B-General_Concept
fetch	O
cycles	O
and	O
executing	O
faster	O
.	O
</s>
<s>
Many	O
instructions	B-General_Concept
are	O
coded	O
differently	O
for	O
zero	B-General_Concept
page	I-General_Concept
and	O
non-zero	O
page	B-General_Concept
addresses	O
,	O
this	O
is	O
called	O
zero-page	O
addressing	O
in	O
6502	B-General_Concept
terminology	O
(	O
it	O
was	O
called	O
direct	B-Language
addressing	I-Language
in	O
6800	O
terminology	O
)	O
:	O
</s>
<s>
The	O
above	O
two	O
instructions	B-General_Concept
both	O
accomplish	O
the	O
same	O
thing	O
:	O
they	O
load	O
the	O
value	O
of	O
memory	B-General_Concept
location	O
$00	O
into	O
the	O
.A	O
register	B-General_Concept
(	O
accumulator	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
first	O
instruction	O
is	O
only	O
two	O
bytes	B-Application
long	O
and	O
requires	O
three	O
clock	O
cycles	O
to	O
complete	O
.	O
</s>
<s>
The	O
second	O
instruction	O
is	O
three	O
bytes	B-Application
in	O
length	O
and	O
requires	O
four	O
clock	O
cycles	O
to	O
execute	O
.	O
</s>
<s>
Some	O
CPUs	O
like	O
the	O
Motorola	B-Device
6809	I-Device
extend	O
this	O
concept	O
by	O
allowing	O
fast	O
accesses	O
to	O
whichever	O
256-byte	O
page	B-General_Concept
,	O
the	O
so	O
called	O
direct	O
page	B-General_Concept
,	O
is	O
selected	O
by	O
a	O
special	O
8-bit	O
"	O
Direct	O
Page	B-General_Concept
"	O
(	O
DP	O
)	O
register	B-General_Concept
.	O
</s>
<s>
This	O
is	O
called	O
direct	O
page	B-General_Concept
addressing	O
.	O
</s>
<s>
Contrary	O
to	O
the	O
zero	B-General_Concept
page	I-General_Concept
's	O
original	O
preferential	O
use	O
,	O
some	O
modern	O
operating	O
systems	O
such	O
as	O
FreeBSD	B-Operating_System
,	O
Linux	B-Application
and	O
Microsoft	B-Application
Windows	I-Application
actually	O
make	O
the	O
zero	B-General_Concept
page	I-General_Concept
inaccessible	O
to	O
trap	O
uses	O
of	O
null	O
pointers	O
.	O
</s>
<s>
Such	O
pointer	O
values	O
may	O
legitimately	O
indicate	O
uninitialized	O
values	O
or	O
sentinel	B-Data_Structure
nodes	I-Data_Structure
,	O
but	O
they	O
do	O
not	O
point	O
to	O
valid	O
objects	O
.	O
</s>
<s>
Buggy	B-Error_Name
code	I-Error_Name
may	O
try	O
to	O
access	O
an	O
object	O
via	O
a	O
null	O
pointer	O
,	O
and	O
this	O
can	O
be	O
trapped	O
at	O
the	O
operating	O
system	O
level	O
as	O
a	O
memory	B-General_Concept
access	B-Error_Name
violation	I-Error_Name
.	O
</s>
<s>
Some	O
computer	O
architectures	O
still	O
reserve	O
the	O
beginning	O
of	O
address	B-General_Concept
space	I-General_Concept
for	O
other	O
purposes	O
,	O
though	O
;	O
for	O
instance	O
,	O
Intel	B-Operating_System
x86	I-Operating_System
systems	O
reserve	O
the	O
first	O
256	O
double-words	O
of	O
address	B-General_Concept
space	I-General_Concept
for	O
the	O
interrupt	B-General_Concept
vector	I-General_Concept
table	I-General_Concept
(	O
IVT	O
)	O
if	O
they	O
run	O
in	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
A	O
similar	O
technique	O
of	O
using	O
the	O
zero	B-General_Concept
page	I-General_Concept
for	O
hardware	O
related	O
vectors	O
was	O
employed	O
in	O
the	O
ARM	O
architecture	O
.	O
</s>
<s>
In	O
badly	O
written	O
programs	O
this	O
could	O
lead	O
to	O
"	O
ofla	O
"	O
behaviour	O
,	O
where	O
a	O
program	O
tries	O
to	O
read	O
information	O
from	O
an	O
unintended	O
memory	B-General_Concept
area	O
,	O
and	O
treats	O
executable	O
code	O
as	O
data	O
or	O
vice	O
versa	O
.	O
</s>
<s>
This	O
is	O
especially	O
problematic	O
if	O
the	O
zero	B-General_Concept
page	I-General_Concept
area	O
is	O
used	O
to	O
store	O
system	O
jump	O
vectors	O
and	O
the	O
firmware	O
is	O
tricked	O
into	O
overwriting	O
them	O
.	O
</s>
<s>
In	O
8-bit	O
CP/M	B-Application
,	O
the	O
zero	B-General_Concept
page	I-General_Concept
is	O
used	O
for	O
communication	O
between	O
the	O
running	O
program	O
and	O
the	O
operating	O
system	O
.	O
</s>
<s>
In	O
some	O
processor	O
architectures	O
,	O
like	O
that	O
of	O
the	O
Intel	B-General_Concept
4004	I-General_Concept
4-bit	O
processor	O
,	O
memory	B-General_Concept
was	O
divided	O
into	O
(	O
256	O
byte	B-Application
)	O
pages	O
and	O
special	O
precautions	O
had	O
to	O
be	O
taken	O
when	O
the	O
control	O
flow	O
crossed	O
page	B-General_Concept
boundaries	O
,	O
as	O
some	O
machine	B-Language
instructions	I-Language
exhibited	O
different	O
behaviour	O
if	O
located	O
in	O
the	O
last	O
few	O
instructions	B-General_Concept
of	O
a	O
page	B-General_Concept
,	O
so	O
that	O
only	O
few	O
instructions	B-General_Concept
were	O
recommended	O
to	O
jump	O
between	O
pages	O
.	O
</s>
