<s>
In	O
IBM	O
terminology	O
,	O
an	O
Access	B-Application
Register	I-Application
(	O
AR	O
)	O
is	O
a	O
hardware	B-General_Concept
register	I-General_Concept
in	O
ESA/370	O
and	O
later	O
processors	B-General_Concept
.	O
</s>
<s>
Access	B-Application
registers	I-Application
work	O
in	O
conjunction	O
with	O
the	O
general	O
purpose	O
registers	O
,	O
giving	O
a	O
program	O
transparent	O
access	O
to	O
up	O
to	O
sixteen	O
2GB	O
address	B-General_Concept
spaces	I-General_Concept
simultaneously	O
.	O
</s>
<s>
In	O
IBM	B-Device
System/360	I-Device
architecture	I-Device
all	O
instructions	O
address	O
memory	O
by	O
specifying	O
a	O
12-bit	O
offset	O
(	O
4096	O
bytes	O
)	O
from	O
a	O
value	O
in	O
a	O
"	O
base	O
register	O
"	O
with	O
optional	O
indexing	O
.	O
</s>
<s>
System/370	O
-XA	O
extended	O
the	O
architecture	O
to	O
allow	O
31-bit	O
addressing	O
and	O
address	B-General_Concept
spaces	I-General_Concept
of	O
up	O
to	O
2GB	O
.	O
</s>
<s>
Enterprise	O
Systems	O
Architecture/370	O
further	O
expanded	O
addressing	O
capabilities	O
with	O
access	B-Application
registers	I-Application
.	O
</s>
<s>
Sixteen	O
32-bit	O
access	B-Application
registers	I-Application
"	O
shadow	O
"	O
the	O
sixteen	O
general-purpose	O
registers	O
.	O
</s>
<s>
In	O
a	O
processor	O
mode	O
called	O
access-register	O
mode	O
the	O
access	B-Application
register	I-Application
corresponding	O
to	O
the	O
specified	O
base	O
register	O
designates	O
the	O
operand	O
address	B-General_Concept
space	I-General_Concept
to	O
be	O
accessed	O
.	O
</s>
<s>
The	O
contents	O
of	O
an	O
access	B-Application
register	I-Application
is	O
called	O
an	O
"	O
Access-list	O
entry	O
token	O
"	O
(	O
ALET	O
)	O
,	O
which	O
contains	O
an	O
index	O
into	O
a	O
system	O
table	O
identifying	O
the	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
