<s>
An	O
index	B-General_Concept
register	I-General_Concept
in	O
a	O
computer	O
's	O
CPU	B-General_Concept
is	O
a	O
processor	B-General_Concept
register	I-General_Concept
(	O
or	O
an	O
assigned	O
memory	O
location	O
)	O
used	O
for	O
pointing	O
to	O
operand	O
addresses	O
during	O
the	O
run	O
of	O
a	O
program	O
.	O
</s>
<s>
It	O
is	O
useful	O
for	O
stepping	O
through	O
strings	O
and	O
arrays	B-Data_Structure
.	O
</s>
<s>
In	O
some	O
architectures	B-General_Concept
it	O
is	O
used	O
for	O
read/writing	O
blocks	O
of	O
memory	O
.	O
</s>
<s>
Depending	O
on	O
the	O
architecture	O
it	O
maybe	O
a	O
dedicated	O
index	B-General_Concept
register	I-General_Concept
or	O
a	O
general-purpose	O
register	O
.	O
</s>
<s>
Some	O
instruction	B-General_Concept
sets	I-General_Concept
allow	O
more	O
than	O
one	O
index	B-General_Concept
register	I-General_Concept
to	O
be	O
used	O
;	O
in	O
that	O
case	O
additional	O
instruction	O
fields	O
may	O
specify	O
which	O
index	B-General_Concept
registers	I-General_Concept
to	O
use	O
.	O
</s>
<s>
Generally	O
,	O
the	O
contents	O
of	O
an	O
index	B-General_Concept
register	I-General_Concept
is	O
added	O
to	O
(	O
in	O
some	O
cases	O
subtracted	O
from	O
)	O
an	O
immediate	O
address	O
(	O
that	O
can	O
be	O
part	O
of	O
the	O
instruction	O
itself	O
or	O
held	O
in	O
another	O
register	O
)	O
to	O
form	O
the	O
"	O
effective	O
"	O
address	O
of	O
the	O
actual	O
data	O
(	O
operand	O
)	O
.	O
</s>
<s>
Special	O
instructions	O
are	O
typically	O
provided	O
to	O
test	O
the	O
index	B-General_Concept
register	I-General_Concept
and	O
,	O
if	O
the	O
test	O
fails	O
,	O
increments	O
the	O
index	B-General_Concept
register	I-General_Concept
by	O
an	O
immediate	O
constant	O
and	O
branches	O
,	O
typically	O
to	O
the	O
start	O
of	O
the	O
loop	O
.	O
</s>
<s>
While	O
normally	O
processors	O
that	O
allow	O
an	O
instruction	O
to	O
specify	O
multiple	O
index	B-General_Concept
registers	I-General_Concept
add	O
the	O
contents	O
together	O
,	O
IBM	O
had	O
a	O
line	O
of	O
computers	O
in	O
which	O
the	O
contents	O
were	O
or	O
'd	O
together	O
.	O
</s>
<s>
Index	B-General_Concept
registers	I-General_Concept
has	O
proved	O
useful	O
for	O
doing	O
vector/array	O
operations	O
and	O
in	O
commercial	O
data	O
processing	O
for	O
navigating	O
from	O
field	O
to	O
field	O
within	O
records	O
.	O
</s>
<s>
In	O
both	O
uses	O
index	B-General_Concept
registers	I-General_Concept
substantially	O
reduced	O
the	O
amount	O
of	O
memory	O
used	O
and	O
increased	O
execution	O
speed	O
.	O
</s>
<s>
In	O
early	O
computers	O
without	O
any	O
form	O
of	O
indirect	O
addressing	O
,	O
array	B-Data_Structure
operations	O
had	O
to	O
be	O
performed	O
by	O
modifying	B-Application
the	O
instruction	O
address	O
,	O
which	O
required	O
several	O
additional	O
program	O
steps	O
and	O
used	O
up	O
more	O
computer	O
memory	O
,	O
a	O
scarce	O
resource	O
in	O
computer	O
installations	O
of	O
the	O
early	O
era	O
(	O
as	O
well	O
as	O
in	O
early	O
microcomputers	O
two	O
decades	O
later	O
)	O
.	O
</s>
<s>
Index	B-General_Concept
registers	I-General_Concept
,	O
commonly	O
known	O
as	O
B-lines	O
in	O
early	O
British	O
computers	O
,	O
as	O
B-registers	O
on	O
some	O
machines	O
and	O
a	O
X-registers	O
on	O
others	O
,	O
were	O
first	O
used	O
in	O
the	O
British	O
Manchester	B-Device
Mark	I-Device
1	I-Device
computer	O
,	O
in	O
1949	O
.	O
</s>
<s>
In	O
general	O
,	O
index	B-General_Concept
registers	I-General_Concept
became	O
a	O
standard	O
part	O
of	O
computers	O
during	O
the	O
technology	O
's	O
second	B-Architecture
generation	I-Architecture
,	O
roughly	O
19541966	O
.	O
</s>
<s>
Most	O
machines	O
in	O
the	O
IBM	B-Device
700/7000	I-Device
mainframe	B-Architecture
series	O
had	O
them	O
,	O
starting	O
with	O
the	O
IBM	B-Device
704	I-Device
in	O
1954	O
,	O
though	O
they	O
were	O
optional	O
on	O
some	O
smaller	O
machines	O
such	O
as	O
the	O
IBM	B-Device
650	I-Device
and	O
IBM	B-Device
1401	I-Device
.	O
</s>
<s>
Early	O
"	O
small	O
machines	O
"	O
with	O
index	B-General_Concept
registers	I-General_Concept
include	O
the	O
AN/USQ	B-Device
-17	I-Device
,	O
around	O
1960	O
,	O
and	O
the	O
9	B-Device
series	I-Device
of	O
real-time	O
computers	O
from	O
Scientific	O
Data	O
Systems	O
,	O
from	O
the	O
early	O
1960s	O
.	O
</s>
<s>
The	O
1964	O
GE-635	B-Device
has	O
8	O
dedicated	O
X-registers	O
;	O
however	O
,	O
it	O
also	O
allows	O
indexing	O
by	O
the	O
instruction	B-General_Concept
counter	I-General_Concept
or	O
by	O
either	O
half	O
of	O
the	O
A	O
or	O
Q	O
register	O
.	O
</s>
<s>
The	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
PDP-6	B-Device
,	O
introduced	O
in	O
1964	O
,	O
and	O
the	O
IBM	B-Application
System/360	I-Application
,	O
announced	O
in	O
1964	O
,	O
do	O
not	O
include	O
dedicated	O
index	B-General_Concept
registers	I-General_Concept
;	O
instead	O
,	O
they	O
have	O
general-purpose	O
registers	O
(	O
called	O
"	O
accumulators	O
"	O
in	O
the	O
PDP-6	B-Device
)	O
that	O
can	O
contain	O
either	O
numerical	O
values	O
or	O
addresses	O
.	O
</s>
<s>
The	O
memory	O
address	O
of	O
an	O
operand	O
is	O
,	O
in	O
the	O
PDP-6	B-Device
,	O
the	O
sum	O
of	O
the	O
contents	O
of	O
a	O
general-purpose	O
register	O
and	O
an	O
18-bit	O
offset	O
and	O
,	O
on	O
the	O
System/360	B-Application
,	O
the	O
sum	O
of	O
the	O
contents	O
of	O
two	O
general-purpose	O
registers	O
and	O
a	O
12-bit	O
offset	O
.	O
</s>
<s>
The	O
compatible	O
PDP-10	B-Device
line	O
of	O
successors	O
to	O
the	O
PDP-6	B-Device
,	O
and	O
the	O
IBM	B-Device
System/370	I-Device
and	O
later	O
compatible	O
successors	O
to	O
the	O
System/360	B-Application
,	O
including	O
the	O
current	O
z/Architecture	B-Device
,	O
work	O
in	O
the	O
same	O
fashion	O
.	O
</s>
<s>
The	O
1969	O
Data	B-Device
General	I-Device
Nova	I-Device
and	O
successor	O
Eclipse	B-Device
,	O
and	O
1970	O
DEC	B-Device
PDP-11	I-Device
,	O
minicomputers	B-Architecture
also	O
provided	O
general-purpose	O
registers	O
(	O
called	O
"	O
accumulators	O
"	O
in	O
the	O
Nova	O
and	O
Eclipse	B-Device
)	O
,	O
rather	O
than	O
separate	O
accumulators	O
and	O
index	B-General_Concept
registers	I-General_Concept
,	O
as	O
did	O
their	O
Eclipse	B-Device
MV	I-Device
and	O
VAX	B-Device
32-bit	O
superminicomputer	B-Device
successors	O
.	O
</s>
<s>
In	O
the	O
PDP-11	B-Device
and	O
VAX	B-Device
,	O
all	O
registers	O
could	O
be	O
used	O
when	O
calculating	O
the	O
memory	O
address	O
of	O
an	O
operand	O
;	O
in	O
the	O
Nova	O
,	O
Eclipse	B-Device
,	O
and	O
Eclipse	B-Device
MV	I-Device
,	O
only	O
registers	O
2	O
and	O
3	O
could	O
be	O
used	O
.	O
</s>
<s>
The	O
1971	O
CDC	B-Device
STAR-100	I-Device
has	O
a	O
register	O
file	O
of	O
256	O
64-bit	O
registers	O
,	O
9	O
of	O
which	O
are	O
reserved	O
.	O
</s>
<s>
Unlike	O
most	O
computers	O
,	O
the	O
STAR-100	O
instructions	O
only	O
have	O
register	O
fields	O
and	O
operand	O
fields	O
,	O
so	O
the	O
registers	O
serve	O
more	O
as	O
pointer	B-General_Concept
registers	I-General_Concept
than	O
as	O
traditional	O
index	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
While	O
the	O
Intel	B-General_Concept
8080	I-General_Concept
allowed	O
indirect	O
addressing	O
via	O
a	O
register	O
,	O
the	O
first	O
microprocessor	B-Architecture
with	O
a	O
true	O
index	B-General_Concept
register	I-General_Concept
appears	O
to	O
have	O
been	O
the	O
1974	O
Motorola	B-Device
6800	I-Device
.	O
</s>
<s>
In	O
1975	O
,	O
the	O
8-bit	O
MOS	B-General_Concept
Technology	I-General_Concept
6502	I-General_Concept
processor	O
had	O
two	O
index	B-General_Concept
registers	I-General_Concept
'	O
X	O
 '	O
and	O
'	O
Y	O
 '	O
.	O
</s>
<s>
In	O
1978	O
,	O
the	O
Intel	B-General_Concept
8086	I-General_Concept
,	O
the	O
first	O
x86	B-Operating_System
processor	O
,	O
had	O
eight	O
16-bit	O
registers	O
,	O
referred	O
to	O
as	O
"	O
general-purpose	O
"	O
,	O
all	O
of	O
which	O
can	O
be	O
used	O
as	O
integer	O
data	O
registers	O
in	O
most	O
operations	O
;	O
four	O
of	O
them	O
,	O
'	O
SI	O
 '	O
(	O
source	O
index	O
)	O
,	O
'	O
DI	O
 '	O
(	O
destination	O
index	O
)	O
,	O
'	O
BX	O
 '	O
(	O
base	O
)	O
,	O
and	O
'	O
BP	O
 '	O
(	O
base	O
pointer	O
)	O
,	O
can	O
also	O
be	O
used	O
when	O
computing	O
the	O
memory	O
address	O
of	O
an	O
operand	O
,	O
which	O
is	O
the	O
sum	O
of	O
one	O
of	O
those	O
registers	O
and	O
a	O
displacement	O
,	O
or	O
the	O
sum	O
of	O
one	O
of	O
'	O
BX	O
 '	O
or	O
'	O
BP	O
"	O
,	O
one	O
of	O
'	O
SI	O
 '	O
or	O
'	O
DI	O
 '	O
,	O
and	O
a	O
displacement	O
.	O
</s>
<s>
The	O
1979	O
Intel	B-Device
8088	I-Device
,	O
and	O
the	O
16-bit	O
Intel	B-Device
80186	I-Device
,	O
Intel	B-Device
80188	I-Device
,	O
and	O
Intel	B-General_Concept
80286	I-General_Concept
successors	O
work	O
the	O
same	O
.	O
</s>
<s>
In	O
1985	O
,	O
the	O
i386	B-General_Concept
,	O
a	O
32-bit	O
successor	O
to	O
those	O
processors	O
,	O
introducing	O
the	O
IA-32	B-Device
32-bit	O
version	O
of	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
,	O
extended	O
the	O
eight	O
16-bit	O
registers	O
to	O
32	O
bits	O
,	O
with	O
"	O
E	O
"	O
added	O
to	O
the	O
beginning	O
of	O
the	O
register	O
name	O
;	O
in	O
IA-32	B-Device
,	O
the	O
memory	O
address	O
of	O
an	O
operand	O
is	O
the	O
sum	O
of	O
one	O
of	O
those	O
eight	O
registers	O
,	O
one	O
of	O
seven	O
of	O
those	O
registers	O
(	O
the	O
stack	O
pointer	O
is	O
not	O
allowed	O
as	O
the	O
second	O
register	O
here	O
)	O
multiplied	O
by	O
a	O
power	O
of	O
2	O
between	O
1	O
and	O
8	O
,	O
and	O
a	O
displacement	O
.	O
</s>
<s>
The	O
Advanced	O
Micro	O
Devices	O
Opteron	B-General_Concept
,	O
the	O
first	O
model	O
of	O
which	O
was	O
released	O
in	O
2003	O
,	O
introduced	O
x86-64	B-Device
,	O
the	O
64-bit	O
version	O
of	O
the	O
x86	B-Operating_System
instruction	B-General_Concept
set	I-General_Concept
;	O
in	O
x86-64	B-Device
,	O
the	O
general-purpose	O
registers	O
were	O
extended	O
to	O
64	O
bits	O
,	O
and	O
eight	O
additional	O
general-purpose	O
registers	O
were	O
added	O
;	O
the	O
memory	O
address	O
of	O
an	O
operand	O
is	O
the	O
sum	O
of	O
two	O
of	O
those	O
16	O
registers	O
and	O
a	O
displacement	O
.	O
</s>
<s>
The	O
reduced	B-Architecture
instruction	I-Architecture
set	I-Architecture
computing	I-Architecture
(	O
RISC	B-Architecture
)	O
instruction	B-General_Concept
sets	I-General_Concept
introduced	O
in	O
the	O
1980s	O
and	O
1990s	O
all	O
provide	O
general-purpose	O
registers	O
that	O
can	O
contain	O
either	O
numerical	O
values	O
or	O
address	O
values	O
.	O
</s>
<s>
In	O
most	O
of	O
those	O
instruction	B-General_Concept
sets	I-General_Concept
,	O
there	O
are	O
32	O
general-purpose	O
registers	O
(	O
in	O
some	O
of	O
those	O
instruction	B-General_Concept
sets	I-General_Concept
,	O
the	O
value	O
of	O
one	O
of	O
those	O
registers	O
is	O
hardwired	O
to	O
zero	O
)	O
could	O
be	O
used	O
to	O
calculate	O
the	O
operand	O
address	O
;	O
they	O
did	O
not	O
have	O
dedicated	O
index	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
In	O
the	O
32-bit	O
version	O
of	O
the	O
ARM	B-Architecture
architecture	I-Architecture
,	O
first	O
developed	O
in	O
1985	O
,	O
there	O
are	O
only	O
16	O
registers	O
designated	O
as	O
"	O
general-purpose	O
registers	O
"	O
,	O
but	O
only	O
13	O
of	O
them	O
can	O
be	O
used	O
for	O
all	O
purposes	O
,	O
with	O
register	O
R15	O
containing	O
the	O
program	B-General_Concept
counter	I-General_Concept
.	O
</s>
<s>
In	O
the	O
64-bit	O
version	O
of	O
the	O
ARM	B-Architecture
architecture	I-Architecture
,	O
there	O
are	O
31	O
64-bit	O
general-purpose	O
registers	O
plus	O
a	O
stack	O
pointer	O
and	O
a	O
zero	O
register	O
;	O
the	O
memory	O
address	O
of	O
a	O
load	O
or	O
store	O
instruction	O
is	O
the	O
sum	O
of	O
any	O
of	O
the	O
31	O
registers	O
and	O
either	O
a	O
displacement	O
or	O
another	O
of	O
the	O
registers	O
.	O
</s>
<s>
Here	O
is	O
a	O
simple	O
example	O
of	O
index	B-General_Concept
register	I-General_Concept
use	O
in	O
assembly	O
language	O
pseudo-code	O
that	O
sums	O
a	O
100	O
entry	O
array	B-Data_Structure
of	O
4-byte	O
words	O
:	O
</s>
