<s>
In	O
a	O
computer	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
(	O
ISA	O
)	O
,	O
an	O
execute	B-General_Concept
instruction	I-General_Concept
is	O
a	O
machine	B-Language
language	I-Language
instruction	I-Language
which	O
treats	O
data	O
as	O
a	O
machine	B-Language
instruction	I-Language
and	O
executes	O
it	O
.	O
</s>
<s>
It	O
can	O
be	O
considered	O
a	O
fourth	O
mode	O
of	O
instruction	O
sequencing	O
after	O
ordinary	B-General_Concept
sequential	I-General_Concept
execution	I-General_Concept
,	O
branching	B-General_Concept
,	O
and	O
interrupting	B-Application
.	O
</s>
<s>
Since	O
it	O
is	O
an	O
instruction	O
that	O
operates	O
on	O
other	O
instructions	O
like	O
the	O
repeat	B-General_Concept
instruction	I-General_Concept
,	O
it	O
has	O
also	O
been	O
classified	O
as	O
a	O
meta-instruction	O
.	O
</s>
<s>
Many	O
computer	O
families	O
introduced	O
in	O
the	O
1950s	O
and	O
1960s	O
include	O
execute	B-General_Concept
instructions	I-General_Concept
:	O
the	O
IBM	B-Device
709	I-Device
and	O
IBM	B-Device
7090	I-Device
(	O
op	O
code	O
mnemonic	O
:	O
)	O
,	O
the	O
IBM	B-Device
7030	I-Device
Stretch	I-Device
(	O
,	O
)	O
,	O
the	O
PDP-1/-4/-9/	O
-15	B-Device
(	O
)	O
,	O
the	O
UNIVAC	B-Device
1100/2200	I-Device
(	O
)	O
,	O
the	O
CDC	O
924	O
(	O
)	O
,	O
the	O
PDP-6/	O
-10	B-Device
(	O
)	O
,	O
the	O
IBM	B-Application
System/360	I-Application
(	O
)	O
,	O
the	O
GE-600/Honeywell	O
6000	O
(	O
,	O
)	O
,	O
and	O
the	O
SDS-9xx	B-Device
(	O
)	O
.	O
</s>
<s>
Fewer	O
1970s	O
designs	O
include	O
execute	B-General_Concept
instructions	I-General_Concept
:	O
the	O
Nuclear	B-Device
Data	I-Device
812	I-Device
minicomputer	O
(	O
1971	O
)	O
(	O
)	O
,	O
the	O
HP	B-Device
3000	I-Device
(	O
1972	O
)	O
(	O
)	O
,	O
and	O
the	O
Texas	O
Instruments	O
TI-990	B-Device
(	O
1975	O
)	O
and	O
its	B-Application
microprocessor	O
version	O
,	O
the	O
TMS9900	B-General_Concept
(	O
1976	O
)	O
(	O
)	O
.	O
</s>
<s>
An	O
execute	B-General_Concept
instruction	I-General_Concept
was	O
proposed	O
for	O
the	O
PDP-11	B-Device
in	O
1970	O
,	O
but	O
never	O
implemented	O
for	O
it	O
or	O
its	B-Application
successor	O
,	O
the	O
VAX	B-Device
.	O
</s>
<s>
Modern	O
instruction	B-General_Concept
sets	I-General_Concept
do	O
not	O
include	O
execute	B-General_Concept
instructions	I-General_Concept
because	O
they	O
interfere	O
with	O
pipelining	B-General_Concept
,	O
prefetching	B-General_Concept
,	O
and	O
other	O
optimizations	O
.	O
</s>
<s>
Some	O
architectures	O
allow	O
the	O
target	O
instruction	O
to	O
itself	O
be	O
an	O
execute	B-General_Concept
instruction	I-General_Concept
;	O
others	O
do	O
not	O
.	O
</s>
<s>
The	O
target	O
instruction	O
is	O
executed	O
as	O
if	O
it	O
were	O
in	O
the	O
memory	O
location	O
of	O
the	O
execute	B-General_Concept
instruction	I-General_Concept
.	O
</s>
<s>
If	O
,	O
for	O
example	O
,	O
it	O
is	O
a	O
subroutine	O
call	O
instruction	O
,	O
execution	O
is	O
transferred	O
to	O
the	O
subroutine	O
,	O
with	O
the	O
return	O
location	O
being	O
the	O
location	O
after	O
the	O
execute	B-General_Concept
instruction	I-General_Concept
.	O
</s>
<s>
However	O
,	O
some	O
architectures	O
implement	O
variants	O
of	O
the	O
execute	B-General_Concept
instruction	I-General_Concept
which	O
inhibit	O
branches	O
.	O
</s>
<s>
The	O
System/360	B-Application
supports	O
variable-length	O
target	O
instructions	O
.	O
</s>
<s>
The	O
target	O
instruction	O
must	B-Application
start	I-Application
on	I-Application
an	I-Application
even-numbered	I-Application
byte	I-Application
.	O
</s>
<s>
The	O
GE-600	B-Device
series	I-Device
supports	O
execution	O
of	O
two-instruction	O
sequences	O
,	O
which	O
must	O
be	O
doubleword-aligned	O
.	O
</s>
<s>
Some	O
architectures	O
support	O
an	O
execute	B-General_Concept
instruction	I-General_Concept
which	O
operates	O
in	O
a	O
different	O
protection	B-Operating_System
and	I-Operating_System
address	I-Operating_System
relocation	I-Operating_System
mode	I-Operating_System
.	O
</s>
<s>
For	O
example	O
,	O
the	O
ITS	B-Application
PDP-10	B-Device
paging	O
device	O
supports	O
a	O
privileged-mode	B-Operating_System
'	O
execute	O
relocated	O
 '	O
instruction	O
which	O
allows	O
memory	O
reads	O
,	O
writes	O
,	O
or	O
both	O
to	O
use	O
the	O
user-mode	B-Operating_System
page	O
mappings	O
.	O
</s>
<s>
Similarly	O
,	O
the	O
KL10	B-Device
variant	O
of	O
the	O
PDP-10	B-Device
supports	O
the	O
privileged	O
instruction	O
'	O
previous	O
context	O
XCT	B-General_Concept
 '	O
.	O
</s>
<s>
The	O
execute	B-General_Concept
instruction	I-General_Concept
can	O
cause	O
several	O
problems	O
when	O
one	O
execute	B-General_Concept
instruction	I-General_Concept
points	O
to	O
another	O
one	O
and	O
so	O
on	O
:	O
</s>
<s>
the	O
processor	O
may	O
be	O
uninterruptable	O
for	O
multiple	O
clock	O
cycles	O
if	O
the	O
execute	B-General_Concept
instruction	I-General_Concept
cannot	O
be	O
interrupted	O
in	O
the	O
middle	O
of	O
execution	O
;	O
</s>
<s>
similarly	O
,	O
the	O
processor	O
may	O
go	O
into	O
an	O
infinite	O
loop	O
if	O
the	O
series	O
of	O
execute	B-General_Concept
instructions	I-General_Concept
is	O
circular	O
and	O
uninterruptable	O
;	O
</s>
<s>
if	O
the	O
execute	B-General_Concept
instructions	I-General_Concept
are	O
on	O
different	O
swap	B-General_Concept
pages	I-General_Concept
,	O
all	O
of	O
the	O
pages	O
need	O
to	O
be	O
swapped	O
in	O
for	O
the	O
instruction	O
to	O
complete	O
,	O
which	O
can	O
cause	O
thrashing	B-General_Concept
.	O
</s>
<s>
The	O
execute	B-General_Concept
instruction	I-General_Concept
has	O
several	O
applications	O
:	O
</s>
<s>
Implementation	O
of	O
call	O
by	O
name	O
and	O
other	O
thunks	B-Application
.	O
</s>
<s>
A	O
table	O
of	O
execute	O
targets	O
may	O
be	O
used	O
for	O
dynamic	O
dispatch	O
of	O
the	O
methods	B-Language
or	O
virtual	B-Application
functions	I-Application
of	O
an	O
object	O
or	O
class	O
,	O
especially	O
when	O
the	O
method	O
or	O
function	O
may	O
often	O
be	O
implementable	O
as	O
a	O
single	O
instruction	O
.	O
</s>
<s>
An	O
execute	O
target	O
may	O
contain	O
a	O
hook	O
for	O
adding	O
functionality	O
or	O
for	O
debugging	O
;	O
it	O
is	O
normally	O
initialized	O
as	O
a	O
NOP	B-Language
which	O
may	O
be	O
overridden	O
dynamically	O
.	O
</s>
<s>
This	O
may	O
maintain	O
a	O
pseudo-program	O
counter	O
,	O
leaving	O
the	O
normal	O
program	B-General_Concept
counter	I-General_Concept
unchanged	O
.	O
</s>
<s>
Executing	O
dynamically	O
generated	O
code	O
,	O
especially	O
when	O
memory	B-General_Concept
protection	I-General_Concept
prevents	O
executable	O
code	O
from	O
being	O
writable	O
.	O
</s>
<s>
Emulating	O
self-modifying	O
code	O
,	O
especially	O
when	O
it	O
must	O
be	O
reentrant	B-Operating_System
or	O
read-only	O
.	O
</s>
<s>
In	O
the	O
IBM	B-Application
System/360	I-Application
,	O
the	O
execute	B-General_Concept
instruction	I-General_Concept
can	O
modify	O
bits	O
8-15	O
of	O
the	O
target	O
instruction	O
,	O
effectively	O
turning	O
an	O
instruction	O
with	O
a	O
fixed	O
argument	O
(	O
e.g.	O
,	O
a	O
length	O
field	O
)	O
into	O
an	O
instruction	O
with	O
a	O
variable	O
argument	O
.	O
</s>
<s>
Privileged-mode	B-Operating_System
execute	B-General_Concept
instructions	I-General_Concept
as	O
on	O
the	O
KL10	B-Device
are	O
used	O
by	O
operating	B-Operating_System
system	I-Operating_System
kernels	I-Operating_System
to	O
execute	B-General_Concept
operations	I-General_Concept
such	O
as	O
block	O
copies	O
within	O
the	O
virtual	O
space	O
of	O
user	O
processes	O
.	O
</s>
