<s>
In	O
computing	O
,	O
an	O
opcode	B-Language
(	O
abbreviated	O
from	O
operation	B-Language
code	I-Language
,	O
also	O
known	O
as	O
instruction	B-Language
machine	I-Language
code	I-Language
,	O
instruction	B-Language
code	I-Language
,	O
instruction	B-Language
syllable	I-Language
,	O
instruction	B-Language
parcel	I-Language
or	O
opstring	B-Language
)	O
is	O
the	O
portion	O
of	O
a	O
machine	B-Language
language	I-Language
instruction	O
that	O
specifies	O
the	O
operation	O
to	O
be	O
performed	O
.	O
</s>
<s>
Beside	O
the	O
opcode	B-Language
itself	O
,	O
most	O
instructions	O
also	O
specify	O
the	O
data	O
they	O
will	O
process	O
,	O
in	O
the	O
form	O
of	O
operands	O
.	O
</s>
<s>
In	O
addition	O
to	O
opcodes	B-Language
used	O
in	O
the	O
instruction	B-General_Concept
set	I-General_Concept
architectures	I-General_Concept
of	O
various	O
CPUs	B-Device
,	O
which	O
are	O
hardware	O
devices	O
,	O
they	O
can	O
also	O
be	O
used	O
in	O
abstract	O
computing	O
machines	O
as	O
part	O
of	O
their	O
byte	O
code	O
specifications	O
.	O
</s>
<s>
Specifications	O
and	O
format	O
of	O
the	O
opcodes	B-Language
are	O
laid	O
out	O
in	O
the	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
(	O
ISA	B-General_Concept
)	O
of	O
the	O
processor	O
in	O
question	O
,	O
which	O
may	O
be	O
a	O
general	O
CPU	B-General_Concept
or	O
a	O
more	O
specialized	O
processing	O
unit	O
.	O
</s>
<s>
Opcodes	B-Language
for	O
a	O
given	O
instruction	B-General_Concept
set	I-General_Concept
can	O
be	O
described	O
through	O
the	O
use	O
of	O
an	O
opcode	B-Language
table	I-Language
detailing	O
all	O
possible	O
opcodes	B-Language
.	O
</s>
<s>
Apart	O
from	O
the	O
opcode	B-Language
itself	O
,	O
an	O
instruction	O
normally	O
also	O
has	O
one	O
or	O
more	O
specifiers	O
for	O
operands	O
(	O
i.e.	O
</s>
<s>
There	O
are	O
instruction	B-General_Concept
sets	I-General_Concept
with	O
nearly	O
uniform	O
fields	O
for	O
opcode	B-Language
and	O
operand	O
specifiers	O
,	O
as	O
well	O
as	O
others	O
(	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
for	O
instance	O
)	O
with	O
a	O
more	O
complicated	O
,	O
variable-length	O
structure	O
.	O
</s>
<s>
Instruction	B-General_Concept
sets	I-General_Concept
can	O
be	O
extended	O
through	O
the	O
use	O
of	O
opcode	B-Language
prefixes	O
which	O
add	O
a	O
subset	O
of	O
new	O
instructions	O
made	O
up	O
of	O
existing	O
opcodes	B-Language
following	O
reserved	O
byte	O
sequences	O
.	O
</s>
<s>
Depending	O
on	O
architecture	O
,	O
the	O
operands	O
may	O
be	O
register	B-General_Concept
values	O
,	O
values	O
in	O
the	O
stack	B-General_Concept
,	O
other	O
memory	O
values	O
,	O
I/O	B-Architecture
ports	I-Architecture
(	O
which	O
may	O
also	O
be	O
memory	B-Architecture
mapped	I-Architecture
)	O
,	O
etc.	O
,	O
specified	O
and	O
accessed	O
using	O
more	O
or	O
less	O
complex	O
addressing	B-Language
modes	I-Language
.	O
</s>
<s>
The	O
types	O
of	O
operations	O
include	O
arithmetic	O
,	O
data	O
copying	O
,	O
logical	O
operations	O
,	O
and	O
program	O
control	O
,	O
as	O
well	O
as	O
special	O
instructions	O
(	O
such	O
as	O
CPUID	B-Architecture
and	O
others	O
)	O
.	O
</s>
<s>
Assembly	B-Language
language	I-Language
,	O
or	O
just	O
assembly	O
,	O
is	O
a	O
low-level	B-Language
programming	I-Language
language	I-Language
,	O
which	O
uses	O
mnemonic	O
instructions	O
and	O
operands	O
to	O
represent	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
This	O
enhances	O
the	O
readability	O
while	O
still	O
giving	O
precise	O
control	O
over	O
the	O
machine	B-Language
instructions	I-Language
.	O
</s>
<s>
Most	O
programming	O
is	O
currently	O
done	O
using	O
high-level	B-Language
programming	I-Language
languages	I-Language
,	O
which	O
are	O
typically	O
easier	O
for	O
humans	O
to	O
understand	O
and	O
write	O
.	O
</s>
<s>
These	O
languages	O
need	O
to	O
be	O
compiled	B-Language
(	O
translated	O
into	O
assembly	B-Language
language	I-Language
)	O
by	O
a	O
system-specific	O
compiler	B-Language
,	O
or	O
run	O
through	O
other	O
compiled	B-Language
programs	O
.	O
</s>
<s>
Opcodes	B-Language
can	O
also	O
be	O
found	O
in	O
so-called	O
byte	O
codes	O
and	O
other	O
representations	O
intended	O
for	O
a	O
software	O
interpreter	O
rather	O
than	O
a	O
hardware	O
device	O
.	O
</s>
<s>
These	O
software-based	O
instruction	B-General_Concept
sets	I-General_Concept
often	O
employ	O
slightly	O
higher-level	O
data	O
types	O
and	O
operations	O
than	O
most	O
hardware	O
counterparts	O
,	O
but	O
are	O
nevertheless	O
constructed	O
along	O
similar	O
lines	O
.	O
</s>
<s>
Examples	O
include	O
the	O
byte	O
code	O
found	O
in	O
Java	B-Language
class	I-Language
files	I-Language
which	O
are	O
then	O
interpreted	O
by	O
the	O
Java	B-Language
Virtual	I-Language
Machine	I-Language
(	O
JVM	B-Language
)	O
,	O
the	O
byte	O
code	O
used	O
in	O
GNU	B-Application
Emacs	I-Application
for	O
compiled	B-Language
Lisp	B-Language
code	O
,	O
.NET	O
Common	O
Intermediate	O
Language	O
(	O
CIL	O
)	O
,	O
and	O
many	O
others	O
.	O
</s>
