<s>
In	O
computing	O
,	O
Java	B-Language
bytecode	I-Language
is	O
the	O
bytecode-structured	O
instruction	B-General_Concept
set	I-General_Concept
of	O
the	O
Java	B-Language
virtual	I-Language
machine	I-Language
(	O
JVM	B-Language
)	O
,	O
a	O
virtual	B-Architecture
machine	I-Architecture
that	O
enables	O
a	O
computer	O
to	O
run	O
programs	O
written	O
in	O
the	O
Java	B-Language
programming	I-Language
language	I-Language
and	O
several	O
other	O
programming	O
languages	O
,	O
see	O
List	B-Language
of	I-Language
JVM	I-Language
languages	I-Language
.	O
</s>
<s>
A	O
Java	B-Language
programmer	O
does	O
not	O
need	O
to	O
be	O
aware	O
of	O
or	O
understand	O
Java	B-Language
bytecode	I-Language
at	O
all	O
.	O
</s>
<s>
However	O
,	O
as	O
suggested	O
in	O
the	O
IBM	O
developerWorks	O
journal	O
,	O
"	O
Understanding	O
bytecode	O
and	O
what	O
bytecode	O
is	O
likely	O
to	O
be	O
generated	O
by	O
a	O
Java	B-Language
compiler	I-Language
helps	O
the	O
Java	B-Language
programmer	O
in	O
the	O
same	O
way	O
that	O
knowledge	O
of	O
assembly	B-Language
helps	O
the	O
C	B-Language
or	O
C++	B-Language
programmer.	O
"	O
</s>
<s>
The	O
JVM	B-Language
is	O
both	O
a	O
stack	B-Application
machine	I-Application
and	O
a	O
register	B-Application
machine	I-Application
.	O
</s>
<s>
The	O
operand	O
stack	O
is	O
used	O
for	O
operands	O
to	O
computations	O
and	O
for	O
receiving	O
the	O
return	O
value	O
of	O
a	O
called	O
method	O
,	O
while	O
local	O
variables	O
serve	O
the	O
same	O
purpose	O
as	O
registers	B-General_Concept
and	O
are	O
also	O
used	O
to	O
pass	O
method	O
arguments	O
.	O
</s>
<s>
Each	O
bytecode	O
is	O
composed	O
of	O
one	O
byte	O
that	O
represents	O
the	O
opcode	B-Language
,	O
along	O
with	O
zero	O
or	O
more	O
bytes	O
for	O
operands	O
.	O
</s>
<s>
Of	O
the	O
256	O
possible	O
byte-long	O
opcodes	B-Language
,	O
,	O
202	O
are	O
in	O
use	O
(	O
~	O
79%	O
)	O
,	O
51	O
are	O
reserved	O
for	O
future	O
use	O
(	O
~	O
20%	O
)	O
,	O
and	O
3	O
instructions	O
(	O
~	O
1%	O
)	O
are	O
permanently	O
reserved	O
for	O
JVM	B-Language
implementations	O
to	O
use	O
.	O
</s>
<s>
The	O
n	O
for	O
the	O
load	B-General_Concept
and	I-General_Concept
store	I-General_Concept
instructions	I-General_Concept
specifies	O
the	O
index	O
in	O
the	O
local	O
variable	O
array	O
to	O
load	O
from	O
or	O
store	O
to	O
.	O
</s>
<s>
The	O
aload_0	O
instruction	O
pushes	O
the	O
object	O
in	O
local	O
variable	O
0	O
onto	O
the	O
stack	O
(	O
this	B-Application
is	O
usually	O
the	O
this	B-Application
object	O
)	O
.	O
</s>
<s>
Consider	O
the	O
following	O
Java	B-Language
code	I-Language
:	O
</s>
<s>
A	O
Java	B-Language
compiler	I-Language
might	O
translate	O
the	O
Java	B-Language
code	I-Language
above	O
into	O
bytecode	O
as	O
follows	O
,	O
assuming	O
the	O
above	O
was	O
put	O
in	O
a	O
method	O
:	O
</s>
<s>
The	O
most	O
common	O
language	O
targeting	O
Java	B-Language
virtual	I-Language
machine	I-Language
by	O
producing	O
Java	B-Language
bytecode	I-Language
is	O
Java	B-Language
.	O
</s>
<s>
Originally	O
only	O
one	O
compiler	O
existed	O
,	O
the	O
javac	B-General_Concept
compiler	O
from	O
Sun	O
Microsystems	O
,	O
which	O
compiles	O
Java	B-Language
source	I-Language
code	I-Language
to	O
Java	B-Language
bytecode	I-Language
;	O
but	O
because	O
all	O
the	O
specifications	O
for	O
Java	B-Language
bytecode	I-Language
are	O
now	O
available	O
,	O
other	O
parties	O
have	O
supplied	O
compilers	O
that	O
produce	O
Java	B-Language
bytecode	I-Language
.	O
</s>
<s>
GNU	B-Language
Compiler	I-Language
for	I-Language
Java	I-Language
(	O
GCJ	O
)	O
,	O
compiles	O
from	O
Java	B-Language
to	O
Java	B-Language
bytecode	I-Language
;	O
it	O
can	O
also	O
compile	O
to	O
native	O
machine	B-Language
code	I-Language
and	O
was	O
part	O
of	O
the	O
GNU	B-Application
Compiler	I-Application
Collection	I-Application
(	O
GCC	B-Application
)	O
up	O
until	O
version	O
6	O
.	O
</s>
<s>
Some	O
projects	O
provide	O
Java	B-Language
assemblers	B-Language
to	O
enable	O
writing	O
Java	B-Language
bytecode	I-Language
by	O
hand	O
.	O
</s>
<s>
Assembly	B-Language
code	I-Language
may	O
be	O
also	O
generated	O
by	O
machine	O
,	O
for	O
example	O
by	O
a	O
compiler	O
targeting	O
a	O
Java	B-Language
virtual	I-Language
machine	I-Language
.	O
</s>
<s>
Notable	O
Java	B-Language
assemblers	B-Language
include	O
:	O
</s>
<s>
Jamaica	O
,	O
a	O
macro	O
assembly	B-Language
language	I-Language
for	O
the	O
Java	B-Language
virtual	I-Language
machine	I-Language
.	O
</s>
<s>
Java	B-Language
syntax	O
is	O
used	O
for	O
class	O
or	O
interface	O
definition	O
.	O
</s>
<s>
Krakatau	O
Bytecode	O
Tools	O
,	O
currently	O
contains	O
three	O
tools	O
:	O
a	O
decompiler	O
and	O
disassembler	O
for	O
Java	B-Language
classfiles	B-Language
and	O
an	O
assembler	B-Language
to	O
create	O
classfiles	B-Language
.	O
</s>
<s>
Lilac	O
,	O
an	O
assembler	B-Language
and	O
disassembler	O
for	O
the	O
Java	B-Language
virtual	I-Language
machine	I-Language
.	O
</s>
<s>
Others	O
have	O
developed	O
compilers	O
,	O
for	O
different	O
programming	O
languages	O
,	O
to	O
target	O
the	O
Java	B-Language
virtual	I-Language
machine	I-Language
,	O
such	O
as	O
:	O
</s>
<s>
Kawa	B-Language
,	O
an	O
implementation	O
of	O
the	O
Scheme	B-Language
programming	I-Language
language	I-Language
,	O
also	O
a	O
dialect	O
of	O
Lisp	B-Language
.	O
</s>
<s>
Object	B-Language
Pascal	I-Language
source	O
code	O
is	O
compiled	O
to	O
Java	B-Language
bytecode	I-Language
using	O
the	O
Free	B-Operating_System
Pascal	I-Operating_System
3.0	O
+	O
compiler	O
.	O
</s>
<s>
There	O
are	O
several	O
Java	B-Language
virtual	I-Language
machines	I-Language
available	O
today	O
to	O
execute	O
Java	B-Language
bytecode	I-Language
,	O
both	O
free	O
and	O
commercial	O
products	O
.	O
</s>
<s>
If	O
executing	O
bytecode	O
in	O
a	O
virtual	B-Architecture
machine	I-Architecture
is	O
undesirable	O
,	O
a	O
developer	O
can	O
also	O
compile	O
Java	B-Language
source	I-Language
code	I-Language
or	O
bytecode	O
directly	O
to	O
native	O
machine	B-Language
code	I-Language
with	O
tools	O
such	O
as	O
the	O
GNU	B-Language
Compiler	I-Language
for	I-Language
Java	I-Language
(	O
GCJ	O
)	O
.	O
</s>
<s>
Some	O
processors	O
can	O
execute	O
Java	B-Language
bytecode	I-Language
natively	O
.	O
</s>
<s>
Such	O
processors	O
are	O
termed	O
Java	B-Language
processors	I-Language
.	O
</s>
<s>
The	O
Java	B-Language
virtual	I-Language
machine	I-Language
provides	O
some	O
support	O
for	O
dynamically	O
typed	O
languages	O
.	O
</s>
<s>
Most	O
of	O
the	O
extant	O
JVM	B-Language
instruction	B-General_Concept
set	I-General_Concept
is	O
statically	O
typed	O
-	O
in	O
the	O
sense	O
that	O
method	O
calls	O
have	O
their	O
signatures	O
type-checked	O
at	O
compile	B-Application
time	I-Application
,	O
without	O
a	O
mechanism	O
to	O
defer	O
this	B-Application
decision	O
to	O
run	B-Library
time	I-Library
,	O
or	O
to	O
choose	O
the	O
method	O
dispatch	O
by	O
an	O
alternative	O
approach	O
.	O
</s>
<s>
JSR	B-Application
292	I-Application
(	O
Supporting	O
Dynamically	O
Typed	O
Languages	O
on	O
the	O
Java	B-Language
Platform	O
)	O
added	O
a	O
new	O
invokedynamic	O
instruction	O
at	O
the	O
JVM	B-Language
level	O
,	O
to	O
allow	O
method	O
invocation	O
relying	O
on	O
dynamic	O
type	O
checking	O
(	O
instead	O
of	O
the	O
extant	O
statically	O
type-checked	O
invokevirtual	O
instruction	O
)	O
.	O
</s>
<s>
The	O
Da	B-Application
Vinci	I-Application
Machine	I-Application
is	O
a	O
prototype	O
virtual	B-Architecture
machine	I-Architecture
implementation	O
that	O
hosts	O
JVM	B-Language
extensions	O
aimed	O
at	O
supporting	O
dynamic	O
languages	O
.	O
</s>
<s>
All	O
JVMs	B-Language
supporting	O
JSE	B-Language
7	O
also	O
include	O
the	O
invokedynamic	O
opcode	B-Language
.	O
</s>
