<s>
The	O
ModR/M	B-Device
byte	O
is	O
an	O
important	O
part	O
of	O
instruction	B-Language
encoding	I-Language
for	O
the	O
x86	B-Device
instruction	I-Device
set	I-Device
.	O
</s>
<s>
Opcodes	B-Language
in	O
x86	O
are	O
generally	O
one-byte	O
,	O
though	O
two-byte	O
instructions	O
and	O
prefixes	O
exist	O
.	O
</s>
<s>
ModR/M	B-Device
is	O
the	O
byte	O
following	O
the	O
opcode	B-Language
and	O
adds	O
additional	O
information	O
for	O
how	O
the	O
instruction	O
is	O
executed	O
.	O
</s>
<s>
Some	O
instructions	O
cannot	O
make	O
use	O
of	O
the	O
REG	O
portion	O
of	O
the	O
ModR/M	B-Device
byte	O
.	O
</s>
<s>
Many	O
of	O
these	O
instructions	O
are	O
"	O
multiplexed	O
"	O
using	O
this	O
field	O
,	O
where	O
a	O
single	O
opcode	B-Language
can	O
refer	O
to	O
multiple	O
instructions	O
,	O
and	O
the	O
REG	O
field	O
determines	O
the	O
instruction	O
.	O
</s>
<s>
In	O
opcode	B-Language
listings	O
,	O
these	O
are	O
specified	O
by	O
following	O
the	O
opcode	B-Language
with	O
a	O
slash	O
(	O
)	O
and	O
a	O
digit	O
0-7	O
.	O
</s>
<s>
The	O
ModR/M	B-Device
byte	O
is	O
central	O
to	O
the	O
changes	O
introduced	O
with	O
AMD	O
's	O
64-bit	B-Device
extension	I-Device
to	O
the	O
original	O
instruction	O
set	O
.	O
</s>
<s>
In	O
long	B-Application
mode	I-Application
,	O
any	O
opcode	B-Language
whose	O
highest	O
four	O
bits	O
are	O
0100	O
(	O
decimal	O
4	O
)	O
are	O
considered	O
to	O
be	O
a	O
new	O
prefix	O
,	O
the	O
REX	O
prefix	O
.	O
</s>
<s>
The	O
lowest	O
four	O
bits	O
of	O
the	O
prefix	O
byte	O
serve	O
various	O
purposes	O
,	O
including	O
an	O
extra	O
bit	O
for	O
the	O
REG	O
and	O
R/M	O
fields	O
of	O
the	O
ModR/M	B-Device
byte	O
that	O
follows	O
.	O
</s>
<s>
Among	O
other	O
changes	O
,	O
expanding	O
these	O
values	O
from	O
three	O
bits	O
to	O
four	O
doubles	O
the	O
number	O
of	O
available	O
processor	B-General_Concept
registers	I-General_Concept
from	O
eight	O
to	O
sixteen	O
.	O
</s>
<s>
The	O
SIB	O
byte	O
is	O
an	O
optional	O
post-opcode	O
byte	O
in	O
x86	B-Device
assembly	I-Device
,	O
used	O
for	O
complex	O
addressing	O
.	O
</s>
<s>
SIB	O
bytes	O
are	O
formatted	O
similarly	O
to	O
ModR/M	B-Device
bytes	O
,	O
and	O
take	O
the	O
form	O
of	O
(	O
scale	O
*	O
index	O
)	O
+	O
base	O
+	O
displacement	O
,	O
where	O
the	O
SCALE	O
is	O
1	O
,	O
2	O
,	O
4	O
,	O
or	O
8	O
.	O
</s>
<s>
An	O
REX	O
prefix	O
can	O
optionally	O
allow	O
the	O
SIB	O
byte	O
to	O
use	O
SSE	B-General_Concept
registers	O
.	O
</s>
