<s>
INT	B-Device
is	O
an	O
assembly	B-Language
language	I-Language
instruction	O
for	O
x86	B-Operating_System
processors	B-General_Concept
that	O
generates	O
a	O
software	O
interrupt	B-Application
.	O
</s>
<s>
It	O
takes	O
the	O
interrupt	B-Application
number	O
formatted	O
as	O
a	O
byte	B-Application
value	O
.	O
</s>
<s>
When	O
written	O
in	O
assembly	B-Language
language	I-Language
,	O
the	O
instruction	O
is	O
written	O
like	O
this	O
:	O
</s>
<s>
where	O
X	O
is	O
the	O
software	O
interrupt	B-Application
that	O
should	O
be	O
generated	O
(	O
0-255	O
)	O
.	O
</s>
<s>
As	O
is	O
customary	O
with	O
machine	O
binary	O
arithmetic	O
,	O
interrupt	B-Application
numbers	O
are	O
often	O
written	O
in	O
hexadecimal	O
form	O
,	O
which	O
can	O
be	O
indicated	O
with	O
a	O
prefix	O
0x	O
or	O
with	O
the	O
suffix	O
h	O
.	O
For	O
example	O
,	O
INT	B-Device
13H	I-Device
will	O
generate	O
the	O
20th	O
software	O
interrupt	B-Application
(	O
0x13	O
is	O
the	O
number	O
19	O
--	O
nineteen	O
--	O
written	O
in	O
hexadecimal	O
notation	O
,	O
and	O
the	O
count	O
starts	O
with	O
0	O
)	O
,	O
causing	O
the	O
function	O
pointed	O
to	O
by	O
the	O
20th	O
vector	O
in	O
the	O
interrupt	B-General_Concept
table	I-General_Concept
to	O
be	O
executed	O
.	O
</s>
<s>
When	O
generating	O
a	O
software	O
interrupt	B-Application
,	O
the	O
processor	O
calls	O
one	O
of	O
the	O
256	O
functions	O
pointed	O
to	O
by	O
the	O
interrupt	B-Application
address	O
table	O
,	O
which	O
is	O
located	O
in	O
the	O
first	O
1024	O
bytes	B-Application
of	O
memory	O
while	O
in	O
real	B-Application
mode	I-Application
(	O
see	O
Interrupt	B-General_Concept
vector	I-General_Concept
)	O
.	O
</s>
<s>
It	O
is	O
therefore	O
entirely	O
possible	O
to	O
use	O
a	O
far-call	O
instruction	O
to	O
start	O
the	O
interrupt-function	O
manually	O
after	O
pushing	O
the	O
flag	O
register	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
useful	O
DOS	O
software	O
interrupt	B-Application
was	O
interrupt	B-Application
0x21	O
.	O
</s>
<s>
Most	O
Unix	B-Application
systems	I-Application
and	O
derivatives	O
do	O
not	O
use	O
software	O
interrupts	B-Application
,	O
with	O
the	O
exception	O
of	O
interrupt	B-Application
0x80	O
,	O
used	O
to	O
make	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
This	O
is	O
accomplished	O
by	O
entering	O
a	O
32-bit	O
value	O
corresponding	O
to	O
a	O
kernel	O
function	O
into	O
the	O
EAX	O
register	O
of	O
the	O
processor	O
and	O
then	O
executing	O
INT	B-Operating_System
0x80	I-Operating_System
.	O
</s>
<s>
The	O
INT3instruction	O
is	O
a	O
one-byte-instruction	O
defined	O
for	O
use	O
by	O
debuggers	B-Application
to	O
temporarily	O
replace	O
an	O
instruction	O
in	O
a	O
running	O
program	O
in	O
order	O
to	O
set	O
a	O
code	O
breakpoint	O
.	O
</s>
<s>
The	O
more	O
general	O
INT	B-Device
XXh	O
instructions	O
are	O
encoded	O
using	O
two	O
bytes	B-Application
.	O
</s>
<s>
This	O
makes	O
them	O
unsuitable	O
for	O
use	O
in	O
patching	O
instructions	O
(	O
which	O
can	O
be	O
one	O
byte	B-Application
long	O
)	O
;	O
see	O
SIGTRAP	B-General_Concept
.	O
</s>
<s>
The	O
opcode	O
for	O
INT3	O
is	O
0xCC	O
,	O
as	O
opposed	O
to	O
the	O
opcode	O
for	O
INT	B-Device
immediate8	O
,	O
which	O
is	O
0xCD	O
immediate8	O
.	O
</s>
<s>
Since	O
the	O
dedicated	O
0xCC	O
opcode	O
has	O
some	O
desired	O
special	O
properties	O
for	O
debugging	O
,	O
which	O
are	O
not	O
shared	O
by	O
the	O
normal	O
two-byte	O
opcode	O
for	O
an	O
INT3	O
,	O
assemblers	B-Language
do	O
not	O
normally	O
generate	O
the	O
generic	O
0xCD	O
0x03	O
opcode	O
from	O
mnemonics	O
.	O
</s>
<s>
The	O
INTOinstruction	O
is	O
another	O
one-byte-instruction	O
.	O
</s>
<s>
It	O
is	O
a	O
conditional	O
interrupt	B-Application
which	O
is	O
triggered	O
when	O
the	O
overflow	O
flag	O
is	O
set	O
at	O
the	O
time	O
of	O
executing	O
this	O
opcode	O
.	O
</s>
<s>
This	O
implicitly	O
indicates	O
interrupt	B-Application
#4	O
.	O
</s>
<s>
The	O
opcode	O
for	O
INTO	O
is	O
0xCE	O
,	O
however	O
it	O
is	O
unavailable	O
in	O
x86-64	O
mode	O
.	O
</s>
