<s>
In	O
computer	O
engineering	O
,	O
Halt	B-Language
and	I-Language
Catch	I-Language
Fire	I-Language
,	O
known	O
by	O
the	O
assembly	O
mnemonic	O
HCF	B-Language
,	O
is	O
an	O
idiom	O
referring	O
to	O
a	O
computer	O
machine	B-Language
code	I-Language
instruction	O
that	O
causes	O
the	O
computer	O
's	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
to	O
cease	O
meaningful	O
operation	O
,	O
typically	O
requiring	O
a	O
restart	O
of	O
the	O
computer	O
.	O
</s>
<s>
It	O
originally	O
referred	O
to	O
a	O
fictitious	O
instruction	O
in	O
IBM	B-Device
System/360	I-Device
computers	O
(	O
introduced	O
in	O
1964	O
)	O
,	O
making	O
a	O
joke	O
about	O
its	O
numerous	O
non-obvious	O
instruction	O
mnemonics	O
.	O
</s>
<s>
With	O
the	O
advent	O
of	O
the	O
MC6800	B-Device
(	O
introduced	O
in	O
1974	O
)	O
,	O
a	O
design	O
flaw	O
was	O
discovered	O
by	O
programmers	O
.	O
</s>
<s>
Due	O
to	O
incomplete	O
opcode	B-Language
decoding	O
,	O
two	O
illegal	B-Language
opcodes	I-Language
,	O
0x9D	O
and	O
0xDD	O
,	O
will	O
cause	O
the	O
program	B-General_Concept
counter	I-General_Concept
on	O
the	O
processor	O
to	O
increment	O
endlessly	O
,	O
which	O
locks	O
the	O
processor	O
until	O
reset	O
.	O
</s>
<s>
Those	O
codes	O
have	O
been	O
unofficially	O
named	O
HCF	B-Language
.	O
</s>
<s>
During	O
the	O
design	O
process	O
of	O
the	O
MC6802	B-Device
,	O
engineers	O
originally	O
planned	O
to	O
remove	O
this	O
instruction	O
,	O
but	O
kept	O
it	O
as-is	O
for	O
testing	O
purposes	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
HCF	B-Language
was	O
officially	O
recognized	O
as	O
a	O
real	O
instruction	O
.	O
</s>
<s>
Later	O
,	O
HCF	B-Language
became	O
a	O
humorous	O
catch-all	O
term	O
for	O
instructions	O
that	O
may	O
freeze	O
a	O
processor	O
,	O
including	O
intentional	O
instructions	O
for	O
testing	O
purposes	O
,	O
and	O
unintentional	O
illegal	B-Language
instructions	I-Language
.	O
</s>
<s>
Some	O
are	O
considered	O
hardware	O
defects	O
,	O
and	O
if	O
the	O
system	B-Operating_System
is	I-Operating_System
shared	I-Operating_System
,	O
a	O
malicious	O
user	O
can	O
execute	O
it	O
to	O
launch	O
a	O
denial-of-service	O
attack	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
real	O
instructions	O
,	O
the	O
implication	O
of	O
this	O
expression	O
is	O
that	O
,	O
whereas	O
in	O
most	O
cases	O
in	O
which	O
a	O
CPU	O
executes	O
an	O
unintended	B-Language
instruction	I-Language
(	O
a	O
bug	O
in	O
the	O
code	O
)	O
the	O
computer	O
may	O
still	O
be	O
able	O
to	O
recover	O
,	O
in	O
the	O
case	O
of	O
an	O
HCF	B-Language
instruction	O
there	O
is	O
,	O
by	O
definition	O
,	O
no	O
way	O
for	O
the	O
system	O
to	O
recover	O
without	O
a	O
restart	O
.	O
</s>
<s>
The	O
expression	O
catch	O
fire	O
is	O
a	O
facetious	O
exaggeration	O
of	O
the	O
speed	O
with	O
which	O
the	O
CPU	B-Architecture
chip	I-Architecture
would	O
be	O
switching	O
some	O
bus	O
circuits	O
,	O
causing	O
them	O
to	O
overheat	O
and	O
burn	O
.	O
</s>
<s>
The	O
Z1	B-Device
(	O
1938	O
)	O
and	O
Z3	B-Device
(	O
1941	O
)	O
computers	O
built	O
by	O
Konrad	O
Zuse	O
contained	O
illegal	O
sequences	O
of	O
instructions	O
which	O
damaged	O
the	O
hardware	O
if	O
executed	O
by	O
accident	O
.	O
</s>
<s>
Apocryphal	O
stories	O
connect	O
this	O
term	O
with	O
an	O
illegal	B-Language
opcode	I-Language
in	O
IBM	B-Device
System/360	I-Device
.	O
</s>
<s>
A	O
processor	O
,	O
upon	O
encountering	O
the	O
instruction	O
,	O
would	O
start	O
switching	O
bus	B-General_Concept
lines	I-General_Concept
very	O
fast	O
,	O
potentially	O
leading	O
to	O
overheating	O
.	O
</s>
<s>
In	O
a	O
computer	O
's	O
assembly	B-Language
language	I-Language
,	O
mnemonics	O
are	O
used	O
that	O
are	O
directly	O
equivalent	O
to	O
machine	B-Language
code	I-Language
instructions	O
.	O
</s>
<s>
The	O
HCF	B-Language
instruction	O
was	O
originally	O
a	O
fictitious	O
assembly	B-Language
language	I-Language
instruction	O
,	O
said	O
to	O
be	O
under	O
development	O
at	O
IBM	O
for	O
use	O
in	O
their	O
System/360	B-Device
computers	O
,	O
along	O
with	O
many	O
other	O
amusing	O
three-letter	O
acronyms	O
like	O
XPR	O
(	O
Execute	O
Programmer	O
)	O
and	O
CAI	O
(	O
Corrupt	O
Accounting	O
Information	O
)	O
,	O
and	O
similar	O
to	O
other	O
joke	O
mnemonics	O
such	O
as	O
"	O
SDI	O
"	O
for	O
"	O
Self	O
Destruct	O
Immediately	O
"	O
and	O
"	O
CRN	O
"	O
for	O
"	O
Convert	O
to	O
Roman	O
Numerals	O
"	O
.	O
</s>
<s>
A	O
list	O
of	O
such	O
mnemonics	O
,	O
including	O
HCF	B-Language
,	O
shows	O
up	O
as	O
"	O
Overextended	O
Mnemonics	O
"	O
in	O
the	O
April	O
1980	O
Creative	B-General_Concept
Computing	I-General_Concept
flip-side	O
parody	O
issue	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/360	I-Device
already	O
included	O
numerous	O
non-obvious	O
mnemonics	O
like	O
ZAP	O
(	O
Zero	O
and	O
Add	O
Packed	O
)	O
,	O
EDMK	O
(	O
EDit	O
and	O
MarK	O
)	O
,	O
TRT	O
(	O
TRanslate	O
and	O
Test	O
)	O
,	O
and	O
Read	O
Backward	O
(	O
an	O
I/O	O
channel	O
command	O
)	O
,	O
and	O
programmers	O
began	O
creating	O
similarly	O
cryptic	O
,	O
but	O
fictitious	O
,	O
instructions	O
in	O
a	O
humorous	O
vein	O
.	O
</s>
<s>
In	O
a	O
1990	O
USENET	B-Application
discussion	O
,	O
it	O
was	O
claimed	O
that	O
HCF	B-Language
dated	O
back	O
to	O
before	O
1977	O
.	O
</s>
<s>
In	O
Rick	O
Cook	O
's	O
science	O
fiction/fantasy	O
novel	O
,	O
The	O
Wizardry	O
Compiled	O
,	O
about	O
programmers	O
transported	O
to	O
a	O
universe	O
where	O
magic	O
could	O
be	O
programmed	O
,	O
one	O
of	O
them	O
refers	O
to	O
the	O
command	O
as	O
HMCF	O
,	O
for	O
"	O
Halt	B-Language
,	O
Melt	O
and	O
Catch	O
Fire	O
"	O
.	O
</s>
<s>
In	O
TIS-100	B-Application
,	O
a	O
2015	O
puzzle	O
video	O
game	O
made	O
by	O
Zachtronics	O
,	O
there	O
is	O
an	O
achievement	O
called	O
HALT_AND_CATCH_FIRE	O
for	O
crashing	O
the	O
machine	O
with	O
a	O
hidden	O
opcode	B-Language
.	O
</s>
<s>
CPU	O
designers	O
sometimes	O
incorporate	O
one	O
or	O
more	O
undocumented	O
machine	B-Language
code	I-Language
instructions	O
for	O
testing	O
purposes	O
,	O
such	O
as	O
the	O
IBM	B-Device
System/360	I-Device
DIAGnose	O
instruction	O
.	O
</s>
<s>
The	O
Motorola	B-Device
6800	I-Device
microprocessor	B-Architecture
was	O
the	O
first	O
for	O
which	O
an	O
undocumented	O
assembly	O
mnemonic	O
HCF	B-Language
became	O
widely	O
known	O
.	O
</s>
<s>
The	O
operation	B-Language
codes	I-Language
(	O
opcodes	B-Language
—	O
the	O
portions	O
of	O
the	O
machine	B-Language
language	I-Language
instructions	O
that	O
specify	O
an	O
operation	O
to	O
be	O
performed	O
)	O
are	O
hexadecimal	O
9D	O
and	O
DD	O
,	O
and	O
were	O
reported	O
and	O
given	O
the	O
unofficial	O
mnemonic	O
HCF	B-Language
in	O
an	O
article	O
written	O
by	O
Gerry	O
Wheeler	O
in	O
the	O
December	O
1977	O
issue	O
of	O
BYTE	B-Application
magazine	O
on	O
undocumented	B-Language
opcodes	I-Language
.	O
</s>
<s>
Wheeler	O
noted	O
that	O
Motorola	O
reported	O
197	O
valid	O
operation	B-Language
codes	I-Language
for	O
the	O
M6800	B-Device
processor	O
,	O
and	O
so	O
inferred	O
that	O
with	O
256	O
possible	O
8	O
bit	O
combinations	O
,	O
there	O
must	O
be	O
59	O
"	O
invalid	B-Language
instructions	I-Language
"	O
.	O
</s>
<s>
He	O
goes	O
on	O
to	O
describe	O
the	O
HCF	B-Language
as	O
a	O
"	O
big	O
surprise	O
"	O
,	O
and	O
saying	O
of	O
the	O
Catch	O
Fire	O
portion	O
of	O
the	O
moniker	O
,	O
"	O
Well	O
,	O
almost	O
"	O
:	O
</s>
<s>
That	O
is	O
,	O
either	O
opcode	B-Language
made	O
the	O
processor	O
enter	O
a	O
mode	B-General_Concept
,	O
continuously	O
performing	O
memory	B-Architecture
read	O
cycles	O
from	O
successive	O
addresses	O
with	O
no	O
intervening	O
instruction	O
fetches	O
.	O
</s>
<s>
Hence	O
,	O
the	O
address	B-Architecture
bus	I-Architecture
effectively	O
became	O
a	O
counter	O
,	O
allowing	O
the	O
operation	O
of	O
all	O
address	B-General_Concept
lines	O
to	O
be	O
quickly	O
verified	O
.	O
</s>
<s>
Once	O
the	O
processor	O
entered	O
this	O
mode	B-General_Concept
,	O
it	O
was	O
not	O
responsive	O
to	O
interrupts	B-Application
,	O
so	O
normal	O
operation	O
could	O
only	O
be	O
restored	O
by	O
a	O
reset	O
(	O
hence	O
the	O
"	O
Drop	B-Language
Dead	I-Language
"	O
and	O
"	O
Halt	B-Language
and	I-Language
Catch	I-Language
Fire	I-Language
"	O
monikers	O
)	O
.	O
</s>
<s>
Other	O
HCF-like	O
instructions	O
were	O
found	O
later	O
on	O
the	O
Motorola	B-Device
6800	I-Device
when	O
executing	O
undocumented	B-Language
opcodes	I-Language
FD	O
(	O
cycling	O
twice	O
slower	O
than	O
9D/DD	O
)	O
or	O
CD/ED	O
(	O
cycling	O
at	O
a	O
human-readable	O
very	O
low	O
frequency	O
on	O
a	O
limited	O
number	O
of	O
high-address	O
lines	O
)	O
.	O
</s>
<s>
The	O
mnemonic	O
HCF	B-Language
is	O
believed	O
to	O
be	O
the	O
first	O
built-in	O
self-test	O
feature	O
on	O
a	O
Motorola	O
microprocessor	B-Architecture
.	O
</s>
<s>
The	O
Intel	O
8086	O
and	O
subsequent	O
processors	O
in	O
the	O
x86	O
series	O
had	O
an	O
HLT	B-Language
(	O
halt	B-Language
)	O
instruction	O
,	O
opcode	B-Language
F4	O
,	O
which	O
stopped	O
instruction	O
execution	O
and	O
placed	O
the	O
processor	O
in	O
a	O
HALT	B-Language
state	O
.	O
</s>
<s>
An	O
enabled	O
interrupt	B-Application
,	O
a	O
debug	O
exception	O
,	O
the	O
BINIT	O
signal	O
,	O
the	O
INIT	O
signal	O
,	O
or	O
the	O
RESET	O
signal	O
resumed	O
execution	O
,	O
which	O
meant	O
the	O
processor	O
could	O
always	O
be	O
restarted	O
.	O
</s>
<s>
Some	O
of	O
the	O
early	O
Intel	O
DX4	O
chips	O
had	O
a	O
problem	O
with	O
the	O
HLT	B-Language
instruction	I-Language
and	O
could	O
not	O
be	O
restarted	O
after	O
this	O
instruction	O
was	O
used	O
,	O
which	O
disabled	O
the	O
computer	O
and	O
turned	O
HLT	B-Language
into	O
more	O
of	O
an	O
HCF	B-Language
instruction	O
.	O
</s>
<s>
The	O
Linux	O
kernel	O
added	O
a	O
"	O
no-hlt	O
"	O
option	O
telling	O
Linux	O
to	O
run	O
an	O
infinite	O
loop	O
instead	O
of	O
using	O
HLT	B-Language
,	O
which	O
allowed	O
users	O
of	O
these	O
broken	O
chips	O
to	O
use	O
Linux	O
.	O
</s>
<s>
The	O
80286	O
has	O
the	O
undocumented	B-Language
opcode	I-Language
0F	O
04	O
,	O
causing	O
the	O
CPU	O
to	O
hang	O
when	O
executed	O
.	O
</s>
<s>
In	O
some	O
implementations	O
,	O
the	O
opcode	B-Language
was	O
emulated	O
through	O
BIOS	B-Operating_System
as	O
a	O
halting	O
sequence	O
.	O
</s>
<s>
Many	O
computers	O
in	O
the	O
Intel	O
Pentium	O
line	O
could	O
be	O
locked	O
up	O
by	O
executing	O
an	O
invalid	B-Language
instruction	I-Language
(	O
F00F	B-Error_Name
C7C8	O
)	O
,	O
which	O
caused	O
the	O
computer	O
to	O
lock	O
up	O
.	O
</s>
<s>
This	O
became	O
known	O
as	O
the	O
Pentium	B-Error_Name
F00F	I-Error_Name
bug	I-Error_Name
.	O
</s>
<s>
During	O
Black	O
Hat	O
USA	O
2017	O
,	O
Christopher	O
Domas	O
showed	O
that	O
he	O
has	O
found	O
a	O
new	O
currently	O
unknown	O
"	O
Halt	B-Language
and	I-Language
Catch	I-Language
Fire	I-Language
"	O
instruction	O
on	O
a	O
particular	O
x86	O
processor	O
model	O
using	O
his	O
own	O
x86	O
processor	O
fuzzer	O
called	O
sandsifter	O
.	O
</s>
<s>
The	O
MOS	B-General_Concept
Technology	I-General_Concept
6502	I-General_Concept
has	O
12	O
invalid	B-Language
instructions	I-Language
which	O
will	O
freeze	O
the	O
CPU	O
.	O
</s>
<s>
On	O
the	O
Zilog	B-General_Concept
Z80	I-General_Concept
,	O
executing	O
DI	O
(	O
disable	O
interrupts	B-Application
)	O
followed	O
by	O
HALT	B-Language
(	O
wait	O
for	O
an	O
interrupt	B-Application
)	O
results	O
in	O
the	O
CPU	O
staying	O
frozen	O
indefinitely	O
,	O
waiting	O
for	O
an	O
interrupt	B-Application
that	O
cannot	O
happen	O
.	O
</s>
<s>
The	O
Z80	B-General_Concept
also	O
supports	O
a	O
non-maskable	O
interrupt	O
.	O
</s>
<s>
Since	O
a	O
non-maskable	O
interrupt	O
will	O
regain	O
control	O
of	O
the	O
CPU	O
even	O
after	O
executing	O
the	O
instruction	O
sequence	O
DI	O
/	O
HALT	B-Language
,	O
that	O
pair	O
does	O
not	O
represent	O
a	O
true	O
HCF	B-Language
.	O
</s>
<s>
It	O
will	O
only	O
result	O
in	O
a	O
HCF	B-Language
condition	O
if	O
either	O
the	O
/NMI	O
pin	O
is	O
connected	O
directly	O
to	O
the	O
+5V	O
rail	O
,	O
making	O
the	O
generation	O
of	O
that	O
signal	O
impossible	O
,	O
or	O
if	O
the	O
interrupt	B-Application
routine	O
that	O
services	O
/NMI	O
ends	O
with	O
a	O
return	O
,	O
placing	O
it	O
back	O
in	O
the	O
HALT	B-Language
state	O
.	O
</s>
<s>
The	O
SM83	O
processor	O
core	O
in	O
the	O
Game	B-Device
Boy	I-Device
's	O
LR35902	O
system	O
on	O
chip	O
has	O
a	O
similar	O
issue	O
,	O
triggered	O
by	O
two	O
consecutive	O
HALTs	O
with	O
interrupts	B-Application
disabled	O
.	O
</s>
<s>
The	O
core	O
itself	O
contains	O
11	O
opcodes	B-Language
that	O
fully	O
lock	O
the	O
CPU	O
when	O
executed	O
.	O
</s>
<s>
The	O
Hitachi	O
SC61860	O
mainly	O
used	O
in	O
Sharp	O
pocket	B-Device
computers	I-Device
in	O
the	O
1980	O
–	O
1990	O
also	O
have	O
an	O
undocumented	O
HCF	B-Language
instruction	O
with	O
the	O
opcode	B-Language
7B	O
.	O
</s>
