<s>
The	O
Cyrix	B-Error_Name
coma	I-Error_Name
bug	I-Error_Name
is	O
a	O
design	O
flaw	O
in	O
Cyrix	B-General_Concept
6x86	I-General_Concept
(	O
introduced	O
in	O
1996	O
)	O
,	O
6x86L	B-General_Concept
,	O
and	O
early	O
6x86MX	B-General_Concept
processors	B-General_Concept
that	O
allows	O
a	O
non-privileged	O
program	O
to	O
hang	B-General_Concept
the	O
computer	O
.	O
</s>
<s>
According	O
to	O
Andrew	O
Balsa	O
,	O
around	O
the	O
time	O
of	O
the	O
discovery	O
of	O
the	O
F00F	B-Error_Name
bug	I-Error_Name
on	O
Intel	B-General_Concept
Pentium	I-General_Concept
,	O
Serguei	O
Shtyliov	O
from	O
Moscow	O
found	O
a	O
flaw	O
in	O
a	O
Cyrix	O
processor	O
while	O
developing	O
an	O
IDE	B-Protocol
disk	O
driver	O
in	O
assembly	B-Language
language	I-Language
.	O
</s>
<s>
Alexandr	O
Konosevich	O
,	O
from	O
Omsk	O
,	O
further	O
researched	O
the	O
bug	O
and	O
coauthored	O
an	O
article	O
with	O
Uwe	O
Post	O
in	O
the	O
German	O
technology	O
magazine	O
c't	O
,	O
calling	O
it	O
the	O
"	O
hidden	B-Error_Name
CLI	I-Error_Name
bug	I-Error_Name
"	O
(	O
CLI	O
is	O
the	O
instruction	O
that	O
disables	O
interrupts	B-Application
in	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
)	O
.	O
</s>
<s>
Balsa	O
,	O
as	O
a	O
member	O
on	O
the	O
Linux	B-Application
kernel	I-Application
mailing	I-Application
list	I-Application
,	O
confirmed	O
that	O
the	O
following	O
C	B-Language
program	I-Language
(	O
which	O
uses	O
inline	O
x86-specific	O
assembly	B-Language
language	I-Language
)	O
could	O
be	O
compiled	O
and	O
run	O
by	O
an	O
unprivileged	O
user	O
:	O
</s>
<s>
Execution	O
of	O
this	O
program	O
renders	O
the	O
processor	O
completely	O
useless	O
until	O
it	O
is	O
rebooted	O
,	O
as	O
it	O
enters	O
an	O
infinite	B-Algorithm
loop	I-Algorithm
that	O
cannot	O
be	O
interrupted	O
.	O
</s>
<s>
It	O
is	O
similar	O
to	O
execution	O
of	O
a	O
Halt	B-Language
and	I-Language
Catch	I-Language
Fire	I-Language
instruction	O
,	O
although	O
the	O
coma	B-Error_Name
bug	I-Error_Name
is	O
not	O
any	O
one	O
particular	O
instruction	O
.	O
</s>
<s>
What	O
causes	O
the	O
bug	O
is	O
not	O
an	O
interrupt	B-Application
mask	I-Application
,	O
nor	O
are	O
interrupts	B-Application
being	O
explicitly	O
disabled	O
.	O
</s>
<s>
Instead	O
,	O
an	O
anomaly	O
in	O
the	O
Cyrix	O
's	O
instruction	B-General_Concept
pipeline	I-General_Concept
prevents	O
interrupts	B-Application
from	O
being	O
serviced	O
for	O
the	O
duration	O
of	O
the	O
loop	O
;	O
since	O
the	O
loop	O
never	O
ends	O
,	O
interrupts	B-Application
will	O
never	O
be	O
serviced	O
.	O
</s>
<s>
The	O
xchg	O
instruction	O
is	O
atomic	B-General_Concept
,	O
meaning	O
that	O
other	O
instructions	O
are	O
not	O
allowed	O
to	O
change	O
the	O
state	O
of	O
the	O
system	O
while	O
it	O
is	O
executed	O
.	O
</s>
<s>
In	O
order	O
to	O
ensure	O
this	O
atomicity	B-General_Concept
,	O
the	O
designers	O
at	O
Cyrix	O
made	O
the	O
xchg	O
uninterruptible	O
.	O
</s>
<s>
Due	O
to	O
pipelining	B-General_Concept
and	O
branch	B-General_Concept
predicting	I-General_Concept
,	O
however	O
,	O
another	O
xchg	O
enters	O
the	O
pipeline	O
before	O
the	O
previous	O
one	O
completes	O
,	O
causing	O
a	O
deadlock	B-Operating_System
.	O
</s>
<s>
A	O
fix	O
for	O
unintentional	O
instances	O
of	O
the	O
bug	O
is	O
to	O
insert	O
another	O
instruction	O
in	O
the	O
loop	O
,	O
the	O
nop	B-Language
instruction	O
being	O
a	O
good	O
candidate	O
.	O
</s>
