<s>
Computer	O
hardware	O
or	O
software	O
is	O
said	O
to	O
be	O
bug	B-General_Concept
compatible	I-General_Concept
if	O
it	O
exactly	O
replicates	O
even	O
an	O
undesirable	O
feature	O
of	O
a	O
previous	O
version	O
.	O
</s>
<s>
An	O
aspect	O
of	O
maintaining	O
backward	B-General_Concept
compatibility	I-General_Concept
with	O
an	O
older	O
system	O
is	O
that	O
such	O
systems	O
 '	O
client	O
programs	O
often	O
do	O
not	O
only	O
depend	O
on	O
their	O
specified	O
interfaces	O
but	O
also	O
bugs	B-Error_Name
and	O
unintended	O
behaviour	O
.	O
</s>
<s>
Besides	O
the	O
significantly	O
higher	O
complexity	O
that	O
needs	O
to	O
be	O
maintained	O
during	O
the	O
natural	O
evolution	O
of	O
the	O
code	O
or	O
interface	O
,	O
it	O
can	O
sometimes	O
cause	O
performance	O
or	O
security	O
issues	O
,	O
and	O
the	O
inconsistencies	O
in	O
the	O
behaviour	O
of	O
interfaces	O
can	O
sometimes	O
lead	O
to	O
new	O
bugs	B-Error_Name
in	O
the	O
software	O
using	O
it	O
,	O
creating	O
difficult	O
to	O
resolve	O
multi-directional	O
cross	O
dependencies	O
between	O
various	O
pieces	O
of	O
code	O
.	O
</s>
<s>
Examples	O
can	O
be	O
found	O
in	O
MS-DOS/PC	O
DOS	O
:	O
</s>
<s>
When	O
MS-DOS/PCDOS	O
3.1	O
and	O
higher	O
(	O
including	O
Windows	B-Application
9x	O
)	O
and	O
OS/2	B-Application
detect	O
certain	O
FAT	O
OEM	O
labels	O
,	O
they	O
do	O
not	O
trust	O
some	O
BIOS	B-Device
Parameter	I-Device
Block	I-Device
(	O
BPB	O
)	O
values	O
and	O
recalculate	O
them	O
from	O
other	O
disk	O
geometry	O
parameters	O
in	O
order	O
to	O
work	O
around	O
several	O
off-by-one	B-Error_Name
calculation	O
errors	O
caused	O
by	O
some	O
of	O
their	O
formatter	O
software	O
under	O
earlier	O
issues	O
of	O
these	O
systems	O
.	O
</s>
<s>
When	O
MS-DOS/PCDOS	O
5.0	O
and	O
higher	O
are	O
running	O
on	O
286	O
or	O
higher	O
processors	O
,	O
the	O
resident	O
executable	B-Operating_System
loader	I-Operating_System
contains	O
code	O
specially	O
designed	O
to	O
detect	O
and	O
fix	O
certain	O
widespread	O
applications	O
and	O
stub	O
loaders	B-Operating_System
(	O
such	O
as	O
programs	O
linked	O
with	O
older	O
versions	O
of	O
Microsoft	O
's	O
EXEPACK	O
or	O
Rational	O
Systems	O
 '	O
386	O
DOS	O
extenders	O
)	O
by	O
patching	O
the	O
loaded	O
program	B-Application
image	I-Application
before	O
executing	O
it	O
.	O
</s>
<s>
Under	O
certain	O
conditions	O
an	O
underlying	O
DOS	O
also	O
patches	O
Windows	B-Application
(	O
WINA20.386	O
)	O
.	O
</s>
<s>
Over	O
the	O
course	O
of	O
development	O
,	O
DR-DOS	B-Application
also	O
had	O
to	O
be	O
modified	O
to	O
not	O
only	O
emulate	O
many	O
undocumented	O
peculiarities	O
and	O
undesirable	O
properties	O
of	O
MS-DOS	B-Application
and	O
PCDOS	B-Operating_System
(	O
like	O
having	O
to	O
use	O
certain	O
misleading	O
filenames	O
such	O
as	O
IBMBIO.COM	O
,	O
IBMDOS.COM	O
or	O
COUNTRY.SYS	O
for	O
files	O
which	O
do	O
not	O
follow	O
the	O
specs	O
for	O
executables	B-Application
under	O
DOS	O
,	O
or	O
having	O
to	O
introduce	O
a	O
directory	B-Application
path	I-Application
length-limited	O
Current	O
Directory	O
Structure	O
(	O
CDS	O
)	O
internally	O
)	O
,	O
but	O
also	O
actual	O
bugs	B-Error_Name
in	O
the	O
kernel	O
and	O
several	O
drivers	O
,	O
in	O
order	O
to	O
make	O
certain	O
other	O
drivers	O
and	O
applications	O
run	O
on	O
DR-DOS	B-Application
,	O
when	O
they	O
were	O
tested	O
on	O
specific	O
versions	O
of	O
MS-DOS	B-Application
only	O
.	O
</s>
<s>
Windows	B-Application
,	O
which	O
has	O
traditionally	O
emulated	O
many	O
old	O
system	O
bugs	B-Error_Name
to	O
allow	O
older	O
low-level	O
programs	O
to	O
run	O
,	O
is	O
another	O
example	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
Wine	B-Application
,	O
which	O
makes	O
it	O
possible	O
to	O
run	O
many	O
Windows	B-Application
applications	I-Application
on	O
other	O
platforms	O
,	O
also	O
needs	O
to	O
maintain	O
bug	B-General_Concept
compatibility	I-General_Concept
with	O
Windows	B-Application
.	O
</s>
<s>
When	O
Microsoft	O
phased	O
out	O
support	O
for	O
16-bit	O
code	O
in	O
Windows	B-Application
by	O
no	O
longer	O
including	O
NTVDM	O
in	O
64-bit	O
editions	O
of	O
the	O
operating	O
system	O
,	O
the	O
executable	B-Operating_System
loader	I-Operating_System
was	O
modified	O
to	O
recognize	O
some	O
specific	O
16-bit	O
stub	O
launchers	O
and	O
installers	O
and	O
replace	O
them	O
on-the-fly	O
with	O
equivalent	O
code	O
stubs	O
that	O
run	O
on	O
64-bit	O
processors	O
.	O
</s>
<s>
During	O
development	O
of	O
its	O
IBM	O
PC	O
compatible	O
,	O
Compaq	O
engineers	O
found	O
that	O
Microsoft	B-Operating_System
Flight	I-Operating_System
Simulator	I-Operating_System
would	O
not	O
run	O
because	O
of	O
what	O
subLOGIC	O
's	O
Bruce	O
Artwick	O
described	O
as	O
"	O
a	O
bug	B-Error_Name
in	O
one	O
of	O
Intel	O
's	O
chips	O
"	O
,	O
forcing	O
them	O
to	O
make	O
their	O
computer	O
bug-compatible	O
with	O
the	O
IBM	O
PC	O
.	O
</s>
<s>
Another	O
hardware	O
example	O
is	O
found	O
in	O
the	O
design	O
of	O
the	O
IBM	B-Operating_System
Personal	I-Operating_System
Computer/AT	I-Operating_System
A20	B-Device
address	I-Device
line	I-Device
to	O
emulate	O
the	O
behaviour	O
in	O
older	O
processors	O
.	O
</s>
<s>
Microsoft	B-Application
Excel	I-Application
has	O
always	O
had	O
a	O
deliberate	O
leap	O
year	O
bug	B-Error_Name
,	O
which	O
falsely	O
treats	O
29	O
February	O
1900	O
as	O
an	O
actual	O
date	O
,	O
to	O
ensure	O
backward	B-General_Concept
compatibility	I-General_Concept
with	O
Lotus	B-Application
1-2-3	I-Application
.	O
</s>
<s>
Hyrum	O
Wright	O
,	O
an	O
engineer	O
at	O
Google	B-Application
,	O
talks	O
about	O
this	O
problem	O
that	O
he	O
observed	O
firsthand	O
while	O
working	O
on	O
C++	O
core	O
libraries	O
.	O
</s>
<s>
It	O
was	O
Titus	O
Winters	O
,	O
also	O
an	O
engineer	O
at	O
Google	B-Application
,	O
who	O
popularized	O
this	O
concept	O
on	O
a	O
larger	O
scale	O
as	O
"	O
Hyrum	O
's	O
Law	O
"	O
.	O
</s>
