<s>
MacsBug	B-Operating_System
is	O
a	O
low-level	O
(	O
assembly	O
language/machine	O
-level	O
)	O
debugger	B-Application
for	O
the	O
classic	B-Application
Mac	I-Application
OS	I-Application
operating	O
system	O
.	O
</s>
<s>
MacsBug	B-Operating_System
is	O
an	O
acronym	O
for	O
Motorola	O
Advanced	O
Computer	O
Systems	O
Debugger	B-Application
,	O
as	O
opposed	O
to	O
Macintosh	O
debugger	B-Application
(	O
The	O
Motorola	B-Device
68000	I-Device
Microprocessor	I-Device
is	O
imprinted	O
with	O
the	O
MACSS	O
acronym	O
)	O
.	O
</s>
<s>
The	O
original	O
version	O
was	O
developed	O
by	O
Motorola	O
as	O
a	O
general	O
debugger	B-Application
for	O
its	O
68000	B-Device
systems	O
.	O
</s>
<s>
MacsBug	B-Operating_System
is	O
invoked	O
by	O
hitting	O
the	O
Macintosh	O
's	O
"	O
Programmer	B-Application
's	I-Application
Key	I-Application
"	O
or	O
,	O
as	O
it	O
became	O
later	O
known	O
,	O
the	O
"	O
Interrupt	O
Key	O
"	O
or	O
by	O
pressing	O
"	O
Command-Power	O
"	O
.	O
</s>
<s>
MacsBug	B-Operating_System
offers	O
many	O
commands	O
for	O
disassembling	O
,	O
searching	O
,	O
and	O
viewing	O
data	O
as	O
well	O
as	O
control	O
over	O
processor	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
MacsBug	B-Operating_System
is	O
not	O
installed	O
by	O
default	O
with	O
Mac	B-Application
OS	I-Application
,	O
although	O
every	O
Macintosh	O
since	O
the	O
Macintosh	O
Plus	O
includes	O
a	O
debugger	B-Application
in	O
ROM	O
known	O
as	O
MicroBug	O
.	O
</s>
<s>
Users	O
who	O
stumble	O
into	O
MacsBug	B-Operating_System
by	O
accident	O
need	O
only	O
to	O
enter	O
G	O
and	O
press	O
return	O
to	O
escape	O
from	O
MacsBug	B-Operating_System
;	O
however	O
,	O
MacsBug	B-Operating_System
is	O
not	O
installed	O
by	O
default	O
,	O
requiring	O
a	O
system	O
extension	O
,	O
so	O
a	O
typical	O
user	O
environment	O
does	O
not	O
include	O
it	O
.	O
</s>
<s>
As	O
the	O
classic	B-Application
Mac	I-Application
OS	I-Application
lacked	O
memory	B-General_Concept
protection	I-General_Concept
,	O
"	O
hard	O
crashes	O
"	O
where	O
an	O
application	O
crash	O
simply	O
froze	O
the	O
entire	O
system	O
were	O
n't	O
uncommon	O
.	O
</s>
<s>
With	O
MacsBug	B-Operating_System
installed	O
,	O
instead	O
of	O
an	O
unresponsive	O
system	O
,	O
the	O
user	O
would	O
be	O
dumped	O
into	O
MacsBug	B-Operating_System
,	O
where	O
they	O
could	O
type	O
ES	O
to	O
Exit	O
to	O
Shell	O
(	O
force	O
quit	O
the	O
crashed	O
application	O
and	O
return	O
to	O
the	O
Finder	B-Application
)	O
or	O
RB	O
for	O
ReBoot	O
,	O
which	O
restarted	O
the	O
system	O
.	O
</s>
<s>
Such	O
recovery	O
efforts	O
were	O
often	O
not	O
successful	O
,	O
with	O
the	O
only	O
alternative	O
a	O
hard	B-Operating_System
reset	I-Operating_System
.	O
</s>
<s>
In	O
Mac	B-Application
OS	I-Application
versions	O
7.5	O
and	O
later	O
,	O
the	O
presence	O
of	O
MacsBug	B-Operating_System
is	O
indicated	O
at	O
startup	O
;	O
it	O
is	O
present	O
if	O
the	O
user	O
sees	O
the	O
text	O
Debugger	B-Application
installed	O
(	O
although	O
,	O
occasionally	O
,	O
this	O
may	O
indicate	O
the	O
presence	O
of	O
another	O
piece	O
of	O
software	O
loaded	O
into	O
the	O
area	O
of	O
memory	O
reserved	O
for	O
the	O
debugger	B-Application
,	O
instead	O
)	O
.	O
</s>
<s>
MacsBug	B-Operating_System
was	O
originally	O
for	O
the	O
Motorola	B-Device
68000	I-Device
series	I-Device
of	O
processors	O
only	O
.	O
</s>
<s>
When	O
Apple	O
introduced	O
the	O
Power	B-Device
Macintosh	I-Device
in	O
1994	O
,	O
it	O
was	O
followed	O
by	O
an	O
updated	O
MacsBug	B-Operating_System
that	O
supported	O
the	O
PowerPC	B-Architecture
instruction	O
set	O
and	O
architecture	O
.	O
</s>
<s>
The	O
last	O
version	O
of	O
MacsBug	B-Operating_System
was	O
6.6.3	O
,	O
released	O
September	O
14	O
,	O
2000	O
.	O
</s>
<s>
This	O
final	O
version	O
works	O
with	O
all	O
of	O
the	O
machines	O
released	O
in	O
the	O
July	O
–	O
September	O
timeframe	O
of	O
2000	O
,	O
including	O
the	O
Power	B-Device
Mac	I-Device
G4	I-Device
(	O
uni	O
-	O
and	O
multi-processor	O
)	O
,	O
Power	B-Device
Mac	I-Device
G4	I-Device
Cube	I-Device
,	O
the	O
iMac	B-Device
family	O
(	O
Ruby	O
,	O
Indigo	O
,	O
Sage	O
,	O
Graphite	O
,	O
and	O
Snow	O
)	O
,	O
and	O
the	O
iBook	B-Device
family	O
(	O
Indigo	O
,	O
Key	O
Lime	O
,	O
and	O
Graphite	O
)	O
.	O
</s>
<s>
6.6.3	O
includes	O
better	O
support	O
for	O
debugging	O
MP	O
tasks	O
,	O
and	O
fixes	O
some	O
serious	O
bugs	O
in	O
the	O
memory	O
setting	O
commands	O
when	O
used	O
in	O
PCI	B-Protocol
I/O	O
space	O
.	O
</s>
<s>
It	O
can	O
also	O
be	O
used	O
in	O
Classic	O
when	O
running	O
under	O
Mac	B-Application
OS	I-Application
X	O
,	O
where	O
it	O
is	O
invoked	O
by	O
pressing	O
"	O
⌘	O
-⏏	O
 "	O
(	O
or	O
"	O
⌘	O
-F12	O
"	O
on	O
systems	O
without	O
an	O
Eject	O
key	O
)	O
.	O
</s>
<s>
Mac	B-Application
OS	I-Application
X	O
allows	O
programmers	O
to	O
use	O
familiar	O
MacsBug	B-Operating_System
commands	O
in	O
gdb	B-Language
.	O
</s>
<s>
This	O
gdb	B-Language
plugin	O
is	O
included	O
with	O
the	O
OS	O
X	O
Developer	O
Tools	O
,	O
located	O
in	O
the	O
directory	O
/usr/libexec/gdb/plugins/MacsBug/	O
.	O
</s>
<s>
Third	O
party	O
alternatives	O
to	O
MacsBug	B-Operating_System
included	O
ICOM	O
Simulations	O
 '	O
TMON	O
which	O
came	O
with	O
the	O
Darin	O
Adler	O
Extended	O
User	O
Area	O
&	O
Trap	O
Discipline	O
(	O
allowing	O
all	O
documented	O
Mac	O
API	O
parameters	O
to	O
be	O
verified	O
)	O
and	O
the	O
fully	O
symbolic	O
Jasik	B-Operating_System
debugger	I-Operating_System
,	O
which	O
was	O
much	O
more	O
powerful	O
,	O
but	O
harder	O
to	O
use	O
due	O
to	O
the	O
intricate	O
and	O
non-standard	O
user	O
interface	O
.	O
</s>
