<s>
Dynamic	B-Operating_System
Debugging	I-Operating_System
Technique	I-Operating_System
(	O
DDT	B-Operating_System
)	O
is	O
a	O
series	O
of	O
debugger	B-Application
programs	O
originally	O
developed	O
for	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
hardware	O
,	O
initially	O
known	O
as	O
DEC	O
Debugging	O
Tape	O
because	O
it	O
was	O
distributed	O
on	O
paper	O
tape	O
.	O
</s>
<s>
The	O
name	O
is	O
a	O
pun	O
on	O
the	O
insecticide	O
DDT	B-Operating_System
.	O
</s>
<s>
The	O
first	O
version	O
of	O
DDT	B-Operating_System
was	O
developed	O
at	O
MIT	O
for	O
the	O
PDP-1	B-Device
computer	O
in	O
1961	O
,	O
but	O
newer	O
versions	O
on	O
newer	O
platforms	O
continued	O
to	O
use	O
the	O
same	O
name	O
.	O
</s>
<s>
Early	O
versions	O
of	O
Digital	O
Research	O
's	O
CP/M	B-Application
and	O
CP/M	B-Operating_System
-86	I-Operating_System
kept	O
the	O
DEC	O
name	O
DDT	B-Operating_System
(	O
and	O
DDT-86	O
and	O
DDT-68K	B-Operating_System
)	O
for	O
their	O
debugger	B-Application
,	O
however	O
,	O
now	O
meaning	O
Dynamic	O
Debugging	B-Application
Tool	I-Application
.	O
</s>
<s>
The	O
CP/M	B-Application
DDT	B-Operating_System
was	O
later	O
superseded	O
by	O
the	O
Symbolic	O
Instruction	O
Debugger	B-Application
(	O
SID	O
,	O
ZSID	O
,	O
SID86	O
,	O
and	O
GEMSID	O
)	O
in	O
DR	O
DOS	O
and	O
GEM	B-Application
.	O
</s>
<s>
In	O
addition	O
to	O
its	O
normal	O
function	O
as	O
a	O
debugger	B-Application
,	O
DDT	B-Operating_System
was	O
also	O
used	O
as	O
a	O
top-level	O
command	O
shell	O
for	O
the	O
Massachusetts	O
Institute	O
of	O
Technology	O
(	O
MIT	O
)	O
Incompatible	B-Application
Timesharing	I-Application
System	I-Application
(	O
ITS	O
)	O
operating	B-General_Concept
system	I-General_Concept
;	O
on	O
some	O
more	O
recent	O
ITS	O
systems	O
,	O
"	O
PWORD	B-Operating_System
"	O
—	O
which	O
implements	O
a	O
restricted	O
subset	O
of	O
DDT	B-Operating_System
's	O
functionality	O
—	O
is	O
run	O
first	O
and	O
is	O
overlaid	O
with	O
DDT	B-Operating_System
as	O
soon	O
as	O
the	O
user	O
logs	O
in	O
.	O
</s>
<s>
DDT	B-Operating_System
could	O
run	O
and	O
debug	B-Device
up	O
to	O
eight	O
processes	B-Operating_System
(	O
called	O
"	O
jobs	O
"	O
on	O
ITS	O
)	O
at	O
a	O
time	O
,	O
such	O
as	O
several	O
sessions	O
of	O
TECO	B-Application
,	O
and	O
DDT	B-Operating_System
could	O
be	O
run	O
recursively	O
-	O
that	O
is	O
,	O
some	O
or	O
all	O
of	O
those	O
jobs	O
could	O
themselves	O
be	O
DDTs	B-Operating_System
(	O
which	O
could	O
then	O
run	O
another	O
eight	O
jobs	O
,	O
and	O
so	O
on	O
)	O
.	O
</s>
<s>
These	O
eight	O
jobs	O
were	O
all	O
given	O
unique	O
names	O
,	O
and	O
the	O
usual	O
name	O
for	O
the	O
original	O
and	O
top-most	O
DDT	B-Operating_System
was	O
"	O
HACTRN	B-Operating_System
"	O
(	O
"	O
hack-tran	O
"	O
)	O
.	O
</s>
<s>
Guy	O
L	O
.	O
Steele	O
wrote	O
a	O
filk	O
poem	O
parody	O
of	O
Edgar	O
Allan	O
Poe	O
's	O
"	O
The	O
Raven	O
,	O
"	O
entitled	O
The	B-Operating_System
HACTRN	I-Operating_System
.	O
</s>
<s>
DDT	B-Operating_System
(	O
Dynamic	B-Operating_System
Debugging	I-Operating_System
Technique	I-Operating_System
)	O
,	O
as	O
implemented	O
on	O
the	O
DECsystem-10	B-Device
&	O
DECSYSTEM-20	B-Device
allowed	O
references	O
to	O
symbols	O
within	O
the	O
programming	O
being	O
debugged	O
.	O
</s>
