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The Free On-line Dictionary of Computing (foldoc)

1. Generic term for a program that assists in debugging other
programs by showing individual {machine instructions} in a
readable symbolic form and letting the user change them. In
this sense the term DDT is now archaic, having been widely
displaced by "debugger" or names of individual programs like
"{adb}", "{sdb}", "{dbx}", or "{gdb}".

2. Under {MIT}'s fabled {ITS} {operating system}, DDT (running
under the alias HACTRN) was also used as the {shell} or top
level command language used to execute other programs.

3. Any one of several specific debuggers supported on early
{DEC} hardware. The {DEC} {PDP-10} Reference Handbook (1969)
contained a footnote on the first page of the documentation
for DDT that illuminates the origin of the term:

Historical footnote: DDT was developed at {MIT} for the
{PDP-1} computer in 1961. At that time DDT stood for "DEC
Debugging Tape". Since then, the idea of an on-line debugging
program has propagated throughout the computer industry. DDT
programs are now available for all DEC computers. Since media
other than tape are now frequently used, the more descriptive
name "Dynamic Debugging Technique" has been adopted, retaining
the DDT abbreviation. Confusion between DDT-10 and another
well known pesticide, dichloro-diphenyl-trichloroethane
(C14-H9-Cl5) should be minimal since each attacks a different,
and apparently mutually exclusive, class of bugs.

(The "tape" referred to was, incidentally, not magnetic but
paper.) Sadly, this quotation was removed from later editions
of the handbook after the {suits} took over and DEC became
much more "businesslike".

The history above is known to many old-time hackers. But
there's more: Peter Samson, compiler of the original {TMRC}
lexicon, reports that he named "DDT" after a similar tool on
the {TX-0} computer, the direct ancestor of the PDP-1 built at
{MIT}'s Lincoln Lab in 1957. The debugger on that
ground-breaking machine (the first transistorised computer)
rejoiced in the name FLIT (FLexowriter Interrogation Tape).

[{Jargon File}]