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The file extension .TD is associated with the program "Borland Turbo Debugger" a DOS based debugger first introduced in 1989. Borland Turbo debugger was initially marketed as a standalone application and later packaged along with "Turbo assembler" and "Turbo C" to form the "Borland C++" suite of programs.

A "Debugger" is an application that allows the user to run programs and follow every step of their execution, pause or stop the program whilst running, edit the code or content, then restart. This allows closer inspection of the programs behaviours and is an invaluable tool for software development.