Dynamic debugging technique

Dynamic debugging technique, or DDT, and its kin, ODT, which accepted octal base numbers, was a debugger program, a form of Command Line Interpreter. This type of program was designed to run on a character-based computer terminal, such as a Teletype, or a DecWriter, or a VT100. The output of the terminal is familiar to any MS-DOS user: ASCII characters on a limited-sized field, typically 80 characters wide, which is the width of an IBM punch card.

Each 80-character line, up to 20 lines high, would fit on the output of a VT100, before scrolling up out of sight, and was thereafter effectively invisible, unless the data was accessible to an editor, such as TECO.

In order to display each 80-character line, a specific command was required. Thus a programmer could not view a program in any coherent form before the eyes. Everything had to be remembered or felt. For people with eidetic memory, like Bill Gates, that is no problem. But for the vast majority of people, such feats of memory made use of this class of software impractical.

The arrow keys didn't work either, because there were no arrow keys on a typewriter keyboard. Thus one line up, or 2 lines down, were a series of keystrokes which were encoded for a specific meaning at a specific time of use.