DDT Debugging System PDP-7

This document is a Programming Manual for the DDT (DEC Debugging Tape) system designed for the Programmed Data Processor-7 (PDP-7) computer, also compatible with the PDP-4. Its primary purpose is to significantly reduce program debugging time by providing an interactive environment where users can control program execution, examine and modify program code or data, and define symbols directly on the computer via a teleprinter.

Key features and operations include:

• Loading Programs: Users can load object tapes, optionally appending their symbols to DDT's symbol table.
• Program Examination and Modification: Allows opening registers to view and change their contents, with output available in various modes (absolute octal, relative, symbolic, constant).
• Breakpoints and Traps: Enables setting breakpoints to interrupt program execution at specific locations, inspecting the machine state (Accumulator and Link register contents are saved), and continuing the program. A special trap (TRAP$) is available for CAL instructions.
• Symbol Management: DDT maintains a permanent symbol table of standard PDP-7 mnemonics, which can be extended with user-defined symbols from program tapes or the keyboard.
• Searching: Provides functions to search memory for specific words, operand addresses, or parts of instructions using a mask.
• Punching Operations: Facilitates punching corrected program patches directly onto object tapes in FF format, eliminating the need for full reassembly after minor fixes.

DDT itself occupies the highest memory registers, with its symbol table extending downward. The manual includes a comprehensive summary of commands and lists basic and extended symbols used within the system.