DIGITAL-1-3-S ddtDesc Aug64

This document describes the DEC Debugging Tape (DDT-1), an online debugging program for the PDP-1 computer, initially revised on August 13, 1964, by A. Kotok.

DDT was designed to significantly reduce program debugging time by providing an interactive environment, contrasting with the slower traditional offline methods of compilation, assembly, dumps, and manual analysis.

Key features and functionalities of DDT-1 include:

• Online Control & Modification: Users can interact directly with their program at the computer, controlling its execution and modifying program code or data in memory.
• Breakpoints: It features a breakpoint facility allowing users to interrupt program execution at specific points to examine the machine's state and quickly locate errors.
• Immediate Patching: Programmers can insert and test corrections or patches instantly, and generate loadable patch tapes on the spot, eliminating the need for full reassembly after each small fix.
• Symbolic Interaction: DDT maintains a symbol table, enabling users to interact with the system using the program's symbolic language rather than raw memory addresses.
• Memory Operations: It supports detailed examination and modification of memory registers, loading binary and symbol tapes, and punching data and jump blocks. Users can also search memory for specific values, non-values, or effective addresses, utilizing a mask for focused searches.
• Output Modes: Various commands allow the user to control the display format of register addresses and contents (e.g., absolute octal, relative symbolic, octal constants, decimal).

An "Extend DDT-1" appendix details a version designed for PDP-1 systems equipped with Memory Extension Control (supporting up to 16 memory modules) and requiring hardware multiply/divide routines. This extended version occupies different memory locations, features a slightly modified symbol syntax (e.g., period indicates a decimal number, first six characters of symbols are significant), and includes some altered or new commands to manage the multi-module memory environment.