FORTRAN Debugging Technique

This document introduces the FORTRAN Debugging Technique (FDT), a sophisticated interactive tool designed for debugging FORTRAN IV programs on PDP-11 systems running RT-11 or RSTS/E. FDT empowers users with step-by-step control over program execution, enabling them to examine and modify the contents of any program variable during runtime. Successful use requires knowledge of FORTRAN IV, but not intricate details of internal data formats or machine operations.

To initiate a debugging session, programs must be compiled with specific FORTRAN compiler options (threaded code, internal statement numbers enabled), linked to include FDT (preferably in the root segment for overlays), and then executed. FDT automatically takes control, pausing before the program's first executable statement.

FDT commands are categorized into three types:

• Program Control Commands: (e.g., START, STOP, CONTINUE, STEP, PAUSE, WATCH) manage program execution flow, allowing users to halt, continue, step through statements, restart, or end a debugging session.
• Information Transfer Commands: (e.g., NAME, DIMENSION, TYPE, ACCEPT, ERASE) facilitate the inspection and modification of variables and array elements, including defining their data types.
• FDT Control Commands: (e.g., GOTO, IF, MACRO, WHAT) enable control over FDT's own operations, such as defining and executing command macros or implementing conditional branching within FDT commands.

The document details FDT's conventions and terminology, including syntax rules, the concept of the "current procedure," and various methods for specifying program locations (offset, named, relative, subscripted, and indirect addressing). It also explains mode codes, which define data types for location specifications. FDT features five types of pauses (automatic, entry, statement, step, and watch) to suspend program execution and prompt for user input. Advanced techniques cover accessing variables in named common blocks, FDT's internal address generation processes, ensuring necessary format conversion routines are loaded, and notes on using FDT in conjunction with the On-Line Debugging Technique (ODT). The guide concludes with important cautions and potential pitfalls to avoid during debugging, such as proper spacing and handling of ASCIZ strings.