XOD - Extended Octal Debugging Program

This document describes XOD (Extended Octal Debugging Program), a debugging tool developed by Michael S. Wolfberg at the Moore School of Electrical Engineering, University of Pennsylvania, dated September 15, 1967, for the PDP-8 with Extended Memory.

Key Features and Functionality:

• Memory Management: Occupies locations 6430 through 7577 in any memory field. Its starting address is 7000.

• Examination and Modification: Allows users to examine and modify the contents of any memory location via the on-line Teletype.

  • / opens a location and types its contents.
  • CR sets the open location's content to a typed octal number.
  • LF acts as CR then opens the next location.
  • * refers to the last opened location.

• Block Operations:

  • Sets lower (LL) and upper (UL) limits for operations.
  • % allows zeroing (or setting to a specified value) a block of memory (LL through UL).
  • > (positive) and < (negative) perform masked searches within the LL-UL range, typing out matching locations.

• Field Specification: Users can direct XOD's attention to a specific memory field using #, which also initializes LL, UL, and MASK values.

• Elaborate Breakpoint Facility: This is a core feature, designed to preserve the status of the program's interrupt system.

  • Users can assign a single breakpoint (") at any location, which replaces the instruction with 5005.
  • It saves the program's accumulator, link, data field, Teletype output flag, and program interrupt status upon a breakpoint trap.
  • XOD uses locations 0005, 0006, and 0007 of every memory field for this facility.
  • When a trap occurs, XOD types the breakpoint location, link, and accumulator contents.
  • The ! command allows "proceeding" (restoring saved status, executing the original breakpoint instruction, and continuing execution), including "multiple proceeds" (N !).
  • Restrictions: Breakpoints should not be set at certain instructions (CIF, specific EAE instructions) if the user intends to "proceed" due to potential conflicts with instruction field registers or interrupt handling.

• Program Control:

  • Users can start a program at a specified address (octal number ').
  • Ctrl+C exits XOD to the monitor at location 07600.
  • A HLT instruction at 6430 is included to catch wild programs.

• State of XOD: Specific internal locations within XOD (e.g., 6600-6612, 7400-7402) store saved program state and debugger configuration (LL, UL, MASK).

Limitations:

• Does not include the ability to punch binary tapes.