M-1108 BBN-46 Jun61

This document describes "BBN-46; Punchoff; Binary 1," a utility routine for PDP computers, dated June 19, 1961.

Purpose: The primary goal of this routine is to create a binary tape that, when reloaded, will accurately reproduce the entire memory contents of the computer as they were at the time of the punchoff.

Key Features and Usage:

• Automatic Memory Copy: It produces a complete copy of memory without requiring the user to specify exact areas, punching all non-zero words. Clearing memory before loading programs is recommended for efficiency, as it skips blocks of five or more consecutive zeroes.
• Self-Loading Tapes: The output tape includes a RIM loader (7700-7777) that can clear memory before loading and a copy of the punchoff routine itself, allowing for subsequent punchoffs from the loaded tape.

• Loading Options (with Sense Switches):

  • SS1 (up): Prevents the loader from performing a jump at the end of the binary tape.
  • SS2 (up): Skips the memory clear routine and causes the loader to compare the tape contents with memory (for verification) rather than loading.

• Memory Area Selection: Different versions (HI, MID, LO) are available to ensure the punchoff routine occupies a memory area not used by the program being punched off. All versions use 7700-7777.

• Jump Location: TW12-17 can be set to specify a desired jump location after the output tape is loaded.
• Execution Time: Varies from 15 seconds to 3 minutes 45 seconds, depending on the number of non-zero registers in memory.
• Origin: The routine was supplied by Bolt, Beranek and Newman.