Failsafe

This document is a technical write-up for "Failsafe," a computer program developed by Gary B. Jennings for the Digital Equipment Corporation in 1970. Written in the LAP6 source language, this program is designed to run on a PDP-12B system with 8K of core memory.

The purpose of the program is to provide a reliable method for duplicating LINC tapes from Unit 0 onto Unit 1. The duplication process involves transferring 36 octal blocks at a time, with built-in counters for the read, write, and check operations. Once the transfer is complete, the program performs a verification step by comparing checksums of the blocks from both units. If an error is detected, the program halts and displays the incorrect block number in the accumulator, allowing the user to resume checking. After verification, the total number of errors is displayed while the tape units rewind, eventually exiting to the DIAL operating system. The document also provides the full assembly code implementation for this process.