PDP-7 Program Library

This document describes DECtog Jr., a program designed to format and certify DECtapes on PDP-7 and PDP-4 computers using their toggle switches. It formats tapes to the Standard Format for DIGITAL's 18 and 36-bit computers, featuring 1102 (octal) blocks per tape and 1536 data lines (3 bits per line) per block, or 400 (octal) words per block.

Key distinctions of DECtog Jr. from other DECtog versions include calculating checksums by XORing six-bit bytes, writing 199 additional space codes between end-zones and ending blocks for compatibility with PDP-7/9 ADVANCED Software, and exercising all tape blocks (0 through 1101 octal).

The library includes five main programs:

• RWMKTR: Writes the timing and mark tracks on a tape.
• WRNVT: Writes the virgin tape pattern and block mark numbers (must be run immediately after RWMKTR for initial tape formatting). It also sum-checks the tape in reverse.
• EXER: Exercises tapes by repeatedly writing patterns of all ones and all zeros, sum-checking in both forward and reverse directions.
• RBMN: Reads block numbers from the tape in the forward direction into memory.
• RBMNR: Reads block numbers from the tape in the reverse direction into memory.

Usage typically involves setting the computer's accumulator switches and starting the desired program at a specific address. The document details a comprehensive error reporting system, providing error codes, involved block numbers, DECtape status bits, and relevant register information for debugging. The software is written in PDP-7 Basic Assembler and requires a Paper Tape Reader, Teleprinter, and DECtapes.