PDP-9 Readin Mode Loader

This document describes the PDP-9 Readin Mode (RIM) Loader, a minimal routine designed for loading programs from perforated paper tapes into the PDP-9 computer. It requires only 13 (decimal) memory locations and can be loaded either manually using console switches or automatically via the optical paper tape reader with a hardware readin mode formatted tape.

The RIM Loader functions as the primary paper tape loader for the PDP-9, typically residing at the top of memory and remaining intact for use by other programs. When loading a RIM format tape, it reads binary data in pairs of words: the first word indicates the memory address, and the second word contains the data to be stored. The loading process concludes when a specific punch in channel 7 of the tape signals an instruction (usually a JMP or HLT), which then transfers control to the newly loaded program or halts the operation.