BM873 Restart/Loader

This document is a technical manual for the Digital Equipment Corporation (DEC) BM873 Restart/Loader, a hardware module designed for the PDP-11 family of processors, published in the mid-1970s.

The BM873 is a quad-height, extended-length module that functions as a general-purpose initial program loader and system restart mechanism. Key aspects include:

• Functionality: It contains Read-Only Memory (ROM) with various bootstrap loader programs for common peripheral devices (e.g., disks, tapes) and provides initial program load capabilities. It supports loading from multiple sources, including the processor console, JMP instructions, external contact closures/voltage levels (e.g., pushbuttons, MODEM control), and power-fail events.
• Versions: Four versions (YA, YB, YC, YD) exist, differing in ROM size and the specific bootstrap loaders they contain.
• Installation & Configuration: The manual details installation procedures, including required wiring for AC LO/DC LO signals, jumper settings (which vary based on the PDP-11 CPU model), and how to configure device start addresses by adding or cutting diodes on the module. Special considerations for volatile memory systems and power-fail trap addressing are also covered.
• Programming: As a ROM-based device, the BM873 itself requires no programming. However, the manual provides guidance on how system software, particularly power-fail routines, should interact with the BM873's behavior to ensure proper system restart or program reloading.
• Customization: The BM873-YA version allows for user-customized programs by providing extra ROM capacity, enabled by cutting a specific jumper and programming compatible PROMs.
• Technical Details: The document includes specifications, timing diagrams for remote start and memory read operations, and engineering drawings of the module. An appendix lists device starting addresses for various peripherals across different BM873 versions.