Reverse-assembled PDP-11 Diagnostic Monitor

Ian Hammond has reverse-assembled the XXDP PDP-11 diagnostic monitor and posted his efforts for all to see:

XXDP is a minimal disk operating system from DEC for the PDP-11 that was used to host their diagnostic programs. It provides a simple, flat filesystem interface (using the DOS-11 on-disk structure) and a binary loader, as well as a monitor interface for querying the fileystem, running diagnostics, and some configuration tasks. The DOS-11 filesystem is an artifact of a pre-RT-11 real-time DOS for the PDP-11 that was almost totally supplanted by RT-11. Like most DEC PDP-11 operating systems, it uses a 6.3 filenam.typ file name format encoded in RADIX-50 (digits, capital letters, $, and %).

Ian’s disassembly includes substantial commenting and cross-referencing, and looks a lot like original source for MACRO-11 might have. It’s really a work of art.


Many thanks. Originally I just wanted to know how it worked, but as I got to know it I saw a system that deserved a bit of love and to not be forgotten. Seeing the code slowly emerge was like watching a garden grow. Since uploading the disassembly I’ve converted XXDP to use RT-11’s file system instead of the file system it inherited from DOS-11. Ian.


Very cool! Working with the DOS-11 filesystem for XXDP has always been a bit of pain for me, because of having to use foreign filesystem drivers to access it. I look forward to seeing that project!