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.