XXDP diagnostic package
XXDP refers both to instances of the collection of diagnostics produced by DEC for the PDP-11, and, more usually, the minimalist system later provided to load and run them. (The diagnostics themselves were actually developed under another OS, such as RSX-11; XXDP was only used to run them.)
PDP-11 diagnostics were originally distributed on paper tape; later, groups of them were amalgamated onto mass storage media such as RK05 and RL0x disk packs. These were given names such as "RKDP" ('RK Diagnostic Package'); typically, a particular system was delivered with an RKDP pack (say) which contained all the diagnostics appropriate to that particular machine. A simple monitor was created to allow the machine to be booted directly from that pack, and also to make it easy to run them; 'XXDP' was the obvious generalization which was not specific to any particular media.
An XXDP pack would contain diagnostics for all the PDP-11 peripherals and options; this could easily be mass produced, as PDP-11 production increased. The XXDP file system is that of DOS-11.