If you want to print a string (terminated with a NUL (000)) suggest the following:
string: .asciz "the quick brown ..."
mov #1000,sp ; init stack
mov #string,R0 ; pass string as argument
jsr pc,Print ; to Print function
Print:mov r1,-(sp) ; save R1
br 20$ ; jump to NUL test
Although the CDP1802 was implemented as a single chip, a preceding version was implemented as two chips. I don't know exactly how functionality was split, but the architecture would have been very amenable to subdivision into two chips--one of which would hold the 4-bit P and X registers along with the sixteen numbered 16-bit registers and the logic to ...
I'm only guessing, but I would guess in the late '70s and early '80s either
RSTS/E for traditional timesharing use
RSX-11M for real-time application use; maybe this transitioned to RSX-11M-Plus as larger memories became common
I have no data for this, just an impression of what was going on in DEC while I was there. I'd be interested in numbers if ...