So I recently had to reboot my machine, and I took the opportunity to try this; it worked right off, booted 'unix' OK. (I didn't try any of the other Unixes in the root directory.) I had only that pack mounted on DL0, nothing else. So, this turned out to be because I had replaced the csv.o in libc.a with a new one, because the standard V6 one doesn't work with long returns (which use R1 as well as R0, and the V6 cret bashed R1). I put the old csv.o back, and re-linked them, and this time c? all turned out identical. So the source in the distro really is the source for the running compiler on it. What was wierd was that in the new one, the routine csv is one word shorter (and so is csv.o). So now I don't understand what made them the same sizes!? The new ones should have been one word shorter!? Still poking into this... I understand most of the differences between the versions of c? with the old and new csv.o; in all the jumps to cret, the indirect word in the instruction was off by two (because cret was one word lower because csv was one word shorter); that, along with different contents in csv.o, created most of the differences. Why one word shorter? Because in csv: tst -(sp) / creates a temporary on top of the stack jmp (r0) had been replaced with: jsr pc,(r0) (saving one instruction, and making it one word smaller). Noel