[TUHS] Work I've done with a PDP-11 simulator

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue May 13 00:49:44 AEST 2014

    > To boot up the root pack (I don't think I did this at any point; I've
    > always mounted it as a subsidiary drive)
    > ...
    > The disk has a working RL bootstrap in block 0, it should boot OK. 

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

    > So, just for grins, because I was curious (after your question), I did
    > try recompiling the C compiler, to see what I'd get.
    > What I got were three files (c0, c1 and c2) which were _the exact same
    > size_ (down to the byte) as the binaries on the V6 Research distro, but
    > had a number of differences when compared with 'cmp -l'. Odd!
    > ...
    > I'll take a gander tomorrow and try and work it out.

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

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

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).


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