Odd Comments and Strange Doings in Unix

Friday, 19 July, Year 11 d.Tr. | Author: Mircea Popescu

I am preserving herei the original form of an article I happen to need to link somewhere else -- yes, I've had to interrupt working on that to work on this, thank you Team Fucktard -- because the Internet's meanwhile broken. Sorry about that ; I didn't break it myself.

username Ritchie, Dennis
category user
contact dmr@bell-labs.com

Odd Comments and Strange Doings in Unix

Values of Beeta

In Sixth Edition Unix, the mv command could produce this diagnostic:

	values of β will give rise to dom!

This was noticed by some and is recorded in aii WWW page or so, which are mostly copies of the same compilation
of rarely encountered and striking error messages from various systems.

The actual source line in mv.c that produced the message was

	write(1,"values of \016B\017 will give rise to dom!\n",37);

except that in the real source, the \016B\017 was written with literal
ASCII SI and SO control characters. These shifted the Model 37 Teletype
into its optional extended character set, and the B printed as the Greek
letter beta. See below for more about rendering beta amidst changing
hardware and software.

Like most of the messages recorded in these compilations, this one was produced
in some situation that we considered unlikely or as result of abuse; the details
don't matter. I'm recording why the phrase was selected.

The very first use of Unix in the "real business" of Bell Labs was to
type and produce patent applications, and for a while in the early 1970s we had three typists
busily typing away in the grotty lab on the sixth floor. One day someone came
in and observed on the paper sticking out of one of the Teletypes,
displayed in magnificent isolation, this ominous phrase:

	values of β will give rise to dom!

It was of course obvious that the typist had interrupted a printout (generating
the "!" from the ed editor) and moved up the paper, and that the context
must have been something like "varying values of beta will give rise to domain wall movement"
or some other fragment of a physically plausible patent application.

But the phrase itself was just so striking! Utterly meaningless, but it looks like what...
a warning? What is "dom?"

At the same time, we were experimenting with text-to-voice software by Doug McIlroy
and others, and of course the phrase was tried out with it. For whatever reason, its rendition
of "give rise to dom!" accented the last word in a way that emphasized
the phonetic similarity between "doom" and the first syllable of "dominance."
It pronounced "beta" in the British style, "beeta." The entire occurrence became a small, shared

The phrase had to be recorded somewhere, and it was, in the v6 source.iii
Most likely it was Bob Morris who did the deed, but it could just
as easily have been Ken.

I hope that your browser reproduces the β as a Greek beta.
It is written here as '& beta ;', which works on MSIE and
at least some Linux browsers, but not on Netscape 4.6 (at least mine).
Formerly I tried rendering it using 'fontface=symbol b /fontface'
with the appropriate angle-brackets, which works on this old Netscape
and MSIE, but not recent Mozillas. Sigh. If you are using
an old Netscape, with an appropriate fontface, here it is: b.iv

/* You are not expected to understand this */

Every now and then on Usenet or elsewhere I run across a
reference to a certain comment in the source code of the Sixth Edition
Unix operating system.

I've even been given two sweatshirts that quote it.

Most probably just heard about it, but those who saw it
in the flesh either had Sixth Edition Unix (ca. 1975)
or read the annotated version of this system by John Lions
(which was republished in 1996: ISBN 1-57298-013-7, Peer-to-Peer Communications).

It's often quoted as a slur on the quantity or quality of the comments
in the Bell Labs research releases of Unix. Not an unfair
observation in general, I fear, but in this case unjustified.
The actual code and other commentary surrounding it were precisely this:

	 * Switch to stack of the new process and set up
	 * his segmentation registers.
	 * If the new process paused because it was
	 * swapped out, set the stack level to the last call
	 * to savu(u_ssav).  This means that the return
	 * which is executed immediately after the call to aretu
	 * actually returns from the last routine which did
	 * the savu.
	 * You are not expected to understand this.
	if(rp->p_flag&SSWAP) {
		rp->p_flag =& ~SSWAP;
	 * The value returned here has many subtle implications.
	 * See the newproc comments.

So we tried to explain what was going on.
"You are not expected to understand this" was intended as a remark
in the spirit of "This won't be on the exam," rather than as an
impudent challenge.

The real problem is that we didn't understand what was going on either.
The savu/retu mechanism for doing process exchange was fundamentally broken
because it depended on switching to a previous stack frame and executing function
return code in a different procedure from the one that saved the earlier state.
This worked on the PDP-11 because its compiler always used the same context-save
mechanism; with the Interdata compiler, the procedure return code differed
depending on which registers were saved.

So, for Steve Johnson and me, trying to move the kernel for the first time
to a new machine, this code was indeed on the exam.
It took about a week of agonizing before we finally convinced each other that
the mechanism was wrong and no fiddling with the compiler was useful.
We redid the coroutine control-passing primitives altogether, and this code
section, and the comment, passed into history.

Comments I do feel guilty about

Doing 32-bit multiplication and division on a 16-bit machine like
the PDP-11 needs cleverness. My PDP-11 C compiler used subroutines
to do long * and /.
The multiplication routine was

/ 32-bit multiplication routine for fixed pt hardware.
/  Implements * operator
/ Credit to an unknown author who slipped it under the door.
.globl	lmul
.globl	csv, cret

	jsr	r5,csv
	mov	6(r5),r2
	sxt	r1
	sub	4(r5),r1
	mov	10.(r5),r0
	sxt	r3
	sub	8.(r5),r3
	mul	r0,r1
	mul	r2,r3
	add	r1,r3
	mul	r2,r0
	sub	r3,r0
	jmp	cret

which is neat, and I wasn't smart enough to figure it out.
I don't feel guilty, though, because I didn't then know who
suggested it and I did acknowledge the fact.

But I'll carry this one on my conscience for a while.
The division routine included

	mov	r4,-(sp)
	clr	r0
	div	r3,r0
	mov	r0,r4		/high quotient
	mov	r1,r0
	mov	r2,r1
	div	r3,r0
	bvc	1f
	sub	r3,r0		/ this is the clever part
	div	r3,r0
	tst	r1
	sxt	r1
	add	r1,r0		/ cannot overflow!

I almost (or maybe even completely) figured out why it worked.

The spot on the soul is the "this is the clever part" comment.

Addendum 18 Oct 1998

Amos Shapir of nSOF (and of long memory!) just blackened
(or widened) the spot a bit more in a
mail message, to wit:

I gather the "almost" here is because this trick almost worked... It has
a nasty bug which I had to find the hard way!

The "clever part" relies on the fact that if the "bvc 1f" is not taken,
it means that the result could not fit in 16 bits; in that case the long
value in r0,r1 is left unchanged. The bug is that this behavior is not
documented; in later models (I found this on an 11/34) when the result
does fit in 16 bits but not in 15 bits (that is, overflow for signed, but
not unsigned types), the overflow bit is set, but the unsigned result
does overwrite the original values -- which makes this routine provide
very strange results!

A hardware story

Back around 1970-71, Unix on the PDP-11/20 ran on hardware that
not only did not support virtual memory, but didn't support any
kind of hardware memory mapping or protection, for example against
writing over the kernel. This was a pain,
because we were using the machine for multiple users. When anyone
was working on a program, it was considered a courtesy to yell
"A.OUT?" before trying it, to warn others to save whatever they
were editing.
[A substory: at some point several were sitting around working away.
Bob Morris asked, almost conversationally, "what are the arguments to ld?"
Someone told him. We continued typing for the next minute, as a
thought began to percolate, not quite to the top of the brain--
in other words, not quite fast enough. The terminal stopped echoing
before anyone could stop and say "Hold on Bob, what is it you're trying to do?"]

We knew the PDP-11/45, which did support memory mapping and protection
for the kernel and other processes, was coming, but not instantly;
in anticipation, we arranged with Digital Special Systems to buy a
PDP-11/20 with KS-11 add-on. This was an extra system unit bolted to
the processor that made it distinguish kernel from user mode, and provided
a classical PDP-10 style "hi-seg" "low-seg" memory mapping unit. I seem
to recall that maybe 6 of these had been made when we ordered it.

Those who remember the early PDP-11s remember that there were no
multiply, divide, or shift-by-more-than-1 instructions, and that
they had an optional EAE ("extended arithmetic extension") gadget,
the KE-11A,
that appeared in physical memory in the I/O device space: one
stored the operands here and read back the answer there
(in fact at 777302 and following addresses).

One problem the KS-11 had to deal with was that ordinary programs needed to
do multiplies and divides, yet shouldn't be allowed to access the I/O device
space. So it included circuitry that detected just the EAE addresses,
and remapped them to physical, while all other virtual addresses in
user mode were mapped.

When we got the KS-11 machine, the EAE just didn't work at all.
With test programming, we got bus errors, or no action from
the EAE unit. But, in those days
when you bought such things from Digital Special Systems, you also got the
circuit drawings, so after a lot of headscratching and fiddling, they
were consulted.

It turned out that on one page of the drawings, there was an address comparator for the
EAE address, with an out-of-page arrow labelled "EAE ADDRESS DETECTED H".
On another page, there was an in-arrow labelled "EAE ADDRESS DETECTED L".
We couldn't find anything between these.

In the end, we had a visit from an embarrassed Digital Special Systems
guy who found an unused inverter pin
and added some white wires.

These days, such a problem would be harder to fix in the field.

  1. Including the completely idiotic hard newlines of the original, god fucking damn these people and their inept notions of implicit tagging. []
  2. Meanwhile broken, was www.tmk.com/ftp/humor/computer-error-messages.txt []
  3. How about you tards get some fucking blogs.

    And actually maintain them. []

  4. The things these puppies will tolerate, I swear... []
  5. The shit these people put up with! []
Category: Meta psihoza
Comments feed : RSS 2.0. Leave your own comment below, or send a trackback.

4 Responses

  1. That 'lmul' routine is a miniature fixint Karatsuba.

  2. Mircea Popescu`s avatar
    Mircea Popescu 
    Friday, 19 July 2019

    What else could it be, practically.

  3. 3 half-bitness muls in there, so pretty obvious, imho.

  1. [...] not, of cattle we use. [↩]Right, "hacked". It's the contemporaneous term for "magic", that "you are not expected to understand this" of yore, most beloved by all the bleaters who really had no business being there in the first [...]

Add your cents! »
    If this is your first comment, it will wait to be approved. This usually takes a few hours. Subsequent comments are not delayed.