The Risks Digest

The RISKS Digest

Forum on Risks to the Public in Computers and Related Systems

ACM Committee on Computers and Public Policy, Peter G. Neumann, moderator

Volume 10 Issue 36

Wednesday 12 September 1990


o Railway Safe Working - large analogue systems
o Re: BMW Heading Control System
A. L. Bangs
o Re: Robustness of RISC architectures
Andy Glew
Dave Sill
Andrew Taylor
Henry Spencer
Peter Holzer
Robert Cooper
Dik T. Winter
o Re: Computers and Safety
Peter Holzer
o Re: Software doesn't wear out?
Bob Estell
o Re: Software bugs "stay fixed"
Peter da Silva
o Info on RISKS (comp.risks)

Railway Safe Working - large analogue systems

Tue, 11 Sep 90 15:30:07 EDT
One area in which safety and reliability of analogue systems is well-understood
is railway safe working. Risks readers will certainly find L.T.C. Rolt's book
"Red for Danger" interesting and instructive. It covers the development of safe
working in the U.K.  from the first railways to the late Sixties (my edition
anyway).  There are many good examples of risks unnoticed, risks fixed badly,
re-appearing risks, and the work it takes to make people realize the risks and
then pay the price of fixing them.

Given the long experience in the U.K with safety in railway systems, you might
wonder why accidents such as Purley happen. It puzzles me too.  BR has the
technology to detect, on the train, the state of trackside signals, but only
uses it, in the first instance, to inform the driver.  In Australia (at least
NSW) signals are equipped with a small handle which pivots up when the signal
is at red and contacts a trip on the brake system if a train attempts to pass
it. The driver cannot subvert the systemin motion, although he can reset the
system and proceed once the train has fully stopped. This can only be done from
outside the train (by climbing down to the trip and manually resetting it).
This seems to be much more consistent with the rest of the safe working system,
where signal box interlocks are implemented by requiring two largish pieces of
metal to occupy the same space for conflicting events to occur. The only
failure mode here is severe deformation of the metal rods.

Of course, this kind of absolute block working is not always appropriate.  In
places where it is common to want to pass a red signal (dense suburban) extra
arms are installed between signals. These drop in sequence at a rate which
reduces the train speed to around 20 km/h.  Again there is no way to go faster
without overunning a raised arm.

Suburban trains have the usual dead-man's throttle handle (the throttle must be
pressed down continuously for it to work on the brakes not to be applied), but
long distance diesels have a vigilance button which must be pressed every sixty
seconds to keep the brakes off. It's a good thing that brakes are applied
automatically because it is commonly believed that old hands can press this
button even when sleeping. I've observed the automaton-like way that drivers
press this button and I have no doubt that it happens. So I remain surprised
that BR didn't seem to believe that train crew vigilance could be a problem.

Mind you, it all seems very safe compared to Canada where express passenger
trains are managed using CTC and walkie-talkie radios. I've seen passenger
trains following one another, separated by a few hundred yards, and relying
purely on the vision of the driver in the second train. Authorized by radio, no
hardware protection.
                                          -David Skillicorn

Re: BMW Heading Control System

Tue, 11 Sep 90 09:57:21 EDT
Michael Snyder's message about the Road Runner problem, i.e., that the system
may not be able to cope with roads that go into mountains, can be a serious
one.  If people rely on the system enough that they tend to take their hands of
the wheel and stop watching the road, then they could get in big trouble if
they suddenly come to a construction site.  Especially if they are moving along
quickly in their BMW :-)

In other words, if the system is good enough to give people confidence, but is
not good enough to deal with all possible situations, then it is risky.

Alex L. Bangs, Oak Ridge National Laboratory/CESAR, Autonomous Robotic Systems

Re: Robustness of RISC architectures (Minow, RISKS-10.35)

Andy Glew <>
Wed, 12 Sep 90 10:57:51 CDT
>Last August, a note posted to showed how user-mode
>code can crash a RISC (reduced instruction set) architecture machine.
>[...]  several RISC architectures could be crashed, but none of the CISC
>(complex...) architectures that were tried.

Let's dispose of the supposition that this is a RISC/CISC issue.  Arne
Helme <> reposted "crashme" to the comp.arch newsgroup,
and obtained a flurry of reports:

80386, '386 protected mode unix => system crash      Peter da Silva
80386, '286 protected mode unix => system crash      Peter da Silva
DG Aviion (88K),  DG/UX 4.30    => no crash              ?
decsystem 5200 Ultrix V3.1A     => crash         Arne Helme
sun 4 SPARC station sunOS4.03c  => crash         Arne Helme
MIPS R[236]000 RISC/os 4.50     => no crash              Charlie Price
Sun-3/50 SunOS 4.1      => unkillable cpu-bound process
                                                        Andrew Taylor

Obviously, some RISCs crashed, some CISCs crashed, some RISCs
survived, some CISCs survived.

The problem most likely is an OS bug. Chris Torek remembers the discussion as
follows: "on all the machines that crashed *except one*, it was a bug in the OS
and not in the chip.  The one exception?  A CISC."

Re: Robustness of RISC architectures (Minow, RISKS-10.35)

Dave Sill, Oak Ridge National Laboratory <de5@de5.CTD.ORNL.GOV>
Tue, 11 Sep 90 13:14:55 GMT
>... One person, commenting on
>this, noted that one of the ways to speed up RISC architectures is to allow
>certain (possible) instruction sequences to have undefined behavior, and to
>let that behavior include "wedging" the machine.  However, CISC architecture
>specifications make sure that every possible instruction (i.e., every pattern
>of bits that can be loaded into the instruction register) returns the machine
>to a known -- viable -- state.

One person said that, but is it true?  I find it hard to believe that excluding
undefined behavior would necessarily exact a performance penalty.  Further,
another person in that same discussion said that the types of bugs causing the
RISC machines to crash were typical of early hardware bugs in CISC machines
too, and that the reason the RISC machines were crashing was because the simply
weren't as thoroughly debugged as the CISC machines.

I don't know who's right, but both explanations seem equally plausable.  It
seems premature to lose sleep over RISC robustness at this point.

Dave Sill (, Martin Marietta Energy Systems, Workstation Support

Robustness of RISC architectures

Andrew Taylor <>
Tue, 11 Sep 90 15:50:05 +1000
I don't believe this is a RISC versus CISC issue. The "execute-random-data"
program found an *OS* bug in both our (RISC) MIPS boxes and our CISC Sun 3/50s.

On the MIPS the behaviour of some instruction in the conditional branch format
is undefined. If there is an illegal FP instruction in the delay slot of such
an undefined branch instruction. The FP instruction traps to the OS which
determines the FP instruction is in a delay-slot and tries to calculate
the branch destination. The OS routine called to do this detects the branch
instruction is undefined but does the wrong thing, it calls "panic" halting
the machine. Trivial to fix.

On our SUN 3/50s (running SunOS 4.1) some random code sequences result
in a cpu-bound process which can not be killed. Rebooting is the only solution.

Its possible that OS bugs are more prevalent on RISCs because their OSs are


Re: Robustness of RISC architectures

Tue, 11 Sep 90 12:28:09 EDT
In fairness, it should be noted that most of these crashes appear to have
been the result of *software* problems:  the operating system was not
prepared to cope with this bizarre situation when the hardware noticed it.

>... one of the ways to speed up RISC architectures is to allow
>certain (possible) instruction sequences to have undefined behavior, and to
>let that behavior include "wedging" the machine...

Very few RISC designers (none that I know of, in fact) are that stupid.
Yes, RISC architectures often state that the results of certain sequences
are undefined, but wedging the machine is *not* considered a legitimate
result.  (Not in a machine meant to support multi-user systems, at any
rate; the rules are different for some of the more specialized processors.)
Claims to the contrary are the result of ignorance or ulterior motives.

This is simply a question of proper design, not of RISC vs CISC.  Certain
early and buggy releases of a certain CISC processor were notorious for
bugs in the protection system, e.g. circumstances in which a memory
fetch would be done with "system" permissions when it was initiated
from "user" state, or vice-versa.  This was mostly a result of the great
complexity of the processor and its interactions with memory management;
a similar problem would have been rather less likely on a RISC machine.
So it cuts both ways.

>... CISC architecture
>specifications make sure that every possible instruction (i.e., every pattern
>of bits that can be loaded into the instruction register) returns the machine
>to a known -- viable -- state.

It is necessary to understand that "known" and "viable" are two different
criteria, analogous to the frequently-mentioned-on-Risks distinction between
correct functioning and safe functioning.  It is not necessary that the result
of violating the rules be precisely defined and the same for all variants of a
particular architecture; what is required is merely that none of the possible
results endanger system integrity.

                          Henry Spencer at U of Toronto Zoology    utzoo!henry

Re: Robustness of RISC architectures (Minow, RISKS-10.35)

Peter Holzer <hp@vmars.UUCP>
11 Sep 90 16:56:22 GMT
This was shown to be a OS bug on the DECstations (The OS could not correctly
resume a process that trapped just after a delayed branch and instead of
killing the process panicked). One person also reported that his 386 (a CISC
processor) could be crashed with the same program (He did not say which
UNIX he used, however. My 386 running 386/ix did not crash).

And talking of VAXes. You can crash a VAX under ULTRIX by loading the frame
pointer with a negative value and then trapping into the OS.

Peter J. Holzer, Technische Universitaet Wien

Re: Robustness of RISC architectures

Robert Cooper <>
Tue, 11 Sep 90 16:08:59 EDT
Many RISC architectures have a notion of a well-defined instruction
stream, and undefined instruction sequences may exhibit undefined
behaviour. Whether this is a real RISK, as usual, depends on a host
of other factors and assumptions relating to the total system of which the
RISC processor is only a part. Here are a few that come to mind:

 o You are not supposed to write (much) assembly code on a RISC. Therefore
   if the compiler is "safe" (i.e. never generates illegal instruction
   sequences) , and object code is protected read-only, executing undefined
   instruction sequences is unlikely. Note that a "safe" compiler need
   not be correct. This idea is not new: these assumptions were necessary
   for the B5000/B6000 series of Burroughs computers of the '60s which had
   unsafe user-mode object code but relied on certified compilers.

 o If the processor is used in a single user application (e.g. an
   embedded application) then there is little difference between just the
   application program failing and the whole processor failing. Both
   may result in byzantine failures for instance.

 o One can question the complexity needed for operating system and compiler
   software as a result of the RISC approach. OS code must perform much
   more work on traps and interrupts than on a typical CISC machine,
   and compiler optimizations are required to realize most of the
   performance benefits of RISC. Clearly these are not impossible
   requirements but my experience suggests that it takes several years
   *after* a RISC machine is introduced for the OS and compiler software
   to become robust.

A particularly bad scenario could be a multi-user university computer that is
used for a student compiler writing course and for payroll!
                                                             -- Robert Cooper

Re: Robustness of RISC architectures

Dik T. Winter <>
11 Sep 90 23:46:17 GMT
Martin Minow writes about a program consisting of random bytes:
 >                                                         Further discussion
 > showed that several RISC architectures could be crashed, but none of the
 > CISC (complex...) architectures that were tried.
There was more than one report of a CISC machine that crashed.  The blame
was lain by some at the (possibly undefined) behaviour of some instruction
sequences of RISC machines, but no proof was given.  It is much more likely
that the machines crash because of bugs in the operating system.  (I know
at least one sequence of bytes that will crash a Sun 4 in some situations,
but I also know that it is due to an OS bug.)

The risk is obvious, one part of the system gets the blame, while nobody
looks at the remainder of the system.

dik t. winter, cwi, amsterdam, nederland

   [There seemed to be enough novel in each of the preceding 7 messages
   that they are all included.  I hope you are not all suffering from the
   bRISC fRISC.     RISKS OF RISCs, I guess...   PGN]

Re: Computers and Safety (RISKS-10.34)

Peter Holzer <hp@vmars.UUCP>
11 Sep 90 16:11:29 GMT
Robert Trebor Woodhead <> writes:

>J.G. Mainwaring discourses about GOTO's and the infamous C BREAK
>(as in "Here is where your program will BREAK!")

>It has long been my opinion (which as we all know, carries the force
>of law in several of the smaller West African countries.. ;^) that the
>EXIT() command pioneered in UCSD PASCAL was an ideal compromise.

>EXIT(a) exited you from enclosing procedure "a", which made it most
>convenient for getting out of incredibly convoluted nested structures
>without making them hugely more convoluted.  It was the equivalent of
>a restricted GOTO to the end of the current procedure, with the extra
>ability to exit any enclosing procedure (even PROGRAM, the whole
>kit-n-kaboodle).  It gave you the the same abilities as 90% of GOTO
>use, but you always knew exactly what it was going to do, and thus
>it was much less dangerous than BREAK.

I do know what break does: It gets me out of the enclosing switch statement
or loop. This is much less powerfull then EXIT, which does not just leave
this procedure (like C return) but eventually other procedures as well, which
is more than you can do with goto in C (You would have to use setjmp/longjump)
to do this.

Comparing EXIT with break does not make sense. It is a generalized return
(Handy with those nested procedures you have in Pascal).

The danger of break is not that it leaves the switch or loop, but that you
can leave it out were it would belong:

switch (a) {
case A:
    /* code */
case B:
    /* more code */
    /* even more code   */

Now should there be a break just before 'case B:' ? You have to understand
the algorithm to answer the question.

This can happen with EXIT just as easily.

Peter J. Holzer, Technische Universitaet Wien

Re: Software doesn't wear out?

11 Sep 90 08:23:00 PDT
Software doesn't wear out?  Doesn't that depend on your definitions?

Example: I use a program, duly protected by both copyright, and trade secret
(of portions not disclosed in the copyright process).  I use that program
under terms of a valid contract.  The program quits working some first of a
month, because the contract has expired.  I order up a new copy.  When it
comes, it is *not* necessarily backwards compatible with my extant data files.

Now, is my program "broken" [worn out] or not?  It may function well by the
*current* definitions of the vendor; but it does not get my work done any
longer.  (At least, not until I "make the problem fit the new too.")
That sounds like a car that won't start, or run; but no, the trouble
is *not* covered by the 5/50 warranty either.  Sorry.  (Or, perhaps better
analogy, the car won't run; and the necessary repair part is no longer made.)

Apparently, "failure" has varied appearances, in the eye of the designer, (and
perhaps the programmer), the vendor, and the user.

Re: Software bugs "stay fixed" (RISKS-10.33)

peter da silva <>
Sat Sep 8 11:15:29 1990
>      extraneous code he doesn't understand, doesn't see how it could work, or
>      whatever and in an attempt to clean up the program, deletes or changes
>      it.  This turns out to be programmer A's bug fix, and the old bug is
>      reintroduced.

In this situation it seems likely that Programmer A merely covered up or made
allowances for the bug. A real bug fix would have redesigned the code so the
condition that equired the obscure code didn't occur. The original comment is
that fixed bugs stay fixed. Patched bugs can (and often do) resurface.

Peter da Silva    +1 713 274 5180.

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