[pups] Issues of AUUGN

Dave Horsfall dave at horsfall.org
Wed Oct 4 17:00:16 AEST 2006

[ Meant to go to list, but sent to DMR only by mistake. ]

On Wed, 4 Oct 2006 dmr at plan9.bell-labs.com wrote:

> > It contains the famous Thrust Meter, a few papers by Yours Truly, and 
> > I think it has the short assembly program that would bring a PDP-11/70 
> > to its knees (the infamous "SPL" firmware bug).
> Was this the feature (not really a bug; it's in the manual) that SPL 
> suppressed interrupts for one instruction after the SPL?  I suppose it 
> was indeed a bug that this happened even in user mode where SPL was 
> intended to be a no-op.

Yep, that's the one.  I regard it as a bug because it indeed happened in 
user mode...

> I remember trying this.  It depends on completely filling memory with 
> SPLs, which I could not figure out how to do using an instruction 
> sequence.  However, putting a bunch of SPLs into a file and reading it 
> in over the program did the job.

There was a clever assembly program that did it; it relied upon the 
instruction counter wrapping around (I can't remember in which direction, 
or whether it first relocated itself).  Anyone, it managed to fill memory 
with SPLs, so the next instruction after overwriting its last instruction 
was SPL, and for the foreseeable future after that...

If I find the article I'll post it here; I don't think there are too many 
11/70s still in public operation.

> It was a bit hard to break out of--the halt switch didn't work. At first 
> I thought that power-off was the only solution, but it turned out that 
> holding down both reset and halt simultaneously did the job.

I'll remember that, should I ever see an emulator :-)  I still remember 
Ian Johnstone cursing me...

-- Dave

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