🚀 go-pugleaf

In article <11...@csli.Stanford.EDU>, po...@csli.Stanford.EDU (Bill
Poser) writes:
< [In reference to story about code that jumps to itself.]
< 
< A friend of mine told me about how he tried this on an old discrete
< component machine. He tried it and the program merrily looped away.
< Eventually he got tired of this and tried to halt the program. No luck.
< During such jumps, the machine disabled interrupts. So he decided to
< halt the machine. That just sent an interrupt, so it didn't work either.
< He thought cutting the power, but was worried that an abrupt power-down
< would cause a melt-down. Finally, he went and got one of the technicians.
< He thought for a minute and asked at what location the instruction had
< been loaded. Then he opened up the machine, found the logic for that
< location (remember, this is a discrete component machine - you can
< locate individual transistors) and shorted the collector to the
< emitter with a screwdriver. This is what is known as "programming down
< on the bare metal".

On a Data General NOVA (and on the Eclipse also, I believe) you
could have an instruction do an infinite indirect memory reference,
where the contents of the memory was the address of that location.

Interrupts are only acknowledged *between* instructions, and the STOP
switch (toggle switch) is only effective after an instruction is FINISHED,
and since the instruction isn't finished until it reaches the end of the
indirect chain (infinite).... you can't stop the CPU by pressing STOP!

Actually, you can press *RESET*, which DOES allow stopping in the middle
of an instruction (FORTUNATELY!)

_____________________________________
Brian V. Smith    (bvs...@lbl.gov)
Lawrence Berkeley Laboratory
I don't speak for LBL, these non-opinions are all mine.