> On Mar 23, 2016, at 7:32 PM, Ron Natalie <ron at ronnatalie.com> wrote:
> 
> 
>> Closest I've ever been murdered was when I "accidentally" filled the local
>> 11/70 with an uninterruptible instruction sequence."
> 
> SPL instruction.   The PDP-11 was odd that while SPL was a "privileged"
> instruction, rather that trapping if you did it in user mode, it just
> "ignored" it.
> Well, what it ignored was the actual change of the processor level.   What
> it still implemented was the side effect was that interrupts were locked out
> until the next instruction fetch.
> If you filled your instruction space up with SPLs you could lock up the
> computer so that even the HALT key didn't work (you had to do a bus RESET).

This is of course a fairly well known bug in the pdp11. I wonder if this was fixable with a hardware mod, like the “there’s this NAND gate here” sort of fix to the MTPx/MFPx instructions or if it required a change to the microcode, that presumably DEC wasn’t interested in.

> [snip]