On Wed, Sep 1, 2021 at 10:07 AM Ron Natalie <ron@ronnatalie.com> wrote:

> I disagree.  TRAP according to the processor handbook was intended to be
> used for what UNIX calls system calls.    EMT was the emulator trap used to
> simulate other operating systems on the same hardware.    Oddly, for some
> reason, all the DEC OSes use EMT instructions for their system calls.
>  This came in handy when JHU ported BasicPlus from RSTS to UNIX.     That
> executable could run fine on UNIX because we caught the few EMT traps that
> mattered to us and simulated them.   The only thing we had to do other than
> that was to add a "nostack()" system call that got rid of the normal
> UNIX-maintained stack starting at the address space (RSTS executables like
> many DEC OSs used a stack that started around 1000).
>

The various RT-11 emulators use variations on this theme as well, some
inside the kernel, some as a signal handler (fast forward 40-odd years and
I'm catching the SIGSEGV traps in executing 16-bit code to implement the
unix system calls)... It's a very useful and elegant trick that's been
oft-repeated.


> Many of the UNIX signals come straight from PDP-11 traps:  SIGFPE, SIGIOT,
> SIGSEGV, SIGBUS, SIGILL, SIGEMT. and those traps invoked those signals.
>

Yes. They seemed to make perfect sense when I encountered them in Unix
after growing up on RSTS/e and RT-11 before my first contact with Unix....


> FPE - floating point exception
> ILL - illegal exception (either unknown opcode or CERTAIN of the
> privileged instructions, others were ignored)
> BUS - fatal unibus timeout trap.   Usually an attempt to access a
> memory/unibus address that doesn't respond, or to do word accesses on odd
> boundaries.
> SEGV - accessing memory not mapped to you
> IOT - the IOT instruction
> BPT - the BPT instruction
> TRAP, EMT - these instructions
>
>