CA> If there are PC OS that can 'stop' ASM programs from toggling
 CA> PIAs - which ones are these?

OS/2 is the first one that comes to mind...

 CA> If the ASM is allowed to
 CA> execute how (other than in hardware) can the OS intercept the
 CA> execution of ML instruction code?  Interrupt tables can be
 CA> re-written from within ASM?  Interesting!

the OS checks what each program is doing before allowing it to do it... WRT
interrupt tables being written in ASM, that's where we started with these
things oh so many years ago... i used to do things of similar nature with
the peeks and pokes of BASIC but that was before i discovered ASM and
PASCAL OB-)

)\/(ark