Ah yeah. Cool.

I did something similar in a recent Z80 based project, shouldn't be too
difficult to do that on m68k.

Only trouble I can see arising from this is enabling and disabling
interrupts is privileged, and from the 68010 onwards you can't even see the
status register to know if interrupts are on/off outside of supervisor
mode, so you wouldn't be able to execute this code from any application
running in user mode.

I suppose that is why it's better trapped as an invalid instruction and
emulated lower down?


On Fri, Apr 24, 2020, 11:20 Rich Felker <dalias@libc.org> wrote:

> On Fri, Apr 24, 2020 at 11:14:38AM +1000, Tom Storey wrote:
> > Sorry to ask what sounds like a dumb question, but is
> >
> > cli;nonatomic_cas;sti
> >
> > basically "disable interrupts, do something equivalent to cas, re-enable
> > interrupts"?
>
> Yeah. Basically:
>
> static inline int a_cas(volatile int *p, int t, int s)
> {
>         __asm__ __volatile__("cli" : : : "memory");
>         if (*p==t) *p=s; else t=*p;
>         __asm__ __volatile__("sti" : : : "memory");
>         return t;
> }
>
> where cli and sti are replaced with the m68k instructions for those
> operations. Of course if you might have code that already has
> interrupts disabled somewhere calling libc stuff, you may need to make
> it save the old interrupt state and restore it rather than just sti.
> See the sh2 version (runtime selected so it's in __sh_cas_imask in
> src/thread/sh/atomics.s and atomic_arch.h just calls the
> runtime-selected function) for an example of code doing this
>
> Rich
>