m68k problem and libstdc++

[Nathan Myers]

On Fri, Jan 04, 2002 at 10:58:34AM -0500, Peter Barada wrote:
> 
> >CAS is an indivisible instruction intended to allow synchronization
> >of multiple processors (the sequence above is straight out of one of
> >Motorola's examples).  I don't think there's any way to get that
> >functionality on pre-68020 chips, so you'll have to conditionalize
> >it away somehow.  
> 
> Since the straight 68k and 5x0x don't support CAS, you could:
> 
> #if defined(__mc68000__) || defined(__mcf5200__)
>                         "   add%.l %2,%1;"
>                         "   cmp%.l %0,%3;"
> #else
>                         "   add%.l %2,%1;"
>                         "   cas%.l %0,%1,%3;"
> #endif

This is not safe against interrupts -- i.e., between the add and
the cmp instructions, the counter may be incremented by another
thread.  What we want is a single instruction that will decrement 
the target and set a status bit based on the result.

Unfortunately the function interface is more general than such
an instruction is likely to be, despite that we only use this
function to decrement-by-one.  That means we really need to change 
the abstract interface provided in this file to something 
supportable on more CPUs.

Nathan Myers
ncm at cantrip dot org