MPROTECT_VDB, PROC_VDB</tt>
<DD>
May be defined if the corresponding "virtual dirty bit"
implementation in os_dep.c is usable on this platform.  This
allows incremental/generational garbage collection.
<TT>MPROTECT_VDB</tt> identifies modified pages by
write protecting the heap and catching faults.
<TT>PROC_VDB</tt> uses the /proc primitives to read dirty bits.
<DT><TT>PREFETCH, PREFETCH_FOR_WRITE</tt>
<DD>
The collector uses <TT>PREFETCH</tt>(<I>x</i>) to preload the cache
with *<I>x</i>.
This defaults to 