When handwritten code is called from Mercury, the garbage collection scheduler doesn't know anything about the code, so it cannot replace the succip on the stack (if there is one) with the collector's address.

If the handwritten code calls no other code, then this is fine, the scheduler knows it can replace the succip variable and when a proceed() occurs execution will return to mercury code which it knows about.

If handwritten code calls other handwritten code, we have a problem, as succip will be saved on the stack and we don't know where on the stack it is stored. So we use a global variable 'saved_succip' which is succip is saved into. Care must be taken to save saved_succip on the stack so it doesn't get clobbered.
So

	detstackvar(1) = (int) succip;
	
becomes
	detstackvar(1) = (int) saved_succip;
	saved_succip = (int) succip;
	
and, when restoring,
	succip = (int) detstackvar(1);
	
becomes
	succip = saved_succip;
	saved_succip = detstackvar(1);
	
(With appropriate LVALUE_CASTs).

In this way, garbage collection always knows where the succip is stored in handwritten code.

The garbage collection code must check that the current execution is not still in a handwritten predicate - if it is, it must re-schedule (essentially just the same as before).