[digitalinfinity]

It can't make that assumption- in a conservative GC, any pointer sized field in an object could be a pointer. So assuming you're on a 32 bit machine, in the above example, the double would look like 2 pointers to the GC. One way of dealing with this is, the GC keeps a set of all the address ranges that belong to its objects (these are the only objects that could be garbage collected). Then, when it sees what could be a pointer, it'll first look up whether this "maybe pointer" actually belongs to it's address range, and only if it does will it add it to the mark stack for further scanning. In terms of how does it know when to stop looking for references, actually, the GC will know the object size for any objects that it has allocated, so it knows when to stop scanning it's own object. For objects that it hasn't given out, it doesn't matter since it won't scan those. And for the stack, it'll just scan the entire stack looking for pointers to it's own objects.