We analyze the performance of several copying garbage collection algorithms in a large address space offered by modern architectures. In particular, we describe the design and implementation of the RealOF garbage collector, an algorithm explicitly designed to exploit the features of 64-bit environments. This collector maintains a correspondence between object age and object placement in the address space of the heap. It allocates and copies objects within designated regions of memory called zones and performs garbage collection incrementally by collecting one or more ranges of memory called windows. The address-ordered heap allows us to use the same inexpensive write barrier that works for traditional generational collectors. We show that for server applications this algorithm improves throughput and reduces heap size requirements over the best-throughput generational copying algorithms such as the Appel-style generational collector.