We've designed and implemented a copying garbage-collection algorithm that is efficient, real-time, concurrent, runs on commercial uniprocessors and shared-memory multiprocessors, and requires no change to compilers. The algorithm uses standard virtual-memory hardware to detect references to "from space" objects and to synchronize the collector and mutator threads. We've implemented and measured a prototype running on SRC's 5-processor Firefly. It will be straightforward to merge our techniques with generational collection. An incremental, non-concurrent version could be implemented easily on many versions of Unix.