The first true virtual memory system was that implemented at the University of Manchester to create a one-level storage system as part of the Atlas Computer. It used a paging mechanism to map the virtual addresses available to the programmer on to the real memory that consisted of 16,384 words of primary core memory with an additional 98,304 words of secondary drum memory. The first Atlas was commissioned in 1962 but working prototypes of paging had been developed by 1959. In 1961, the Burroughs Corporation independently released the first commercial computer with virtual memory, the B5000, with segmentation rather than paging.