© 1992 IEEE. The recent appearance of architectures with flat 64-bit virtual addressing opens an opportunity to reconsider the way in which operating systems use virtual address spaces. An operating system called Opal is being built for these wide-address architectures. The key feature of Opal is a single global virtual address space that extends to data on long-term storage and across the network. The case for the use of a single virtual address space is outlined, the model of addressing and protection used in Opal is presented, and some of the problems and opportunities raised by the approach are discussed.