The availability of mass storage devices with access times several orders of magnitude less than rotating-device memories will have varying impacts on computer system architectures. These effects will range from simple changes such as direct substitution for disks, where significant productivity increases can be easily achieved, through more radical changes where both hardware and software are changed in structure to exploit the potential of these new devices. Such changes include architectures for transaction-based computing, real-time computing with nonresident programs, and more flexible structures for time-shared computers.