Atomic sixteen-byte memory accesses are provided in a 64-bit system in which eight of the bytes are stored in a 64-bit general-purpose register and eight of the bytes are stored in a 64-bit special-purpose register. A 16-byte load instruction transfers the low eight bytes to an explicitly specified general-purpose register, while the high eight bytes are transferred to the special-purpose register. Likewise, a 16-byte store instruction transfers data from a general-purpose register and the special-purpose register. Also provided is an 8-byte compare conditioning a 16-byte exchange semaphore instruction that can be used to accelerate algorithms that use multiple processors to simultaneously read and update large databases.