A parallel hardware-based multithreaded processor is described. The processor includes a general purpose processor that coordinates system functions and a plurality of microengines that support multiple hardware threads or contexts (THREAD—3 . . . THREAD—0). The processor maintains execution threads (THREAD—3 . . . THREAD—0) access a register set organized into a plurality of relatively addressable windows of registers that are relatively addressable per thread (THREAD—3 . . . THREAD—0).