1. A method for managing transitions between a first number of threads and a second number of threads executing a simultaneous multi-threaded processor, comprising: receiving an instruction indicating a thread mode switch; setting thread enable signals indicating an enable state of multiple threads, wherein said second number of threads is specified for further execution; and reallocating per-thread fixed entries within a rename resource of said processor in conformity with said thread enable signals, by directing an action at a mapper associated with said rename resource that maps said per-thread fixed entries.