FETCH DIRECTOR EMPLOYING BARREL-INCREMENTER-BASED ROUND-ROBIN APPARATUS FOR USE IN MULTITHREADING MICROPROCESSOR

A fetch director in a multithreaded microprocessor that concurrently executes instructions of N threads is disclosed. The N threads request to fetch instructions from an instruction cache. In a given selection cycle, some of the threads may not be requesting to fetch instructions. The fetch director includes a circuit for selecting one of threads in a round-robin fashion to provide its fetch address to the instruction cache. The circuit adds a first addend to a 1-bit left-rotated version of a second addend to generate a sum and a carry-out bit. The circuit includes the carry-out bit as a carry-in bit of the add to generate the sum. The sum is ANDed with the inverse of the first addend to generate a 1-hot vector indicating which of the threads is selected next. The first addend is an N-bit vector where each bit is false if the corresponding thread is requesting to fetch instructions from the instruction cache. The second addend is a 1-hot vector indicating the last selected thread. In one embodiment threads with an empty instruction buffer are selected at highest priority; a last dispatched but not fetched thread at middle priority; all other threads at lowest priority. The threads are selected round-robin within the highest and lowest priorities.