A data processing system includes a multithreaded processor to execute a plurality of selected program threads in parallel. A mode-selectable processor is coupled to the multithreaded processor and executes in either a first mode or a second mode. In the first mode program instructions from a single thread are executed. In the second mode, which is selected when the single program thread is inactive, program instructions forming a plurality of borrowed threads are executed. These borrowed threads are taken from a queue of candidate program threads which is managed by the multithreaded processor.