Multiprocessor system with multiple concurrent modes of execution

A multiprocessor system comprising

• a plurality of processors adapted to run threads of program code in parallel in accordance with speculative execution;
• and
• each processor of said plurality having a cache, each processor configured to:
• allocate at least one identification number to a thread executing speculatively;
• maintain in the cache, a directory based speculation control responsive to the identification number;
• count instances of use of the identification number being active in the multiprocessor system;
• and
• prevent the identification number from being allocated to a new thread until the counting indicates no instances of use of that ID being active in the system.