(a) A thread running on an SPE. Each such thread has its own 128 x 128-bit register file, program counter, and MFC Command Queues, and it can communicate with other execution units (or with effective-address memory through the MFC channel interface). (b) A thread scheduled and run on an SPE. A program has one or more SPE threads. Each thread has its own SPU local store (LS), register file, program counter, and MFC command queues.