Cell Inside, the Future of Processor Architecture - Synergistic Processing Element (SPE)
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Even though the PPE is a powerful processor in its own right, the eight Synergistic Processing Elements (SPEs) are the real workers. After the PPE has hacked data into little chunks and handed the SPEs, this is where 5 + 5 becomes 10.
In that demo Toshiba presented of a Cell processor decoding 48 streams of video, only 6 of the SPEs were used for data decoding. Another was used for scaling the screen, and the last can be used for other tasks. The SPEs can function in unison, like decoding streams together, or they can be dedicated to completely separate jobs.
The SPEs don’t have any cache, but they do have four 64 KB arrays of private memory, or Load Store (LS) units. This gives them a total of 256 KB of private memory. Current processors automate the use of the memory for tasks like data fetch and branch prediction, but this adds complexity and cost to hardware. The SPEs’ memory is not cache, but instead operates as flexible storage for the little processor. In Cell, programs must manage how this memory is used, and this potentially makes these functions far more efficient and specialized for the programs.
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