ISSCC 2005: The CELL Microprocessor

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The POWERPC Processing Element

Neither microarchitectural details nor the performance characteristics of the POWERPC Processing Element were disclosed by IBM during ISSCC 2005. However, what is known is that the PPE processor core is a new core that is fully compliant with the POWERPC instruction set, the VMX instruction set extension inclusive. Additionally, the PPE core is described as a two issue, in-order, 64 bit processor that supports 2 way SMT. The L1 cache sizes of the PPE is reported to be 32KB each, and the unified L2 cache is 512 KB in size. Furthermore, the lineage of the PPE can be traced to a research project commissioned by IBM to examine high speed processor design with aggressive circuit implementations. The results of this research project were published by IBM first in the Journal of Solid State Circuits (JSSC) in 1998, then again in ISSCC 2000.

The paper published in JSSC in 1998 described a processor implementation that supported a subset of the POWERPC instruction set, and the paper published in ISSCC 2000 described a processor that supported the complete POWERPC instruction set and operated at 1 GHz on a 0.25µm process technology. The microarchitecture of the research processor was disclosed in some detail in the ISSCC 2000 paper. However, that processor was a single issue processor whose design goal was to reach high operating frequency by limiting pipestage delay to 13 FO4, and power consumption limitations were not considered. For the PPE, several major changes in the design goal dictated changes in the microarchitecture from the research processor disclosed at ISSCC in 2000. Firstly, to further increase frequency, the per stage circuit delay design target was lowered from 13 FO4 to 11 FO4. Secondly, limiting power consumption and minimize leakage current were added as high priority design goals for the PPE. Collectively, these changes limited the per stage logic depth, and the pipeline was lengthened as a result. The addition of SMT and the two issue design goal completed the metamorphosis of the research processor to the PPE. The result is a processing core that operates at a high frequency with relatively low power consumption, and perhaps relatively poorer scalar performance compared to the beefy POWER5 processor core.

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