ISSCC 2005: The CELL Microprocessor

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Element Interconnect Bus

The element interconnect bus is the on chip interconnect that ties together all of the processing, memory, and I/O elements on the CELL processor. The EIB is implemented as a set of four concentric rings that is routed through portions of the SPE, where each ring is a 128 bit wide interconnect. To reduce coupling noises, the wires are arranged in groups of four and interleaved with ground and power shields. To further reduce coupling noises, the direction of data flow alternates between each adjacent ring pair. Data travels on the EIB through staged buffer/repeaters at the boundaries of each SPE. That is, data is driven by one set of staged buffer and latched by the buffer at the next stage every clock cycle. Data moving from one SPE through other SPE’s requires the use of repeaters in the intermediary SPE’s for the duration of the transfer. Independently from the buffer/repeater elements, separate data on/off ramps exist in the BIU of the SPE, as data targeted for the LS unit of a given SPE can be off-loaded at the BIU. Similarly, outgoing data can be placed onto the EIB by the BIU.

Figure 8 – Counter rotational rings of the EIB – 4 SPE’s shown

The design of the EIB is specifically geared toward the scalability of the CELL processor. That is, signal path lengths on the EIB do not change regardless of the number of SPE’s in a given CELL processor configuration. Since the data travels no more than the width of one SPE, more SPE’s on a given CELL processor simply means that the data transport latency increases by the number of additional hops through those SPE’s. Data transfer through the EIB is controlled by the EIB controller, and the EIB controller works with the DMA engine and the channel controllers to reserve the buffers drivers for certain number of cycles for each data transfer request. The data transfer algorithm works by reserving channel capacity for each data transfer, thus providing support for real time applications. Finally, the design and implementation of the EIB has a curious side effect in that it limits the current version of the CELL processor to expand only along the horizontal axis. Thus, the EIB enables the CELL processor to be highly configurable and SPE’s can be quickly and easily added or removed along the horizontal axis, and the maximum number of SPE’s that can be added is set by the maximum width of the chip allowable by the reticule size of the fabrication equipment.

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