Sun's unreleased 16-core processor called "Rock".
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This design enables resource sharing among cores within a cluster, thus reducing the area requirements. All cores in a cluster share an instruction fetch unit (IFU) that includes the level-one (L1) instruction cache. We decided that all four cores in a cluster should share the IFU because it is relatively simple to fetch a large number of instructions per cycle. Thus, four cores can share one IFU in a round-robin fashion while maintaining full fetch bandwidth. Furthermore, the shared instruction cache enables constructive sharing of common code, as is encountered in shared libraries and operating system routines.
Each core cluster contains two L1 data caches (D cache) and two floating-point units (FGU), each of which is shared by a pair of cores, these structures are relatively large, so sharing them provides significant area savings.
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Shailender Chaudhry, Robert Cypher, Magnus Ekman, Martin Karlsson, Anders Landin, Sherman Yip, Hakan Zeffer, Marc Tremblay - Sun Microsystems
ROCK, SUN’S THIRD-GENERATION CHIP-MULTITHREADING PROCESSOR, CONTAINS 16 HIGH-PERFORMANCE CORES