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The “high-end” water-cooled processors in the IBM Enterprise System/9000™ product family use a CPU organization and cache structure which depart significantly from previous designs. The CPU organization includes multiple execution elements which execute instructions out of sequence, and uses a new virtual register management algorithm to control them. It also contains a branch history table to remember recent branches and their target addresses so that instruction fetching and decoding can be directed more accurately. These models also use a two-level cache structure which provides a level 1 cache associated with each processor and a level 2 cache associated with central storage. The level 1 cache uses a store-through organization, and is split into two separate caches, one used for instruction fetching and the other for operand references. The level 2 cache uses a store-in method to handle stores.
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