Upstream vs. Downstream

Any references to the terms “upstream” or “downstream” should be interpreted as follows:

• Upstream. As in “the request is forwarded upstream to the L2 Cache”. It means to the next higher level in the memory hierarchy. The L1 Data Cache represents the lowest level of the memory hierarchy (i.e., the closest to the processor core). In order, the remaining upstream levels are: the L2 Cache, the L3 Cache (if there is one), and system memory.

• Downstream. As in “the data is forwarded downstream to the L1 Data Cache”. It means to the next lower level in the memory hierarchy. System memory represents the highest level of the memory hierarchy (i.e., the furthest from the processor core). In order, the remaining downstream levels are: the L3 Cache If there is one), the L2 Cache, and the L1 Data Cache.