5.5 Secondary Cache Tag

SCTag(25:4), Physical Tag


The minimum secondary cache size is 512 Kbytes (256 Kbytes per way), so a minimum of 18 bits are required to index a data byte within a selected way. Since the processor supports 40 physical bits, a maximum of 22 bits are required for the physical tag:

40 physical address bits - 18 minimum required = 22

Consequently, the processor supplies the 22 physical address bits, PA(39:18), on SCTag(25:4) for the physical tag.

When the secondary cache is larger than the minimum size, the secondary cache tag array must still maintain the full physical tag supplied by the processor, even though some bits are redundant.




Copyright 1995, MIPS Technologies, Inc. -- 29 JAN 96


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