-Variable-Length Cache Lines

11.3 R4000 Cache Description

Variable-Length Cache Lines

A cache line is the smallest unit of information that can be fetched from the cache, and that is represented by a single tag.*1 A primary cache line can be either 4 or 8 words in length; a secondary cache line can be either 4, 8, 16, or 32 words in length. Primary cache line length is set in the Config register; see Chapter 4 for more information. Secondary cache line length is set at boot time through the boot-mode bits, as described in Chapter 9.

Upon a cache miss in both primary and secondary caches, the missing secondary cache line is loaded first from memory into the secondary cache, whereupon the appropriate subset of the secondary cache line is loaded into the primary cache.

The primary cache line length can never be longer than that of the secondary cache; it must always be less than or equal to the secondary cache line length. This means the secondary cache cannot have a 4-word line length while the primary cache has an 8-word line length.

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