Caching-Related Changes to the Programming Environment

The 486 made the following changes to the programming environment:

  • The CD (Cache Disable) and the NW (Not Write Through) bits were added to CR0 (see Figure 19-2 on page 434). These two bits provide high-level control of the internal cache(s). Also refer to Table 19-7 on page 455.

    Table 19-7. CR0[CD] and CR0[NW] Control the Cache
    CDNWDescription
    00The cache is fully enabled.
    01Reserved.
    10The cache is enabled but is locked:

    • Lines already cached will remain cached.

    • No new lines will be read into the cache.

    • A memory write hit on a line in the cache will update the cache line and is also written through to memory.

    • Any memory access initiated on the FSB by another entity will be snooped in the cache. ...

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