On-Chip Cache Added

General

Refer to Figure 18-7 on page 428. The 486 was the first IA32 processor to integrate a cache on the processor die. The cache had the following characteristics:

  • It was a write-through cache (except for one version that had a write back cache).

    - The write-through cache supported two possible states for a line: S (Shared) or I (Invalid), wherein S merely indicated that the line was valid.

    - The write back cache supported four possible states for a line: M (Modified), E (Exclusive), S (Shared), or I (Invalid).

  • It was a unified code/data cache. A collision on the cache due to the receipt of simultaneous access requests from the Execution Unit data and the Instruction Prefetcher results in stalling the Prefetch Unit.

  • There were ...

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