15.1.5. Dormant mode

Dormant mode is an implementation of a power-down state. In dormant mode, the core logic is powered down, but the cache RAMs are left powered up. Often the RAMs are held in a low-power retention state where they hold their contents but are not otherwise functional. This provides a far faster restart than complete shutdown, as live data and code persists in the caches. Again, in a multi-core system, individual cores can be placed in dormant mode.

In a multi-core system that permits individual cores within the cluster to go into dormant mode, there is no scope for maintaining coherency while the core has its power removed. Such cores must therefore first isolate themselves from the coherence domain. They clean all dirty data before doing this and are typically woken up using another core signaling the external logic to re-apply power.

The woken core must then restore the original core state before rejoining the coherency domain. Because the memory state might have changed while the core was in dormant mode, it might have to invalidate the caches anyway. Dormant mode is therefore much more likely to be useful in a single core environment rather than in a cluster. This is because of the additional expense of leaving and rejoining the coherency domain. In a cluster, dormant mode is typically likely to be used only by the last core when the other cores have already been shut down.

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