9.2.4. Sub-Regions

The eight Sub-Region Disable (SRD) bits of the Region Attribute and Size Register divide a region into eight equal-sized units based on the region size. This enables selectively disabling some of the 1/8th sub-regions. The least significant bit affects the first 1/8th sub-region, and the most significant bits affects the last 1/8th sub-region. A disabled sub-region enables any other region overlapping that range to be matched instead. If no other region overlaps the sub-region, the default behavior is used, no match – a fault. Sub-regions cannot be used with the three smallest regions of size: 32, 64, and 128. If these sub-regions are used, the results are Unpredictable.

Example of SRD use

Two regions with the same base address overlap. One region is 64KB, and the other is 512KB. The bottom 64KB of the 512KB region is disabled so that the attributes from the 64KB apply. This is achieved by setting SRD for the 512KB region to b11111110.

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