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The iCE40 only has ~12k of useable RAM once the other blocks are instantiated.
This isn't enough for us to cache and erase a full 64k block, but it is sufficient
to cache / erase a 4k subsector.  It is currently unknown if hostboot will demand
a write window of more than 12k -- if so, it will need modification to stay within
the 12k limit of the small FPGA device.