Construct the free queue(s) in descending order (by physical address) so that the first 16MB of physical memory is allocated last rather than first.  On large-memory machines, this avoids the exhaustion of low physical memory before isa_dmainit has run.
Construct the free queue(s) in descending order (by physical
address) so that the first 16MB of physical memory is allocated
last rather than first.  On large-memory machines, this avoids
the exhaustion of low physical memory before isa_dmainit has run.

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