mptable_search() base-memory branch, ebda_addr == 0 (verbatim arithmetic, mptable.c:284/368):
  base_memory=0    -> target=0xfffffc00  scan [fffffc00..00000000]: WRAPPED past 4GB, top-of-4GB MMIO hole, not base memory
  base_memory=1    -> target=0x00000000  KERNEL PANIC -> assertion "target != 0" failed in mptable_search_sig at mptable.c:368
  base_memory=2    -> target=0x00000400  scan [00000400..00000800]: sane region (sig not found: 0)
  base_memory=16   -> target=0x00003c00  scan [00003c00..00004000]: sane region (sig not found: 0)
  base_memory=511  -> target=0x0007f800  scan [0007f800..0007fc00]: sane region (sig not found: 0)
  base_memory=638  -> target=0x0009f400  scan [0009f400..0009f800]: sane region (sig not found: 0)
  base_memory=639  -> target=0x0009f800  scan [0009f800..0009fc00]: sane region (sig not found: 0)

result: base_memory==1 -> deterministic boot-time KKASSERT panic; base_memory==0 -> wrong-region scan at 0xFFFFFC00 (crosses the 4GB wrap); panics=1