+Some controllers have limitations with the way they deal with >16MB of memory,
+due to the fact that the ISA bus only has a DMA address space of 24 bits.
+If you do your arithmetic, you'll see that this makes it impossible to do
+direct DMA to any address >16MB. This limitation is even true of some
+EISA controllers (which are normally 32 bit) when they're configured to
+emulate an ISA card, which they then do in *all* respects. This problem
+is avoided entirely by IDE controllers (which do not use DMA), true EISA
+controllers (like the UltraStor or Adaptec 1742A) and most VLB (local bus)
+controllers. In these cases, the system will use "bounce buffers" to
+to talk to the controller, and is generally the default.
+