Regarding scatter/gather, Peter Brewer noted:

> Sounds an awful lot like Mmap or mmap() to me? And then use the memory
> mapped file like a chunk of malloc'ed memory? Isn't this supported in
> BSD thru this mechanism? Or am I looking at a higher level than you are?

The memory mapping facility of the PMMU is available to the CPU when it
accesses memory, yes, but not to the IO system when it goes to memory
via DMA. Therefore, BSD has to translate the memory addresses to real 
addresses to give to the I/O.  That is why scatter/gather is useful - 
to mirror the effects of the PMMU for an I/O operation that crosses 
page boundaries.
______________________________________________________________________
  Steven R. Weiss  |  srw@cyberspace.org   | If you think education is
Computer Scientist | (313) 995-8250 x 5632 | expensive, try ignorance.