[Cryptech Tech] IO interface (was Re: Incremental digest outputs)

[Rob Austein]

The main advantage I can see to the memory mapped approach is that it
makes it easy to do things like FIFOs, since they're basically just
ring buffers (not necessarily of bytes, might be a very small number
of ring buffer slots, each holding a complete block) with a few
control registers.  Adds a bit of complexity but might matter for
throughput.  For that matter, we could encode command+data as packets
in a FIFO if we want semantics like a network protocol.

I've been assuming that knowledge of the grotty details on the C side
will be restricted to a small number of method routines in any case.