The "Xena" co-processor is just an off-the-shelf XMOS XCore microcontroller interfaced to the CPU somehow. I've got a development board for one of their chips kicking around somewhere, though obviously without the CPU interface. It's only "software defined silicon" in the sense that it gives you very powerful ways of implementing custom I/O; it's not that fast at any compute tasks compared to a modern PC. There also doesn't appear to be anyone who's done anything with this, and the official prototyping board for connecting stuff to it is very expensive too: http://amigakit.leamancomputing.com/catalog/product_info.php... (several times the price of a standalone XCore board). Oh, and apparently the toolchain required to compile code for "Xena" doesn't even run on Amiga or PowerPC, but that's not terribly surprising.