FPGAs Aren't Processors (Unless You Want Them to Be) FPGA Deep Dive and Use [video] (youtube.com)

…except this Zync FPGAs, it has a processors in it.

I've never understood why the Zync CPU has to communicate with the rest of the FPGA over external busses. Why not connect a set of latches or LUTs to the instruction decoder and data bus, so you can create custom instructions?