But the strangest thing about Mr Do's Castle board is the third Z80. This CPU has a tiny program ROM - 256 bytes, and most of them empty. It sits there, doing almost nothing. It communicates with the main CPU using the same method described in the previous article, but it doesn't send anything back.
The interesting thing is what the main CPU sends it. It's not just 9 bytes as with the second CPU. It is 0x200 bytes, and it's the sprite data. So essentially what the third CPU does is sit there, wait for the sprite data from the main CPU, and pass it over to the video hardware.
Why couldn't the main CPU just send the data to the video hardware by itself? That's the difficult question. I haven't an answer yet.