I’ve been kicking around the idea of using the on-board FPGA in my Yellowstone Universal Disk Controller as a general-purpose Apple II coprocessor. With an FPGA on a peripheral card connected to the 6502 address and data buses, I can’t make the CPU run any faster, but maybe I can make calculations faster by offloading them to some type of coprocessor that’s constructed within the FPGA. As a simple example, I could implement a fast PicoBlaze soft-CPU or something similar, and provide a way for the 6502 to run/halt the PicoBlaze and read/write from its memory space. The 6502 could upload a short PicoBlaze program and its required data, wait for the PicoBlaze to finish running, and then read the results back out.
The existing Yellowstone hardware can already do this. It would just be a matter of firmware development.
What would this be useful for? I’m not sure. But the Yellowstone firmware already implements a very simple taste of this idea, because I needed it in order to achieve the disk timing requirements. It’s not a coprocessor, but it’s an extension that shaves a few clock cycles where they’re most valuable. There’s a specific magic address in the Yellowstone card’s address space, and when the 6502 reads from that address, Yellowstone performs automatic GCR decoding of the most recent disk byte.
I had a crazy idea to use this like a classic computer hardware emulator, except that the Apple II wouldn’t be emulating an older computer but a newer one. The 6502 program could just sit in a tight loop reading bytes from the FPGA coprocessor and writing them into the Apple II frame buffer, so whatever appeared on the screen would be the result of the coprocessor program. It would still be limited by the Apple’s physical screen resolution and color palette, but panning around a larger virtual screen should be possible. And conveniently I’ve already implemented a Macintosh Plus FPGA core, so in theory we could see MacOS running on an Apple II? Or more realistically, this scheme could turn an Apple II into something like a Commodore 64, or a 1980s video game console. Probably impractical for any serious use, but at least it would make interesting discussion for r/vintagecomputing!