sdspi VS nybbleForth

I have built a lot of open-source C++ tooling for design verification. You can find a lot of my C++ models posted on my Github. Example C++ models include: UART, SPI/DSPI/QSPI Flash, SD-Card (SPI-based interface), VGA Video, Ethernet MDIO, PS/2 mouse, OLED display, SDRAM and more. (I've even simulated PLLs using C++ models ...) I have also written extensively about doing so at ZipCPU.com.

You mean ... sort of like I did in this project? I implemented an SPI based controller, where the controller took care of all the bit-banging for you, but the CPU still needed to issue the commands as appropriate for the protocol?

How about an SD card controller? I know I have a SPI based SD card controller, but the SDIO isn't that much harder. If you look hard enough you can find open source SDIO controllers.

There are also open source versions of many of the pieces you will need. I now use an open source crossbar interconnect for most of my designs. I use AutoFPGA to connect all the pieces together. I mentioned my flash controller above, but I also have a SD Card controller I've used quite successfully. I've also posted a UART to Wishbone bridge and discussed network debugging, both of which I use routinely with the ZipCPU. If for no other reason, these components allow me to load or update software on my CPU even after it's been placed into an FPGA. Of course, many of those components are tied to a Wishbone bus infrastructure. You may find you need a bridge of some type to connect different buses structures together--memory naturally tends to operate at one width and clock, video at another, and your CPU at another, so it helps at times to have a universal bus adapter kit handy.

Beware, however, that you'll get what you pay for. There are some very small 8b CPU's I've seen implemented in less than 1k LUTs (iCE40). (I think this was the one.) The 32b class I work with tends to require about 4k LUTs (iCE40), and while the CPU can often fit nicely in that space the rest any design using a CPU can get kind of tight. It will also cost you more to [pipeline your CPU](). The cache can be quite expensive as well, but still fits nicely within an Artix-7 35T class FPGA--depending on the size and implementation of your cache. There are also lower logic alternatives, but again ... with all of these there are performance tradeoffs. Again, you get what you pay for.