If you’re interested in experimenting with a tiny CPU implemented in an FPGA, we found this interesting article on using the Zylin Tiny CPU. AMR wanted a really small CPU core to handle things like loading ROMs from SD card. It had to take up a minimal number of logic elements and be easy to program using C. AMR finally settled on the Zylin CPU (ZPU), implemented in a DE1 board. He explains, “The ZPU is an interesting design. There’s no register file: instead, it’s stack based, and the instruction set is split into compulsory and optional instructions which allows an implementation to trade speed for resource usage. Unimplemented instructions are emulated by way of an exception table stored in low memory, and if any particular emulated instruction turns out to be a performance killer for your particular application it’s easy to add support for just that one instruction to the CPU implementation.”
You can find the code for this project and details at RetroRamblings. Visit Zylin for information on the official open source Zylin CPU design.