Using the Pi4 to learn how to make cpu's in fpga's?
Making CPU in FPGA is simple, but.. what for? (if you are not a professional working for a CPU making company)
The purpose of making a CPU inside FPGA is:: instead of a lot of different logic circuits you make a simplest possible CPU ("nanoprocessor:) ) using customized instruction set and customized word legth and voila: it does its job using 500 LEs instead of sophisticated logics which costs 5000 LEs. And you don't need to make every job in nanoseconds..
I did an implementation of a retrocomputing machine in an FPGA board: the board costs something about $700... and the CPU works at something about 120 MHz/30 MIPS, then the graphic "chip" (also my project) struggles to display fullHD in 4 colors due to slow onboard RAM used for the framebuffer.
The RPi4/1G costs $35, has already 4 cores of CPU and 1 GB of fast RAM. It can display 4k in true color and I can write an emulator of the retro CPU on it, if I want, using optimized assembly code...
So yes,, FPGAs are fine, but not at the price level they are now.
RPi lacks some realtime high frequency GPIO related stuff but then you can always add a $8 Parallax Propeller chip to it and if you want analog inputs the Atmegas are even cheaper.
RISC-V is becoming popular in China and India, I wonder why?
Maybe because it is open?