What are CPU designs which are not fetch/store but operate directly on RAM?
I only know about the design of the Nintendo Entertainment System (NES), where the CPU does not have registers (AFAIK) and operates directly on RAM, with fast access to low addresses in the RAM.
What CPUs/Systems do you know, which also do not do fetch/store for their operands? Which systems are out there? Why do CPUs like RISC/Arm/AMD64 use fetch/store, what are the tradeoffs? Are there different architectures for CPUs working on operands outside of fetch/store, DMA and stack machines?
As far as I know the 6502 clone in the NES has 3 general-purpose registers: X, Y and accumulator. Many of the instructions operate on them.
https://www.masswerk.at/6502/6502_instruction_set.html
It’s a good question though. I am just not aware of a mainstream CPU architecture without registers.