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A project log for PDP - Processor Design Principles

Distilling my experience and wisdom about the architecture, organisation and design choices of my CPUs

yann-guidon-ygdesYann Guidon / YGDES 10/19/2021 at 01:480 Comments

Don't have instructions that can run only in certain "modes".

I know many CPU do it, even RISC-V, but I can't find a decent reason to accept this.

Remember : Special Registers are where all the configuration, and protection, occurs, so there should be only two instructions that enforce "capabilities" : GET and PUT. They, and only they, clear the pipeline and ensure serialisation, trigger a trap/fault if the address falls in a forbidden range.

The SR space can grow while keeping the very same management opcodes. You don't need to change the decoder if you add features, it's all in the SR circuits.

KISS, guys.