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The Framework Laptop 13 is about to become one of the world’s first RISC-V laptops
(www.theverge.com)
this post was submitted on 19 Jun 2024
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Could someone eli5 risc-v and why the fuss?
Edit: thanks for the replies. Searchingnfurther, this 15 min video is quite well made and told me more than I need to know (for now) https://www.youtube.com/watch?v=Ps0JFsyX2fU
Not an eli5 because I'm still not caught up on it but if my memory serves, RISC-V is an open source architecture for processors, basically like amd64 or arm64, actually I'm pretty sure ARM's chips are RISC derivatives.
Edit: correcting my comment, ARM makes RISC chips, not RISC-V
ARM and RISC-V are entirely different in that neither one is based on the other, but what they have in common is that they're both RISC (Reduced Instruction Set Computing) architectures. RISC is what makes ARM CPUs (in your phone, etc) so efficient and hopefully RISC-V will get there too.
x86 by comparison is Complex Instruction Set Computing, which allows for more performance in some cases, but isn't as efficient.
The original debate from the 80s that defined what RISC and CISC mean has already been settled and neither of those categories really apply anymore. Today all high performance CPUs are superscalar, use microcode, reorder instructions, have variable width instructions, vector instructions, etc. These are exactly the bits of complexity RISC was supposed to avoid in order to achieve higher clock speeds and therefore better performance. The microcode used in modern CPUs is very RISC like, and the instruction sets of ARM64/RISC-V and their extensions would have likely been called CISC in the 80s. All that to say the whole RISC vs CISC thing doesn't really apply anymore and neither does it explain any differences between x86 and ARM. There are differences and they do matter, but by an large it's not due to RISC vs CISC.
As for an example: if we compare the M1 and the 7840u (similar CPUs on a similar process node, one arm64 the other AMD64), the 7840u beats the M1 in performance per watt and outright performance. See https://www.cpu-monkey.com/en/compare_cpu-amd_ryzen_7_7840u-vs-apple_m1. Though the M1 has substantially better battery life than any 7840u laptop, which very clearly has nothing to do with performance per watt but rather design elements adjacent to the CPU.
In conclusion the major benefit of ARM and RISC-V really has very little to do with the ISA itself, but their more open nature allows manufacturers to build products that AMD and Intel can't or don't. CISC-V would be just as exciting.
compressed instruction set /= variable-width. x86 instructions are anything from one to a gazillion bytes, while RISC-V is four bytes or optionally (very commonly supported) two bytes. Much easier to handle.
RISC-V is (as far as I'm aware) the first ISA since Cray to use vector instructions. Certainly the only one that actually made a splash. SIMD isn't vector instructions, most crucially with vector insns the ISA doesn't care about vector length on an opcode level. That's like if you wrote MMX code back in the days and if you run the same code now on a modern CPU it's using just as wide registers as SSE3.
But you're right the old definitions are a bit wonky nowadays, I'd say the main differentiating factor nowadays is having a load/store architecture and disciplined instruction widths. Modern out-of-order CPUs with half a gazillion instructions of a single thread in flight at any time of course don't really care about the load/store thing but both things simplify insn decoding to ludicrous degrees, saving die space and heat. For simpler cores it very much does matter, and "simpler core" here can also could mean barely superscalar, but with insane vector width, like one of 1024 GPU cores consisting mostly of APUs, no fancy branch prediction silicon, supporting enough hardware threads to hide latency and keep those APUs saturated. (Yes the RISC-V vector extension has opcodes for gather/scatter in case you're wondering).
Then, last but not least: RISC-V absolutely deserves the name it has because the whole thing started out at Berkeley. RISC I and II were the originals, II is what all the other RISC architectures were inspired by, III was a Smalltalk machine, IV Lisp. Then a long time nothing, then lecturers noticed that teaching modern microarches with old or ad-hoc insn sets is not a good idea, x86 is out of the question because full of hysterical raisins, ARM is actually quite clean but ARM demands a lot, and I mean a lot of money for the right to implement their ISA in custom silicon, so they started rolling their own in 2010. Calling it RISC V was a no-brainer.
Oh for sure, but before the days of super-scalars I don't think the people pushing RISC would have agreed with you. Non-fixed instruction width is prototypically CISC.
If you can simplify the instruction decoding that's always a benefit - moreso the more cores you have.
You'll get no disagreement from me on that. Maybe you misunderstood what I meant by "CISC-V would be just as exciting"? I meant that if there was a popular, well designed, open source CISC architecture that was looking to be the eventual future of computing instead of RISC-V then that would be just as exciting as RISC-V is now.