this post was submitted on 19 Jun 2024
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[–] [email protected] 7 points 4 months ago* (last edited 4 months ago) (30 children)

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

[–] [email protected] 38 points 4 months ago* (last edited 4 months ago) (13 children)

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.

[–] [email protected] 1 points 4 months ago (4 children)

So is Reduced Instruction Set like in the old assembly days where you couldn't do multiplication, as there wasn't a command for it, so you had to do multiple loops of addition?

[–] [email protected] 6 points 4 months ago (1 children)

Right concept, except you're off in scale. A MULT instruction would exist in both RISC and CISC processors.

The big difference is that CISC tries to provide instructions to perform much more sophisticated subroutines. This video is a fun look at some of the most absurd ones, to give you an idea.

[–] [email protected] 2 points 4 months ago* (last edited 4 months ago)

ARM prominently has an instruction to deal with Javascript. And RISC-V will have those kinds of instructions, too, they're too useful, saving a massive amount of instructions and cycles and the CPU itself doesn't really need any logic added, the insn decoder just has to be taught a bit pattern and which microops to emit, the APUs already can do it.

What that instruction will never do in a RISC CPU though is read from memory.

On the flipside, some RISC-V macroops are CISC, fusing memory access and arithmetic. That's an architecture detail, though, only affecting code to the degree of "if you want to do this stuff, and want it to run faster on some cores, put those instructions in this exact sequence so the core can spot and fuse them).

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