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Could someone elaborate a bit on how RISC-V relates to ARM? From what I understand the former is a completely open source spec, and ARM is a proprietary one. B

Could someone elaborate a bit on how RISC-V relates to ARM? From what I understand the former is a completely open source spec, and ARM is a proprietary one. But both are just specs, right? It’s not as if I can buy a RISC-V CPU from an organization behind it, it’ll have to be from a manufacturer that produces their own RISC CPUs? Is it also an entirely different instruction set? How does it compare to x86 and ARM in terms of performance? Last but not least, other than it being free / opensource, what are some of the reasons why anyone would choose RISC? Phrased differently, if they are going to take marketshare in the CPU world in the next decade, what will be the reason?

2Gkashmiri 5y ago
take my comment with a bag full of sand, i am no expert, still when you talk about ARM, you have to license designs from the ARM, inc. Apple with its apple silicon is licensing from ARM the designs and IP from them. with RISC-v, there is no company to hold you hostage for designs and all. its all open source so you can build your own chip, own design, own tech, everything. this also means i assume that in the long run, if RISC-v and ARM are on equal footing, RISC-v would be more freer, cheaper because there is no licensing and other corporate stuff to go through like with ARM, inc
- goodpoint 5y ago
  > with RISC-v, there is no company to hold you hostage for designs and all No no no. The RISC-V ISA is open. The implementation of RISC-V CPUs is often proprietary.
  - 2Gkashmiri 5y ago
    OK. So can't there be multiple competing implementations? Proprietary but some like those university projects, Foss ?
    - monocasa 5y ago
      Yeah, the spec is free with a permissive license, and individual implementations are expanding to fill the full gamut of free to proprietary.
    - goodpoint 5y ago
      There are open and closed implementations. Unfortunately the license on the ISA does not require implementations to be open.
      - LeFantome 5y ago
- WanderPanda 5y ago
  How to prevent e.g. Alibaba from embrace-extend-extinguishing RISC-V? It seems so obvies that this will happen in one way or another
  - snvzz 5y ago
    Upstream toolchains won't readily accept extensions that aren't part of the standard, and maintaining forks is costly. There is no incentive not to get extensions standardized. In the first place, what's even missing in RISC-V? As of December's 15 extensions, it is pretty much there already, in terms of functionality.
    - brucehoult 5y ago
      Exactly. Extend it with what? Intel and AMD each have instructions in their x86 chips that the other doesn't have. No one cares, pretty much. If you're building your own performance-critical software to run on a specific chip then you might enable some of them (or just do "gcc -march=native") but software distributed in binary form generally gets on fine without them.
- f0xJtpvHYTVQ88B 5y ago
  ARM does develop their own core designs which they license to many companies. However, they have several different licenses available which range from using ARM's designs, tweaking ARM's designs, up to the Architectural license which allows companies to develop their own ARM ISA compatible cores own their own. Apple is one of the companies which has an Architectural license. The current cores used by Apple were designed by Apple and not based on designs from ARM.
jchw 5y ago
Nit: you shouldn’t use RISC as a shorthand for RISC-V. The former is a well-known term. Yes, RISC-V is an open ISA, yes, like ARM there is no fab making canonical RISC-V chips. It does at least have the benefit of having support in the UEFI spec pretty much right out of the gate, which offers a standard boot path of sorts (Arm64 also supports UEFI, but most devices to this day have their own bespoke boot firmware and protocols, AFAIK. The unifying factor tends to be something like uboot, again, AFAIK.) but it also has more flexibility, which has the knock-on effect that it may be harder to know which RISC-V software can run on which RISC-V hardware. I’m very curious to see where it goes. As a tinkerer I definitely have been looking for entire desktop computers that, weak or not, are truly open source and documented from the ground up. RISC-V platforms don’t solve that entirely (in fact it’s probably the remainder of SoC IP that poses the biggest problems) but it does offer an interesting option for a CPU.
monocasa 5y ago
The spec is free, and a lot of cores are free as well. Here's the HDL for AliBaba's cores they've been developing this against: https://github.com/T-head-Semi/openc910 https://github.com/T-head-Semi/openc910 There are also non-free RISC-V cores, but they essentially have to continuously make the argument for their existence against the free cores that are available (support, verification resources, just being better designs in the short term, already being hardened for proprietary process nodes, etc.)
imtringued 5y ago
>Is it also an entirely different instruction set? How does it compare to x86 and ARM in terms of performance? It's a bit of a mixed bag. I'd say if macro op fusion turns out to be viable, it can become the fastest of the three, if not, it will be somewhere between ARM and x86 due to its unique compression scheme. This obviously depends on your particular chip. Apple is way ahead of Qualcomm on the same ISA.
- snvzz 5y ago
  >macro op fusion turns out to be viable Macro-op fusion predates RISC-V. What's new is that RISC-V has been designed with knowledge of Macro-op fusion, which gave it some opportunities. I'd say it weights very little overall; RISC-V is very clever, irrespective of fusion.
mhh__ 5y ago
RISC-V is a nice clean design but ARM is also a fairly clean but less nice design, by people who have their face on the grindstone 24/7. Winning is likely to be a question of luck (a la Unix perhaps) and implementation quality.

2Gkashmiri 5y ago

take my comment with a bag full of sand, i am no expert, still when you talk about ARM, you have to license designs from the ARM, inc. Apple with its apple silicon is licensing from ARM the designs and IP from them. with RISC-v, there is no company to hold you hostage for designs and all. its all open source so you can build your own chip, own design, own tech, everything. this also means i assume that in the long run, if RISC-v and ARM are on equal footing, RISC-v would be more freer, cheaper because there is no licensing and other corporate stuff to go through like with ARM, inc

goodpoint 5y ago
> with RISC-v, there is no company to hold you hostage for designs and all No no no. The RISC-V ISA is open. The implementation of RISC-V CPUs is often proprietary.
- 2Gkashmiri 5y ago
  OK. So can't there be multiple competing implementations? Proprietary but some like those university projects, Foss ?
  - monocasa 5y ago
    Yeah, the spec is free with a permissive license, and individual implementations are expanding to fill the full gamut of free to proprietary.
  - goodpoint 5y ago
    There are open and closed implementations. Unfortunately the license on the ISA does not require implementations to be open.
    - LeFantome 5y ago
WanderPanda 5y ago
How to prevent e.g. Alibaba from embrace-extend-extinguishing RISC-V? It seems so obvies that this will happen in one way or another
- snvzz 5y ago
  Upstream toolchains won't readily accept extensions that aren't part of the standard, and maintaining forks is costly. There is no incentive not to get extensions standardized. In the first place, what's even missing in RISC-V? As of December's 15 extensions, it is pretty much there already, in terms of functionality.
  - brucehoult 5y ago
    Exactly. Extend it with what? Intel and AMD each have instructions in their x86 chips that the other doesn't have. No one cares, pretty much. If you're building your own performance-critical software to run on a specific chip then you might enable some of them (or just do "gcc -march=native") but software distributed in binary form generally gets on fine without them.
f0xJtpvHYTVQ88B 5y ago
ARM does develop their own core designs which they license to many companies. However, they have several different licenses available which range from using ARM's designs, tweaking ARM's designs, up to the Architectural license which allows companies to develop their own ARM ISA compatible cores own their own. Apple is one of the companies which has an Architectural license. The current cores used by Apple were designed by Apple and not based on designs from ARM.

jchw 5y ago

Nit: you shouldn’t use RISC as a shorthand for RISC-V. The former is a well-known term. Yes, RISC-V is an open ISA, yes, like ARM there is no fab making canonical RISC-V chips. It does at least have the benefit of having support in the UEFI spec pretty much right out of the gate, which offers a standard boot path of sorts (Arm64 also supports UEFI, but most devices to this day have their own bespoke boot firmware and protocols, AFAIK. The unifying factor tends to be something like uboot, again, AFAIK.) but it also has more flexibility, which has the knock-on effect that it may be harder to know which RISC-V software can run on which RISC-V hardware. I’m very curious to see where it goes. As a tinkerer I definitely have been looking for entire desktop computers that, weak or not, are truly open source and documented from the ground up. RISC-V platforms don’t solve that entirely (in fact it’s probably the remainder of SoC IP that poses the biggest problems) but it does offer an interesting option for a CPU.

monocasa 5y ago

The spec is free, and a lot of cores are free as well. Here's the HDL for AliBaba's cores they've been developing this against: https://github.com/T-head-Semi/openc910 https://github.com/T-head-Semi/openc910 There are also non-free RISC-V cores, but they essentially have to continuously make the argument for their existence against the free cores that are available (support, verification resources, just being better designs in the short term, already being hardened for proprietary process nodes, etc.)

imtringued 5y ago

>Is it also an entirely different instruction set? How does it compare to x86 and ARM in terms of performance? It's a bit of a mixed bag. I'd say if macro op fusion turns out to be viable, it can become the fastest of the three, if not, it will be somewhere between ARM and x86 due to its unique compression scheme. This obviously depends on your particular chip. Apple is way ahead of Qualcomm on the same ISA.

snvzz 5y ago
>macro op fusion turns out to be viable Macro-op fusion predates RISC-V. What's new is that RISC-V has been designed with knowledge of Macro-op fusion, which gave it some opportunities. I'd say it weights very little overall; RISC-V is very clever, irrespective of fusion.

mhh__ 5y ago

RISC-V is a nice clean design but ARM is also a fairly clean but less nice design, by people who have their face on the grindstone 24/7. Winning is likely to be a question of luck (a la Unix perhaps) and implementation quality.