4 ms·

I hear a lot of people talking about RISC-V but I wonder why people don't think companies would move to other open ISAs like Power or MIPS.

Teknoman117 6y ago
"MIPS Open" came after the RISC-V announcement, and is still currently somewhat of a joke. Half the links on the MIPS Open site are dead. I think one of the major points for RISC-V was to avoid the possibility of patent encumbrance of the ISA so that it can be freely used for educational purposes. My computer architecture courses 5-6 years ago used MIPS I heavily. MIPS was not open at the time, but any patents for the MIPS-I ISA had long since expired. POWER is actually open, but it is tremendously more complicated. RISC-V by comparison feels like it borrows heavily from the early MIPS ISAs, just with a relaxation of the fixed sized instructions and no architectural delay slots and a commitment to an extensible ISA (MIPS had the coprocessor interface, but I digress). The following is my own experience - while obviously high performance CPU cores are the product of intelligent multi-person teams and many resources, I believe RISC-V is simple enough that a college student or two could implement a compliant RV32I core in an HDL course if they knew anything about computer architecture. It wouldn't be a peak performance design by any measure (if it was they should be hired by a CPU company), but I think that's actually a point of RISC-V as an educational platform AND a platform for production CPU cores.
- Symmetry 6y ago
  As a teaching tool RISC-V is clearly great, as it is for companies that want to add custom instructions to their microcontrollers like NVidia or WD. But if I was looking to design a core to run user applications then to me it looks like everything is stacked in favor of Power. The complexity of the ISA is dwarfed by the complexity of a performant superscalar architecture. And to be performant in RISC-V you'd probably be needing extensive instruction fusion and variable length instructions anyways further equalizing things. And you really need the B extension which hasn't been standardized yet. Plus binary compatibility is a big concern on application cores and ISA extensions get in the way of that.
  - Teknoman117 6y ago
    I totally agree.
- rrss 6y ago
  > still currently somewhat of a joke. Half the links on the MIPS Open site are dead. unsurprising given that Wave Computing shut down the project almost a year ago and subsequently declared bankruptcy.
  - Teknoman117 6y ago
    Oh I missed that, thanks for the info.
gumby 6y ago
Because they aren’t as open in reality as they sounded when announced. The advantages of those two (and of ARM) is that there actual Implementations with decades of development behind them, Yes, some technical debt but also many painfully-learned good decisions. RISC V, which I’m really excited about, you can think of as a PRD (customer’s perspective product requirements document). That’s what an ISA is. Each team builds to meet that using their own iomplemetation, none of which is more than a few years old yet. But the teams incorporate decades of learning , and have largely a blank sheet to start with. I think it will be great....but isn’t yet.
seldridge 6y ago
The MIPS Open ISA project was actually shut down after only a year. [^1] POWER has technically only been "open" for a little under a year. OpenPOWER was always a thing, but this used to mean that companies could participate in the ISA development process and then pay license fees to build a chip. This changed last year when POWER went royalty-free (like RISC-V). The real defniition of "open" is can you answer the following questions in the negative: - Do I need to pay someone for a license to implement the ISA? - Is the ISA patent-unencumbered? RISC-V was the only game in town for a long time and thereby attracted large companies (including NVIDIA) and startups that were interested in building their own microprocessors, but didn't want to pay license fees or get sued into oblivion. [^1]: https://www.hackster.io/news/wave-computing-closes-its-mips-open-initiative-with-immediate-effect-zero-warning-e88b0df9acd0 https://www.hackster.io/news/wave-computing-closes-its-mips-...

Teknoman117 6y ago

"MIPS Open" came after the RISC-V announcement, and is still currently somewhat of a joke. Half the links on the MIPS Open site are dead. I think one of the major points for RISC-V was to avoid the possibility of patent encumbrance of the ISA so that it can be freely used for educational purposes. My computer architecture courses 5-6 years ago used MIPS I heavily. MIPS was not open at the time, but any patents for the MIPS-I ISA had long since expired. POWER is actually open, but it is tremendously more complicated. RISC-V by comparison feels like it borrows heavily from the early MIPS ISAs, just with a relaxation of the fixed sized instructions and no architectural delay slots and a commitment to an extensible ISA (MIPS had the coprocessor interface, but I digress). The following is my own experience - while obviously high performance CPU cores are the product of intelligent multi-person teams and many resources, I believe RISC-V is simple enough that a college student or two could implement a compliant RV32I core in an HDL course if they knew anything about computer architecture. It wouldn't be a peak performance design by any measure (if it was they should be hired by a CPU company), but I think that's actually a point of RISC-V as an educational platform AND a platform for production CPU cores.

Symmetry 6y ago
As a teaching tool RISC-V is clearly great, as it is for companies that want to add custom instructions to their microcontrollers like NVidia or WD. But if I was looking to design a core to run user applications then to me it looks like everything is stacked in favor of Power. The complexity of the ISA is dwarfed by the complexity of a performant superscalar architecture. And to be performant in RISC-V you'd probably be needing extensive instruction fusion and variable length instructions anyways further equalizing things. And you really need the B extension which hasn't been standardized yet. Plus binary compatibility is a big concern on application cores and ISA extensions get in the way of that.
- Teknoman117 6y ago
  I totally agree.
rrss 6y ago
> still currently somewhat of a joke. Half the links on the MIPS Open site are dead. unsurprising given that Wave Computing shut down the project almost a year ago and subsequently declared bankruptcy.
- Teknoman117 6y ago
  Oh I missed that, thanks for the info.

gumby 6y ago

Because they aren’t as open in reality as they sounded when announced. The advantages of those two (and of ARM) is that there actual Implementations with decades of development behind them, Yes, some technical debt but also many painfully-learned good decisions. RISC V, which I’m really excited about, you can think of as a PRD (customer’s perspective product requirements document). That’s what an ISA is. Each team builds to meet that using their own iomplemetation, none of which is more than a few years old yet. But the teams incorporate decades of learning , and have largely a blank sheet to start with. I think it will be great....but isn’t yet.

seldridge 6y ago

The MIPS Open ISA project was actually shut down after only a year. [^1] POWER has technically only been "open" for a little under a year. OpenPOWER was always a thing, but this used to mean that companies could participate in the ISA development process and then pay license fees to build a chip. This changed last year when POWER went royalty-free (like RISC-V). The real defniition of "open" is can you answer the following questions in the negative: - Do I need to pay someone for a license to implement the ISA? - Is the ISA patent-unencumbered? RISC-V was the only game in town for a long time and thereby attracted large companies (including NVIDIA) and startups that were interested in building their own microprocessors, but didn't want to pay license fees or get sued into oblivion. [^1]: https://www.hackster.io/news/wave-computing-closes-its-mips-open-initiative-with-immediate-effect-zero-warning-e88b0df9acd0 https://www.hackster.io/news/wave-computing-closes-its-mips-...