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.
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?
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.
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).
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.
vector instructions,
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.
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).
If you can simplify the instruction decoding that’s always a benefit - moreso the more cores you have.
Then, last but not least: RISC-V absolutely deserves the name it has because the whole thing started out at Berkeley.
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.
If you still have commenting motivation, what are the top 5 differences between x86 and ARM?
Up until your post I had thought it exactly was the size of the instruction set with x86 having lots of very specific multi-step-in-a-single instruction as well as crufty instruction for backwards compatibility (like MPSADBW).
I’m more familiar with RISC-V than I am with ARM though it’s my understanding they’re quite similar.
ARM/RISC-V are load-store architectures, meaning they divide instructions between loading/storing and doing computation. x86 on the other hand is a register-memory architecture, having instructions that do both computation as well as loading/storing.
ARM/RISC-V also have weaker guarantees as to memory ordering allowing for less synchronization between cores, however RISC-V has an extension to enforce the same guarantees as x86 and Apple’s M-series CPU have a similar extension for ARM. If you want to emulate x86 applications on ARM/RISC-V these kinds of extensions are essential for performance.
ARM/RISC-V instructions are variable width but only in a limited sense. They have “compressed instructions” - 2 bytes instead of 4 - to increase instruction density in order to compete with x86’s true variable width instructions. They’re fairly close in instruction density, though compressed instructions are annoying for compilers to handle due to instruction alignment. 4 byte instructions must be aligned to 4 bytes, so if you have 3 instructions A, B and C but only B has a compressed version then you can’t actually use it because there must be 4 bytes between instructions A and C.
ARM/RISC-V also makes backwards compatibility entirely optional, Apple’s M-series don’t implement 32-bit mode for instance, whereas x86-64 still has “real mode” for running 16 bit operating systems.
There’s also a number of other differences, like the number of registers, page table formats, operating modes, etc, but those are the more fundamental ones I can think of.
Up until your post I had thought it exactly was the size of the instruction set with x86 having lots of very specific multi-step-in-a-single instruction as well as crufty instruction for backwards compatibility (like MPSADBW).
The MPSADBW thing likely comes from the hackaday article on why “x86 needs to die”. The kinda funny thing about that is MPSADBW is actually a really important instruction for (apparently) video decoding; ARM even has a similar instruction called SABD.
x86 does have a large number of instructions (even more so if you want to count the variants of each), but ARM does not have a small number of instructions and a lot of that instruction complexity stops at the decoder. There’s a whole lot more to a CPU than the decoder.
The CISC vs RISC thing is dead. Also modern ARM ISAs aren’t even RISC anymore even if that’s what they started out as. People have no idea what’s going on with modern technology.
X86 can actually be quite low power (see LPE cores and Intel Atom). The producers of x86 don’t specialize in that though, unlike a lot of RISC-V and ARM producers. It’s not that it’s impossible, just that it isn’t typically done that way.
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.
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?
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.
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).
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.
Thank you so much for this information.
If you still have commenting motivation, what are the top 5 differences between x86 and ARM?
Up until your post I had thought it exactly was the size of the instruction set with x86 having lots of very specific multi-step-in-a-single instruction as well as crufty instruction for backwards compatibility (like MPSADBW).
I’m more familiar with RISC-V than I am with ARM though it’s my understanding they’re quite similar.
ARM/RISC-V are load-store architectures, meaning they divide instructions between loading/storing and doing computation. x86 on the other hand is a register-memory architecture, having instructions that do both computation as well as loading/storing.
ARM/RISC-V also have weaker guarantees as to memory ordering allowing for less synchronization between cores, however RISC-V has an extension to enforce the same guarantees as x86 and Apple’s M-series CPU have a similar extension for ARM. If you want to emulate x86 applications on ARM/RISC-V these kinds of extensions are essential for performance.
ARM/RISC-V instructions are variable width but only in a limited sense. They have “compressed instructions” - 2 bytes instead of 4 - to increase instruction density in order to compete with x86’s true variable width instructions. They’re fairly close in instruction density, though compressed instructions are annoying for compilers to handle due to instruction alignment. 4 byte instructions must be aligned to 4 bytes, so if you have 3 instructions A, B and C but only B has a compressed version then you can’t actually use it because there must be 4 bytes between instructions A and C.
ARM/RISC-V also makes backwards compatibility entirely optional, Apple’s M-series don’t implement 32-bit mode for instance, whereas x86-64 still has “real mode” for running 16 bit operating systems.
There’s also a number of other differences, like the number of registers, page table formats, operating modes, etc, but those are the more fundamental ones I can think of.
The MPSADBW thing likely comes from the hackaday article on why “x86 needs to die”. The kinda funny thing about that is MPSADBW is actually a really important instruction for (apparently) video decoding; ARM even has a similar instruction called SABD.
x86 does have a large number of instructions (even more so if you want to count the variants of each), but ARM does not have a small number of instructions and a lot of that instruction complexity stops at the decoder. There’s a whole lot more to a CPU than the decoder.
You can pay ARM to build and sell cores, you can’t do that for x86.
The CISC vs RISC thing is dead. Also modern ARM ISAs aren’t even RISC anymore even if that’s what they started out as. People have no idea what’s going on with modern technology.
X86 can actually be quite low power (see LPE cores and Intel Atom). The producers of x86 don’t specialize in that though, unlike a lot of RISC-V and ARM producers. It’s not that it’s impossible, just that it isn’t typically done that way.