3 ms·

That's the question I have as well, but since microarchitecture is in large parts dictated by isa how custom it can really be? On the other hand custom isa woul

That's the question I have as well, but since microarchitecture is in large parts dictated by isa how custom it can really be? On the other hand custom isa would require rolling the entire software stack, which is a complete non-starter. Really weird article and announcement

mhh__ 5y ago
CPU design isn't like software. A microarchitecture is some you design rather than adding features in a software-like manner, there is an enormous scope for design choices within a given ISA. And using a custom uarch is also a statement to your customers, shareholders, competitors etc. that you mean business rather than just going the well-trodden path with ARM cores.
- jholman 5y ago
  I'm not disagreeing with either of your paragraphs, they're both correct... but the thing to note is that, to some degree, they argue in opposite directions. That's because a consequence of the second paragraph is that it's potentially business-profitable to make a "custom uarch" even if you make no use of the enormous scope for design changes.
  - mhh__ 5y ago
    I'm not sure what you mean.
    - jholman 5y ago
      ON THE ONE HAND..... The comment you replied to said "with a given ISA, how custom can the uarch be", implying that it would not be very custom. You replied, in your first paragraph, that on the contrary, it could be pretty custom, with the implication being that it probably would be, because that's the value proposition that Ampere is presumably offering. So your paragraph 1 is suggesting that the sentence "Ampere will now use a custom uarch" is an argument that Ampere's custom arch might well be pretty highly customized. ON THE OTHER HAND.... Your second paragraph points out that simply being custom is a social signal (the signal of being, in your words, "that you mean business rather than just going the well-trodden path"). You argue (and I agree) that this social signal might make you more attractive to some customers. I.e. the signal might increase sales. This implies that it would be reasonable for Ampere to spend a certain amount on a barely-custom custom uarch, purely for sales/marketting reasons, as long as it didn't substantially harm performance. You know, because of the implication. So your paragraph 2 is suggesting that the sentence "Ampere will now use a custom uarch" is NOT an argument that Ampere's custom arch has any particular need to be highly customized.
quanticle 5y ago
>but since microarchitecture is in large parts dictated by ISA Well, Apple uses the ARM ISA for their A and M series chips, and but not ARM's microarchitecture, and their chips have significantly higher IPC than comparable parts from Qualcomm or Samsung. Similarly, Intel and AMD both implement the same x86 ISA, but the performance advantage has swung repeatedly between the two. Microarchitecture might be "in large parts" dictated by the ISA, but there's still enough left over for hardware designers to have a significant influence on the overall performance.
ajb 5y ago
"microarchitecture is in large parts dictated by isa " This is very much not the case. Just consider that, for example, Intel still supports running the same ISA that ran on 8086. The microarchitecture now running those instructions isn't at all like the 8086 microarchitecture. (Its running a bunch of other instructions too, but the points stands) An example in the reverse direction, is that a some MIPS chip teams saw that MIPS was dying, and reused their MIPS microarchitectural designs to build ARM processors. Sure, ISA can be designed assuming some things about microarchitecture, and then it can be inconvenient to change it. But not impossible. And during the transition to ARM64, ARM took the opportunity to remove things that were inconvenient for different microarchitectures (eg, directly accessible PC reg, changes to how condition codes work, etc)
snvzz 5y ago
>but since microarchitecture is in large parts dictated by isa Refer to RISC-V unprivileged spec, chapter one. Not dictating microarchitecture is one of its goals.

mhh__ 5y ago

CPU design isn't like software. A microarchitecture is some you design rather than adding features in a software-like manner, there is an enormous scope for design choices within a given ISA. And using a custom uarch is also a statement to your customers, shareholders, competitors etc. that you mean business rather than just going the well-trodden path with ARM cores.

jholman 5y ago
I'm not disagreeing with either of your paragraphs, they're both correct... but the thing to note is that, to some degree, they argue in opposite directions. That's because a consequence of the second paragraph is that it's potentially business-profitable to make a "custom uarch" even if you make no use of the enormous scope for design changes.
- mhh__ 5y ago
  I'm not sure what you mean.
  - jholman 5y ago
    ON THE ONE HAND..... The comment you replied to said "with a given ISA, how custom can the uarch be", implying that it would not be very custom. You replied, in your first paragraph, that on the contrary, it could be pretty custom, with the implication being that it probably would be, because that's the value proposition that Ampere is presumably offering. So your paragraph 1 is suggesting that the sentence "Ampere will now use a custom uarch" is an argument that Ampere's custom arch might well be pretty highly customized. ON THE OTHER HAND.... Your second paragraph points out that simply being custom is a social signal (the signal of being, in your words, "that you mean business rather than just going the well-trodden path"). You argue (and I agree) that this social signal might make you more attractive to some customers. I.e. the signal might increase sales. This implies that it would be reasonable for Ampere to spend a certain amount on a barely-custom custom uarch, purely for sales/marketting reasons, as long as it didn't substantially harm performance. You know, because of the implication. So your paragraph 2 is suggesting that the sentence "Ampere will now use a custom uarch" is NOT an argument that Ampere's custom arch has any particular need to be highly customized.

quanticle 5y ago

>but since microarchitecture is in large parts dictated by ISA Well, Apple uses the ARM ISA for their A and M series chips, and but not ARM's microarchitecture, and their chips have significantly higher IPC than comparable parts from Qualcomm or Samsung. Similarly, Intel and AMD both implement the same x86 ISA, but the performance advantage has swung repeatedly between the two. Microarchitecture might be "in large parts" dictated by the ISA, but there's still enough left over for hardware designers to have a significant influence on the overall performance.

ajb 5y ago

"microarchitecture is in large parts dictated by isa " This is very much not the case. Just consider that, for example, Intel still supports running the same ISA that ran on 8086. The microarchitecture now running those instructions isn't at all like the 8086 microarchitecture. (Its running a bunch of other instructions too, but the points stands) An example in the reverse direction, is that a some MIPS chip teams saw that MIPS was dying, and reused their MIPS microarchitectural designs to build ARM processors. Sure, ISA can be designed assuming some things about microarchitecture, and then it can be inconvenient to change it. But not impossible. And during the transition to ARM64, ARM took the opportunity to remove things that were inconvenient for different microarchitectures (eg, directly accessible PC reg, changes to how condition codes work, etc)

snvzz 5y ago

>but since microarchitecture is in large parts dictated by isa Refer to RISC-V unprivileged spec, chapter one. Not dictating microarchitecture is one of its goals.