Comment by snvzz

There is no doubt that Aarch64 is a much higher quality ISA than RISC-V. The good or bad licensing model used for an ISA has nothing to do with the technical quality of an ISA.

Even the Intel/AMD ISA, despite its horrible encoding, after excluding many hundreds of obsolete instructions that are not needed any more and after excluding from the instruction encoding the prefix bytes that are needed only for backward compatibility, would be a higher quality ISA than RISC-V, especially for expressing any task that is computationally intensive. RISC-V is particularly bad for expressing computations with big integers.

The modern subset of the Intel/AMD ISA is better than RISC-V even from the point of view of some of the design criteria on which RISC-V is based.

For instance, the designers of RISC-V have omitted many useful features under the pretext that for good performance those features would require additional read and write ports to the register file.

The Intel/AMD ISA, by allowing one of the three operands of an instruction to be a memory operand, allows identical performance with an ISA with 3 register operands, while having one less read port and one less write port in the register file.

Having instructions with one memory operand works well only in CPUs with out-of-order execution. Nevertheless, the same performance as for an ISA with a memory operand can be achieved in a low-cost in-order CPU if there are a set of explicitly addressable load buffer registers, and one or more operands of an instruction can be a load buffer, instead of a general-purpose register.

So there would have been many better ways of accomplishing the design criteria of RISC-V. RISC-V is a simple ISA that is great for teaching, but its only advantage over any ISA that a competent designer could conceive in a few days is the large amount of already existing software tools, i.e. compilers, linkers, debuggers and so on, which would take years to duplicate for any brand-new ISA.

I’ve heard really good things about SuperH (sh-2, sh-4 etc.), designed by Hitachi for their processors including those used in the Sega Saturn and Sega Dreamcast. Really high code density was one big thing, but it was covered by patents until recently. There was a group trying to make a modern open source processor in VHDL for FPGAs and later on ASICs based on it, but I think it may have mostly fizzled out.

WDC 65C02, or even WDC 65C816.

Or how about MMIX?

Well first you need to define your criteria for "high quality"