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RISC-V: An Open Standard for SoCs

The case for an open ISA
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pattrsn
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Re: Why not build on OpenRISC
pattrsn   8/7/2014 12:38:36 PM
If you want to learn more about the RISC-V ISA, go to riscv.org

pattrsn
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Re: Why not build on OpenRISC
pattrsn   8/7/2014 12:31:58 PM
We started in 2010, when OpenCore only had a 32-bit address space, which was a fatal flaw that was later corrected. It is still missing the small code size option, which is requirement for IoT. 

And I am not sure if everyone understands the importance of the "Base+Extension" approach to instruction sets. This is a new approach to coping with software compatability of instruction sets. As we wrote in the associated technical report:



"RISC-V is aimed at SoCs, with a base that should never change given the longevity of the basic RISC ideas; a standard set of optional extensions that will evolve slowly; and unique instructions per SoC that never need to be reused."

Software compatability with controlled evolution.

(And it's really hard in 2014 to embrace an ISA that offers delayed branches:)

jeremybennett
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Why not build on OpenRISC
jeremybennett   8/7/2014 12:07:07 PM
NO RATINGS
I'm not sold on the idea that we have to start from scratch to get an industry standard open ISA. As the article notes, we already have OpenRISC, which comes with an open bus standard (WishBone). The architecture is based on something well proven (DLX, of which David Patterson was half the design team), for which there is plenty of tutorial material.

It takes a long time to build all the software infrastructure around a new ISA. Surely far better to start with something like OpenRISC that has spent 15 years invested in its software.

The OpenRISC architecture doesn't tick all the author's boxes, but it is extensible, and the missing features (do we really need 128-bit addressing) could be added.

One feature that is not mentioned is multiprocessor support. Thanks to the work of Stefan Wallentowitz at TU Munich and others, this is something that OpenRISC now supports. It seems to me this ought to be a key feature of any new ISA.

The authors are two engineers for whom I have the greatest respect. I wish RISC-V well, because this team is certain to innovate, and that can only be good for the field. But as the basis of an industry standard open ISA? I wish they had built on what was already there, rather than starting again from scratch.

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