I was discussing with a colleague about the concept of architecture license in RISC-V. I realized that, in the open-source world, it can be a little tricky to grasp.

In a traditional processor IP model, there is a clear distinction between an off-the-shelf IP license that gives some level of configuration but no customization, and a fairly expensive architecture license enabling a licensee to use the instruction set with their own custom microarchitecture.

With RISC-V, the complication comes from the fact that it is often described as “an open-source architecture”, so people believe that some source code is licensed. But actually that is not the case at all.

Traditional IP and architecture licensing

In a traditional model, things are quite straight forward. A standard license for Arm or MIPS lets the customer use an RTL design but not change it at all (aside from a few configuration options perhaps).

Meanwhile, for customers willing to spend a lot of money, an architecture license gives them the right to modify how a processor executes instructions (the issue width, the cache size, etc.). However, it does not generally give the right to modify the instructions (with some exceptions such as the Cortex-M33 that supports Arm Custom Instructions, allowing the implementation of bespoke data processing operations, or Cadence Tensilica).

RISC-V based IP and architecture licensing

In an open-source model such as RISC-V, things are somewhat different

The RISC-V architecture is usually described as “open-source”, which implies everyone can use it at no cost.

However, a better description of RISC-V is that it is an “open architecture” or “open standard”. In that sense RISC-V is like C, Wi-Fi or LTE with RISC-V International performing the role of (respectively) ANSI, IEEE 802.11 and 3GPP in defining and managing standards that people are free to implement as they choose. But that is a written standard – not an implementation or a microarchitecture.

Just as is the case with those other open standards,  RISC-V licenses can either be open-source or commercial.

You can download open-source designs and have complete freedom to modify them however you wish. Boom, PULP, SweRV and other open-source designs give absolute freedom. But that comes with a cost: they are not supported, the verification is often troublesome, and they may not be of sufficient quality to use in a commercial design. Some companies do use them accepting those compromises; others are understandably wary.

Or you can buy a commercial RISC-V design. There are many companies offering high quality cores delivered as RTL with warranty and full product support. These can be an excellent solution for many customers. Then we are back to something similar to the traditional model: a sort of black box design – although based on an open-standard ISA – in that it cannot be modified or customized to address specific needs. But for many purposes that generic product will be a good fit.

Codasip, as a RISC-V core vendor, does a lot of business on this basis: customers buy a standard RISC-V processor core delivered as RTL and SDK, with high performance, “best-in-class” verification and full support. That is without any architecture license fees, to use it as it is, off the shelf.

ISA customization enables Design for Differentiation

But Codasip offers another option. This is an architecture license – and more – delivered as a source code in the CodAL processor description language.

Many of our customers buy a standard Codasip processor IP product delivered as CodAL source and then use Codasip Studio™  which enables them to modify it freely. We provide the flexibility to modify both the microarchitecture and the ISA, precisely what one needs to Design for Differentiation. Customization at ISA level brings higher performance and optimization. What is more, the power and elegance of the Codasip Studio toolset makes this very easy.

This is different to, for example, an Arm architecture license in three ways:

Freedom

Arm, even with an architecture license, constrains what you are allowed to do. Codasip does not: it is your core and you have control.

Tooling

Arm cores are developed internally, in traditional ways and not designed to be easy to modify. In contrast, all of Codasip’s cores are developed using Studio and are designed expressly to make customization (both ISA and microarchitecture) straightforward and efficient. That includes automatically creating the software toolchain (customized compiler etc) and verification.

Cost

Traditionally the limitation of architecture licenses has been both the cash cost of the license and the engineering resources (cost) required to take advantage of it. Codasip and Studio now make that far easier and hence more affordable with a complete end-to-end architecture customization solution. This dramatically changes the cost equation both of the architecture license fee and the engineering resources required.

RISC-V offers the promise of openness: Codasip and codasip Studio offer custom compute to make the promise a reality

Codasip offers the best of both worlds: a portfolio of high-quality standard cores that are a good fit for standard applications, with full verification and support. Or you can upgrade to a cost-effective architecture license, and benefit from custom compute: Architecture optimization, hardware/software co-design, and domain-specific acceleration. Doing so, enables you to freely customize the core in an easy-to-use environment and have a unique product for your unique needs.

Rupert Baines

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