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Everybody is familiar with commercial licensing from traditional processor IP vendors such as Arm, Cadence, and Synopsys. But in discussing the RISC-V Open Instruction Set Architecture (ISA), there is widespread confusion of terminology with RISC-V often being described as “open source”. Some have even accused vendors of commercial RISC-V IP such as Codasip or Andes as not being in the spirit of RISC-V. But what is reality?

Let’s look at definitions briefly. An open standard like C, Verilog or HTTP is defined by a document that is maintained by an independent organisation. Thus, C is maintained by ISO, Verilog by IEEE, and HTTP by IETF. These organisations maintain the technical standards using a set of impartial rules. Such open standards are generally freely accessible.

With open source, the source code for a software package or the hardware description language source for a hardware block are made available using a license. Open-source licenses vary from restrictive ones, such as copyleft license, to permissive ones, such as Apache. An open-source license defines rights for using, studying, modifying, and distributing the code. A copyleft license will require that any modifications be open-sourced, while a permissive license will not.

The RISC-V is an open standard, and the ISA does not define any microarchitecture or business model. Therefore, a RISC-V microarchitecture can be licensed either as a commercial IP license or as an open source one. Nothing is prescribed.

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